EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: chipwitch on March 05, 2014, 04:16:30 am
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This probably isn't the place to be asking this question, but I'm hoping someone might be able to point me in the right direction.
We have a 25 HP rotary phase converter. Mostly, it operates small machine tools. The two largest machines are a 6.5 HP Manual lathe and a 10HP CNC mill. Everything else is under two horse, all manual (no fancy computers or electronics).
The problem we've had is that we've burned up the insulation on two of the pony motors, the motor that generates the third leg. Both were brand new Reliance (made by Baldor) 25HP, open frame motors. They're kept outside, in a large doghouse with plenty of ventilation and away from the machine shop environment. Tear down's revealed insulation breakdown in the stator. The single phase electric feeding the motor is a 50A Cutler Hammer breaker. Not the new cheap crap... the old copper bus kind. The motor is located less than ten feet from the panel. Rarely is the pony motor run for more than 4 hours at a time. It's never started under load. The last motor started fine the last time it ran. 30 seconds after starting, before we even turned on a machine, the motor smoked. No load. Nothing powered.
I can't for the life of me figure out what could be causing this to happen short of low voltage. Two motors in two years. Less than 300 hours on each. Short of a heavy load, I can't imagine anything one could do electrically to cause the insulation to burn through. From the outside, there's nothing that would indicate this wasn't still a brand new motor. No discolored paint. No outward appearance of heat exposure.
I've spoken with the manufacturer. He's been very helpful in helping me investigate this. His best guess is that harmonics from the CNC mill is causing the breakdown as the spindle is controlled by a Baldor Drive, a type of VFD. I don't think he's 100% convinced of it being harmonics, but that's all we have to go on at this point. He's suggested a line reactor.
I'd never heard of a line reactor before. Seemed like a good idea at first and I had planned on buying one. What reading I've done since, sounds like sometimes they can cause more problems than they aim to help and that selecting the right one isn't a trivial matter. Plus, while they help smooth harmonic spikes, will they smooth them enough?
When I google 3 phase converters and line reactors, there seems to be a lot of people not using them together. I even called a man who used to be a technician for Fadal (the maker of our mill and no longer in business). He's since opened his own service company. He said he's seen lots of people running these machines on phase converters without a problem.
If anyone here knows anything about these things, I'd sure be appreciative. I'm just not sure where else to turn.
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The motors are probably not designed for that use.
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There is a lot of stuff going on in dynamic phase converter systems. Most of them have gone away in favor of static phase converters in recent years. I actually just demonstrated a small one for Chris Gammel the other day actually. Let me start out by saying I use KBVF series inverters to drive 3 phase equipment from 120. I use a small inverter for each machine and that gives me individual speed control to boot! Anyways, most older shops that used old Phase-O-Matics would just have a 3 phase motor connected to it, lets say 2 horse or so with a 50 pound flywheel that just ran constantly and that served as a dynamic filter of sort for all the equipment. Typically in the dynamic systems when a big load is applied, or a motor starts stalling, or something is taking time to get up to speed the phases get out of wack and if one of the phases lags you can get large voltage spikes that take out windings. The motor and flywheel helps bring the load up to speed and keel things smooth and provides a sink for overvoltages. For critical stuff you see those SOLA AC regulator/reactors from time to time, but they are expensive. Especially 3 phase ones. The static converters have all the ramp up, ramp down, over current, and stall prevention built in. KBVF stands for King Bearing Variable Frequency. The ones I use take 120 in and kick out 208 3 phase. They have a whole range of hp and voltages. They are reasonably priced too.
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They're kept outside, in a large doghouse with plenty of ventilation
Are the motors sealed or open?
Plenty of ventilation can also mean plenty of dust/leaves/dirt/crap/rain can be blown inside and get sucked into the motor.
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As I stated in the op, the motor is open frame, in a doghouse away from the machines environment. Rain? Not a chance. Dust? Whatever accumulates in a couple months time in a close dog house.
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Can you measure the 3 phase currents to the motor during operation to check against the nameplate ratings of the motor? Does the system ever have problems starting the motor? If the motor failed to start it would sit there stalled and burn up if the protection devices failed to operate.
What kind of start and run capacitors does the system use? If the starting relay is welded and left the start cap connected, that could potentially burn up the motor, however I'd expect the intermittent duty rated start caps to explode first.
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I would also change the start and run capacitors as well, if one is shorted or open it will cause the motor to overheat. If they have dropped in value you can get a resonance in the winding that builds up voltage until something burns.
The line reactor is put in series with the CNC inverter only, and is generally a part that you get from the inverter manufacturer. It is rated for the inverter only, so is quite small as it is only going to handle the inverter.
Best is to check the phase currents and capacitor currents on the motor as well.
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Thanks for the replies:
@Tesla -- First off, I have a TEFC motor, no nameplate that I intend to use as a replacement. I took it to a motor repair shop for testing. They were kind enough to test it without charge. He estimated the HP at 20 to 25. It was drawing 16A, no load, IIRC. Now, to your question, I'm avoiding connecting it until I'm sure how to proceed. If it means installing a line reactor, then I'll have to order one. So, I'm not able to test the current for that reason only. It's not hooked up yet.
Physically, all the caps look good. Nothing in the control box looks abnormal. Motor has started every time without incident. There's no monitoring equipment connected to the supply or load. There didn't seem to be a reason for it. Had no indication current was a problem, but if I recall, I did put an amprobe on it when I reconnected the second motor. Everything seemed in order, but it was just a momentary evaluation.
As for failing to start, the control box uses a toggle switch for run and a MC switch for start. It's never been held for more than a second, but perhaps a time or two inadvertently pressed after the motor was running. I don't imagine that would have destroyed the motor.
The caps look like any other run and start caps (individual, not dual). I'm not sure what you mean by "what kind?" If the relay were "welded," wouldn't I hear a "buzz?" Once running, the motor has always sounded smooth with just a normal hum.
Another thing regarding monitoring of the current... my Fadal has never indicated anything but clean 3 phase power coming from the converter. Wouldn't it glitch or something? Would the transformer that feeds the machine offer enough isolation to smooth spikes? As far as I'm aware, the only protection on the machine is a thermistor circuit. Burned it up once when I forgot to shut down the power to the machine before turning off the converter. That was on the first pony motor we destroyed.
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@Sean, I don't know what you're calling an "inverter." Are you talking about the spindle drive? I've never heard it called an inverter before. I only know it to be a VFD. If I install a line reactor on the drive would that benefit the pony motor of the 3 phase converter? That's what I'm after. I would think if there's a lot of harmonic distortion on the line, especially from the drive, the CNC computer would go berserk. Is that misguided?
I'm not opposed to replacing all the caps, but before I just go replacing parts willy nilly, I'd like to know that that's the problem. Otherwise, I hook up another motor and a couple months down the road, I'm possibly looking for another one at a minimum of $800.
Which motor are you suggesting checking currents? Pony motor or mill spindle?
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Yes VFD's are commonly called inverters in the UK
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Is this a self starting rotary with start caps switched in at startup or does it use a smaller motor to bring it up to speed? The smaller motors are usually referred to as pony motors.
It's possible it may not be coming up to speed. I've burned up one motor that way. It will draw a heck of a lot more current if it's not at synchronous speed.
What's the rpm rating? If you can find one, I think the lower rpm motors make better phase converters.
I have a 25hp motor waiting for me to turn into a phase converter. I'm thinking of putting a controller on it so it spins up to rated rpm first before applying power.
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is your CNC equipment the kind that controls 90v and or 180v dc motors from a 3 phase dual back to back SCR control or is it all rectified to DC and driven from PWM and or Three phase ac servos driven from an inverter?
regarding a line reactor:
most VFDs are just a 3 phase diode bridge that draws lots of harmonics to keep those 400 volt capacitors charged up to 330vdc
480v units will have ~660 volts on the bus. you can put the three phase line reactor between the inverter and the mains. this is often done for large 50HP and bigger VFDs but that reactor must be made for the 60 Hz side.
the motor side reactors are designed to keep resonances out of the motor.
regarding the motors burning up, either its I^2R loss or its harmonics exciting a resonance and you're blowing the insulation.
in my mind, a 20-25 hp motor has enough volts per turn to make the wire glow red and break from a direct single turn short.
also, which windings are burning out and what does the schematic look like?
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@Tray... those whacky Brits! When will they learn to speak English! ;) JK
Is this a self starting rotary with start caps switched in at startup or does it use a smaller motor to bring it up to speed? The smaller motors are usually referred to as pony motors.
If that's the case, I stand corrected. There's no startup motor. In fact, RPC motor shaft has been cut flush.
It's possible it may not be coming up to speed. I've burned up one motor that way. It will draw a heck of a lot more current if it's not at synchronous speed.
Never put a tach to it, but seems about right to me.
What's the rpm rating? If you can find one, I think the lower rpm motors make better phase converters.
I have a 25hp motor waiting for me to turn into a phase converter. I'm thinking of putting a controller on it so it spins up to rated rpm first before applying power.
1750 rpm. I've been considering the benefit of adding a start up motor, but without a load on the motor, I can't see how it can't handle the inrush.
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is your CNC equipment the kind that controls 90v and or 180v dc motors from a 3 phase dual back to back SCR control or is it all rectified to DC and driven from PWM and or Three phase ac servos driven from an inverter?
Honestly, I don't know. It's beyond me. It's a Baldor Vector Drive. Nameplate shows output as 230V 3ph, 0-500Hz. I can control the spindle much like a stepper(can move and hold at any of the 3600 divisions per revolution), but I don't know by what means that's done electrically.
regarding a line reactor:
most VFDs are just a 3 phase diode bridge that draws lots of harmonics to keep those 400 volt capacitors charged up to 330vdc
480v units will have ~660 volts on the bus. you can put the three phase line reactor between the inverter and the mains. this is often done for large 50HP and bigger VFDs but that reactor must be made for the 60 Hz side.
the motor side reactors are designed to keep resonances out of the motor.
I think "motor" has become ambiguous as we all seem to be using it to refer to either the spindle motor or the RPC motor. If I understand you correctly, you're addressing comments suggesting the physical placement of the reactor? I've always pictured, and assumed it would be installed at the RPC motor. I think you're saying that anywhere on the line side of the CNC and the RPC motor is fine?
regarding the motors burning up, either its I^2R loss or its harmonics exciting a resonance and you're blowing the insulation.
in my mind, a 20-25 hp motor has enough volts per turn to make the wire glow red and break from a direct single turn short.
I suspect you are quite right! While there's no external visual cues that it overheated, the stator is clearly fried in one spot. Insulation was burned away and a short resulted. Boy, the smoke it made!!
also, which windings are burning out and what does the schematic look like?
The stator winding on the RPC motor. Schematic? Of the motor?
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It might be the starting current that's cooking the wiring; it's going to be pretty significant for a 25hp motor until it gets up to speed. Starting a 3ph motor on single phase + start cap is not quite the same as starting on 3 ph which is what the motor is designed for.
For something that large, I'd look at a pony motor to spin it up.
Get a current probe and see what the starting current is, that would be interesting.
Voltage balance when it is running under load would also be interesting. Here's a link:
http://www.phaseconverterinfo.com/phaseconverter_voltagebalance.htm (http://www.phaseconverterinfo.com/phaseconverter_voltagebalance.htm)
Or you could scrap it and go with a Phase Perfect solution. From what I've seen, people really like these.
http://www.phaseperfect.com/ (http://www.phaseperfect.com/)
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Could be... never tested current at start up.
Voltage balance was good. Less than 5%
A Phase Perfect would be great if I had Revenue Perfect! ;) Of course, I can't afford to replace these motors every couple months. Obviously, that isn't an option either.
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The load balance could be off.
This leaves a remanent field in the stator. This could lead to saturating and then the impedance of the coil will be very low leading to high currents.
Why use a RPC at all.
You can buy a single phase input VFD and connect your motor directly to the VFD.
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He has a 10HP CNC mill which implies the spindle already has a VFD to control it.
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For the record, he's a she ;)
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load imbalance won't lead to saturation and remnant fields.
how long did it normally take to come up to speed?
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Under two seconds. Maybe one second.
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i really can't think of anything, short of a shorted capacitor that would burn out a coil in a 3 phase idler motor used to make 3 phase.
the capacitors need to be sized correctly but that doesn't normally lead to huge circulating currents either, just improper phase voltages and phases.
you have a circuit that looks like this right?
http://www.waterfront-woods.com/Articles/phaseconverter.htm (http://www.waterfront-woods.com/Articles/phaseconverter.htm)
what are the uF values you are using?
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Here's an interesting link:
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/20-25-hp-rotary-phase-converter-help-102854/ (http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/20-25-hp-rotary-phase-converter-help-102854/)
Your startup current may be in the 250-400A range. You can avoid the surge with a pony. Still, unless you're starting it 5 times a day, it should be fine.
Do the start caps go out of circuit once it's up to speed? Something's flaky to burn up 2 motors
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Like I said in my previous post. It was not uncommon to use another smaller 3 phase motor with a flywheel as a dynamic filter in these systems to keep things like that from happening. Also a soda can type delta surge arrestor (Available in 3 phase) may help things along.
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Several converters one of them 25 HP You could well be better off with a diesel genset for the 3 phase you are getting fairly close to the break even point for power costs I would think (I don't know the cost of diesel or electricity in the US) But here in the UK if you need more than 40KVA a converter is not cost effective.
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I don't know the cost of diesel or electricity in the US
Much cheaper than in the UK...
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do you have the first motor that got fried?
i'm wondering if its just one coil or two, or three that got cooked.
I would call this guy and ask him if he knows of any customers blowing their converters.
http://cromanconverters.com/home (http://cromanconverters.com/home)
His units are the standard x and 2x run capacitor plus a magnetic start switch to switch out the electrolytic start caps.
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[url]http://www.phase-a-matic.com//url]
These guys are the main ones here in the states for this sort of thing.
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Must say I am a little unsure exactly what this setup is. Say it doesn't have any electronics and then we seem to have variable speed inverter drives?
There seems to be a singe phase to three phase converter using a transformer and capacitors?
If so then they are easy to fault find. All you need is three voltmeters, one across each phase from the three phase converter. The capacitors are used to create the artificial third phase. The three phase balance is quite simply the three voltages being equal. If the voltages are within 5 volts or so then you have created an exact three phase system.
The pony motor just provides a stabiliser for the three phases, startup currents can be supplied from both the capacitors and the pony motor. The only way it can ever burn out is that the voltages are incorrect, use the three voltmeters to check.
Because all that is being run here is induction motors then there can't be strange harmonics. This does assume that there is a 230 to 400V single phase transformer so the pony motor, and all motors on the machines, are connected in Wye, NOT DELTA.
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When you say "it doesn't have any electronics," I don't know what you're talking about. The load has electronics, (ie CNC mill). I stated that in the OP. There've been several suggestions of using phase-a-matics implying there aren't any electronics. I can see how that might make one think there are only induction loads.
The setup is pretty simple. RPC powers a small 3 phase panel from which 5 or 6 machines draw power, usually one at a time. There are single speed motors, multi-speed motors and even a VFD inside the CNC machine along with a transformer. The transformer has several taps to feed the many varied components that are a CNC machine, the VFD and onboard computer in addition to various motors I assume are both A/C and D/C.
The RPC is rather ordinary. If you call capacitor banks "electronic," then the control box is electronic, then I'll go along with the RPC being "electronic." Otherwise, it's just switches and contactors.
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Well, it did say in the first post no computers or electronics, then the implication is that the VFD is running off the phase converter three phase supply? This is not a sensible idea at all.
A phase converter will only work with induction motors, not electronic loads like the VFD. Why? Because the motors will automatically generate the missing phase, this is not so with the VFD, that is just a collection of rectifiers. That the pony motor is burning up is an indication that it really does not like the operating conditions. Is the phase that has the CNC transformer the one created by the 230/400 physical transformer, or one of the capacitor generated phases? It must be the real, transformer, phase. If it is a a three phase transformer then this can't be run from the created three phase supply. The way to visualise it is that the pony motor is acting as an electrical flywheel for the three phase supply, and this requires rotating lumps of iron and copper. A transformer can't do this.
The converter is described as a 25hp unit, and that the pony motor is 25hp, is this really correct? If it is only rated at 25hp then all its power is taken up in running the pony motor and none will be left for the machines. I would have expected the pony to be about 5hp or so, 25%, and that hp rating would then have to be deducted from the converter hp rating. There might be some specmanship here, that the converter rating includes the pony motor?
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Woodchips, don't take this the wrong way, but do you really know what an RPC is?
The point of the idelr or 'pony' motor as you call it, is to generate the third phase. You spin it up, apply power to two of the terminals, and it generates the third phase on the other terminal. Whether it's connected Star or Delta makes no difference, as long as you're applying a suitable voltage.
The purpose of the start capacitor(s) is to generate a false phase in the third leg so the motor starts spinning at which point the start capacitors get disconnected.
Run/balance capacitors are added to help balance the voltages.
Depending on size, some will use a small start (this is what I'd call the pony) motor to spin the main idler up to speed to minimize startup surge, instead of start capacitors.
Also more advanced ones will have automatic switching of the balance capacitors to automatically balance the voltages.
By the sounds of the OPs problem, I'd say there is a major balance problem.
First thing I'd check, is to make sure all the single phase/control items are being powered of the non-generate leg. Another issue might be if any of the equipment is using any kind of braking that dumps power back into the supply, as an standard RPC won't be able to handle it very well, if at all.
After that, I'd be monitoring the phase voltages with various machines running to see if any are causing any balance issues. You can get industrial voltmeter modules that will display all 3 phases at once, which might be a wise investment. One fix may be to install additional run/balance capacitors directly into any machines that do cause balance problems, failing that you'll need to look at some kind of automated balancing system.
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Woodchips, don't take this the wrong way, but do you really know what an RPC is?
The point of the idelr or 'pony' motor as you call it, is to generate the third phase. You spin it up, apply power to two of the terminals, and it generates the third phase on the other terminal.
No, I don't believe this is what the pony motor is. The main phase converter motor is what generates the extra phases from a single phase supply.
The pony motor is a separate small single phase motor coupled to the shaft of the phase converter motor. The job of the pony motor is to spin up the main phase converter from stationary and then help it to run at a stable speed during load variations, to reduce imbalance in the output phases.
However, I believe woodchips may be right about where an RPC is suitable for use. An RPC based on a three phase induction motor is suitable for balanced three phase loads like simple three phase motors. It is not suitable for unbalanced loads like complex machinery with VFDs and transformers on some of the phases. You cannot treat a simple RPC as a substitute for a proper three phase generator set. It doesn't have the ability to generate three fully independent and unbalanced output phases that a three phase generator has.
If you need a general purpose three phase supply, I think you either need to get three phases directly from the utility company, or you need your own three phase diesel generator set, or you need a proper motor-alternator set where a single phase motor drives a three phase generator.
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The OP says a the RPC feeds a CNC mill. In other words, there's a computer/electronics on the load side of the RPC.
The idea that an RPC operating a CNC mill isn't "sensible" is absurd. There's probably well over a hundred thousand CNC machines being powered by RPC. I wouldn't be surprised if it's in the millions.
Now, if you're saying running unbalanced loads isn't sensible, I might be convinced to agree with you. But, any degree of unbalance will be relatively minute. All major loads are balanced. Obviously, there are some power indicators, a brake on the lathe, etc that may not be full 3 phase. Even the CNC... not sure how the axis' are powered, but the spindle is powered by a Vector Drive. I'm guessing that can be accurately called a VFD as the name plate says 3 phase in, 3 phase out 60-500Hz, or something to that effect. The 10 HP spindle is the single greatest load in the shop and the only thing running off a "VFD." The RPC is a 25 HP motor, way over the rule of thumb when sizing an RPC for power. Further, as I mentioned in one of my posts, the CNC has an internal 3 phase transformer from which all loads in the machine are drawn. It's way beyond my pay grade and will defer to you electronics experts as to how much, if any, that transformer might tend to smooth any unbalance in the CNC. Next to the CNC spindle, I'm guessing the greatest load will come from the servos. Don't know for sure, but I'm thinking they're DC. I suppose they could all be coming off one leg of the transformer. I don't drive them hard. Their Jog speed remains set at 50%.
So, no, the loads are NOT 100% balanced. I doubt any CNC machine is. Considering that there are countless people out there with similar setups (including Fadals as I've been informed by a former Fadal technician), without burning up their RPC, I'm inclined to look elsewhere for the problem.
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Running VFDs and CNC machinery from a RPC is pretty common place. It's definetly not ideal, but plenty people do it without any issues.
I suspect the big issue is phase balance, and loading. You've got to remember that all most VFDs are interested in, is xxxVDC on it's internal bus, not what individual phases are doing. If the false leg is a higher voltage, then it will be supplying a larger percentage of any load. Chances are the VFDs internal rectifiers will quite happily handle the out of balance phase, unless you're pushing the VFD capacity to it's limits for extended periods of time, and even then any decent VFD will probably fault due to undervoltage before damage occurs.
Servo drives are essentially just a more advanced version of a VFD, and work on the same principles. They take AC in, convert it to DC, then use that to generate whatever voltage/waveforms the motor requires.
I'm thinking the first thing to do is install a 3 phase voltmeter (http://www.alibaba.com/showroom/3-phase-voltmeter.html (http://www.alibaba.com/showroom/3-phase-voltmeter.html) is the kind of thing I'm thinking, but am struggling to find any reasonably priced non-china based suppliers!), so that you can check voltages at a quick glance. Without knowing what the voltages are doing, it's just total guess work.
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I disagree with woodchips, a 3 phase rectifier doesn't have the ability to pull unreasonable currents through the induction motor.
harmonics yes, and that will make a lot more losses, but the induction motor is only delivering power through one phase.
However, the third leg has a much higher impedance than the other two legs/phases, which are connected to the grid, the third leg will provide some power to the VFD but it won't be able to fully support it, its terminal volts will drop and the amount of current delivered through it will decrease, with the VFD showing a lot of 120HZ ripple on the dc bus, rather than only 360HZ.
furthermore its a 25HP motor, with line resistances of .3 ohms when wired for 480 volts.
if this were a 10HP motor i'd be more inclinded to believe its possible for the vfd to burn it out.
I happen to have a 2 hp 3 phase 240/480v induction motor.. as well as a 480v vfd..
---will be back with some test results...
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Just an idea
One of the differences between this installation and others is the physical layout required.
I read one place where a problem was created just by increasing the distance between the VFD motor driver unit and the motor. Just the additional wire needed for that distance.
I am wondering if the physical layout and the interaction of loads is allowing the build up of what could be called a standing wave that is putting more stress on the insulation of the windings.
There are motors that are built special for variable speed motor drive use to handle these additional extra stresses. I would not expect the 3 phase rotary converter to have these types of changes.
Looking at the power wires with a oscope would be a good idea.
I would not be surprised if there was a big difference when you look at the large load connection end and the connections to the 3 phase converter.
C
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Here's a thought... In the absence of any monitoring equipment, is it reasonable to assume that any harmonics that might be generated would cause all kinds of disruption in the CNC electronics? Seems to me, anything that's going to take out a 25 HP motor 30 feet away is going to show up somewhere in the electronics. I mean, this machine is a 1994... not exactly state of the art. Yet, I've never had a hiccup. I now zilch about harmonics, so take that into account.
Now, anytime I shut down the RPC, I disconnect all equipment. Same for startup. So IF, and that's all capital letters, the machines are never connected, then they aren't exposed to any potential spikes that occur at start and stop. Therefore, they've been just as happy as can be. There was once... maybe twice, we lost power from the grid. IIRC, we blew out a varistor circuit in the CNC. It was either a power outage or when I inadvertently shut down the RPC before the CNC. In any case, the Fadal cannot take the RPC shutting down. I just assumed, at the time, that the voltage went low, and that's what caused it. But, I guess, it would have had to be a high voltage spike to take out the thermistor circuit?
For those reasons, I'm leaning away from harmonics and the cause having to do with start up or shut down. The RPC did die seconds after start up. The suggestion for a pony motor may be in order, but I've been in communication with the owner of the company that manufactures the RPC. He's got 40 HP units out there starting without pony motors with no problem.
C, I think things are getting a bit convoluted, so forgive me if I'm misunderstanding you...
The VFD motor drive is actually in the CNC Mill's cabinet. The motor it operates, the spindle is maybe 4 feet away?
If you're talking about the distance to the RPC motor, that's at best 30'.
<edited> replaced thermistor with varistor, mistyped.
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So, my brain may or may not be working to great here, but i get the impression this is probably not the average shed out in the backyard deal, for the cost of a bunch of efficiency, a 25hp RPC, spare room and hassle, why don't you just get 3 phase installed?
BIG EDIT - EXTRA:
The RPC did die seconds after start up. The suggestion for a pony motor may be in order, but I've been in communication with the owner of the company that manufactures the RPC. He's got 40 HP units out there starting without pony motors with no problem.
If the RPC cooks less that 30 seconds after starting up, with no load connected there is something seriously wrong (and it clearly isn't the VFD or anything related to the VFD because the few seconds it takes to get up to speed, then the few seconds to walk over ot the CNC to turn it on ect is probably more than it would take to cook the RPC, right?) - initial start-up may heat it a bit, but no where near enough to cook a winding unless your start caps/run caps are RS; that is a LOT of VERY thermally conductive metal to heat....
On a similar note, you should probably look into the power distribution in your shop, if you can dump enough power to cook an RPC without popping a breaker, you need that looked into.
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So, my brain may or may not be working to great here, but i get the impression this is probably not the average shed out in the backyard deal, for the cost of a bunch of efficiency, a 25hp RPC, spare room and hassle, why don't you just get 3 phase installed?
I've been holding back on that question too, since it is a bit off topic. But it does seem unusual that a farm or light industrial premises would not have a three phase supply available?
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If the RPC cooks less that 30 seconds after starting up, with no load connected there is something seriously wrong (and it clearly isn't the VFD or anything related to the VFD because the few seconds it takes to get up to speed, then the few seconds to walk over ot the CNC to turn it on ect is probably more than it would take to cook the RPC, right?) - initial start-up may heat it a bit, but no where near enough to cook a winding unless your start caps/run caps are RS; that is a LOT of VERY thermally conductive metal to heat....
On a similar note, you should probably look into the power distribution in your shop, if you can dump enough power to cook an RPC without popping a breaker, you need that looked into.
seems to me the capacitors would explode if its a start cap left in circuit, before the motor gets cooked.
if there is no start cap then the motor won't start... not in 1-2 seconds anyways.
also, if you did cook a thermister then that is evidence of standing waves being excited, they would get through the transformer in the cnc mill because harmonics are going to be 5 and 7th multiple of 60, so they won't cause the iron to saturate and self limit the voltage.
or it could have just been a transient impulse from the grid, lightning, etc.
no, the cnc mill won't care about harmonics. everything gets rectified to dc before anything interesting goes on..
unless its one of those machines that runs dc servos directly from controlled rectifiers running on the grid frequency.
edit yes i new you meant varistor, not sure why it typed thermister.
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I have a 1200 sqft shop in my residence. Even if the power company would bring in 3 phase, they'd want in the neighborhood of $25k. Look, I may not be the smartest person in this thread, but I'm not exactly stupid. Like most people here, I'm a problem solver. I doubt anyone here is going to make decisions without carefully weighing the more obvious choices. How about giving me the benefit of the doubt? As for the 40 HP installation? I know it's on some kind of farm to operate irrigation pumps. I'm assuming if a farmer chooses to go with an RPC, they must have good reason. I don't need to know the reason, I can imagine many different possibilities and I'd probably still not guess the right one.
I had a half dozen different options to solve my 3 phase needs. I picked one. A reasonable one. One that thousands choose. I don't know why that's so hard to accept. And, you're right... it is off topic.
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@peter... I agree with your point about the motor frying without a load, in theory. I'm inclined to it, in fact. But, I think it' safe to say there is a spike with all motors on start up? Isn't it a possibility, even if less likely, that harmonics or some other cause could incrementally breakdown the winding insulation, weakening it? Then, the moment it starts, its the straw that breaks the camels back?
As for the wiring. It's top notch. Cutler Hammer 50A, copper bus. Less than 12 years old. 10 feet away, #4 cu. if memory serves. Breaker did trip when the motor fried.
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You could try a really old school method of phase balancing, a synchronous motor with a flywheel attached to it in circuit. Just left running all the time, other than on start up it takes very little power but will feed power back into the lines as a phase drops. An induction motor will work but with nowhere near the same efficiency as a synchronous motor.
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This is getting a few heads scratched!
A RPC is a rotary phase converter. A static phase converter uses a boost transformer from 230V to 400V with a selection of AC working capacitors to create a third phase. A RPC is simply a static phase converter that has a 400V three phase induction motor permanently connected to the three phase outputs, called either an idler or pony motor. If running just one 3ph motor then the idler motor isn't needed, the load motor will act to create the balanced, with suitable capacitor values, 3ph supply needed. The advantage of the idler motor is that it stabilises the 3ph supply allowing one or more other 3ph motors to be switched in and out at will. This is typically the case with a spindle motor and coolant pump, without the idler motor the coolant pump normally will die a quick death.
So what is it that makes these converters work? Why does switching in a capacitor or two adjust the output voltages to keep them balanced? The induction motor is a rotating energy store, and will on its own tend to balance the three phase voltages. But, even more important, is that as the load comes on the motor, its power factor improves. It is this simple change that allows the converter to work. Running light an induction motor will have a pf of possibly only 0.2, meaning that the line current is about 80% of full load. As the load comes on the pf improves, the motor power output increase, but the line current hardly increases at all. It is this wonderful automatic correction that allows the converter to use just one capacitor value from no load to near 80% load, being that no load is still rotating the spindle etc and taking quite a bit of power doing so.
This is what the idler motor does, provide an awful pf load which tends to swamp other loads, so the capacitor run value doesn't have to be changed every time the machine load changes. You would be far better off if you bought a synchronous motor rather than an induction motor and brought the power factor up to something sensible.
Now it is obvious that you can't run a transformer powered CNC off of a static converter, but you can from a rotary, because of the phase and pf stabilisation done by the idler motor. The transformer will almost certainly power DC servo motors, and the VFD will similarly run a DC bus with the variable frequency switching. What is forgotten here is that both are assuming a solid 3ph supply. There will be a bridge rectifier using six diodes, and the overlapping phases will result in a considerably easier smoothing task than from a single phase supply. The snag is the three phases are not the same. One phase will come from the transformer, the other two from the capacitor, singular. Note that there is one good solid phase and two created phases. The diodes will switch in the few microseconds, and the current they switch, as they start to charge the smoothing capacitors, comes from the idler motor. You guess as to what happens then is a as good as mine. It seems reasonable that the transformer phase will end up supplying most of the real power to run the CNC and VFD. This sounds like harmonics.
This is why I don't think running a CNC/VFD from a RPC is a good idea. Certainly the well over rated idler motor will help to maintain phase balance, but as you say, burning out out seems impossible.
One way you really will burn the idler out is if the start capacitors are in circuit for too long. The idler motor must only have the start caps connected for 1 or 2 seconds. If it hasn't come up to at least half speed by that time then you have too much impedance in the circuit, and need to drop it. The start capacitance is something like 80uF per hp, run capacitance more like 10uF per hp. The enourmous start capacitance still in circuit with the idler running over 1/3 speed, and hence 1/3 back emf, will fry the motor. The fact that the motor died just after start up would suggest that this is indeed the problem.
As I said in my first post, buy three cheap multimeters and put them across the three phase, and watch them as you turn it on and run the machines. In this case an analogue meter is probably better than digital because the over voltage during start will be very obvious.
Be aware that the AC cabling could be critical. Is all the wiring around the idler motor in three/four/five core or individual wires? Don't forget that the loop area of the wiring will introduce inductance which might in 30' of wiring be enough to completely ruin the current rise time in the motor.
Never run a static or rotary phase converter with delta connected motors. This also applies to the transformers in the CNC. Theoretically it is the same as a wye, but practically it most definitely isn't. Why? Circulating currents. The only paths for the various currents in a wye is in the motor windings and back along a different winding. With a delta motor there are also paths around the windings inside the motor as well, and because you will never get it all balanced then there will be circulating currents.
Clever things are phase converters, amazing how complex a bit of iron laminations wrapped with some wire can be.
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Johansen, I mistyped when I wrote "thermistor." I meant to say "varistor." I've gone back and edited that post. Sorry about the confusion.
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Woodchips, you're mostly right. As I understand them, and I'm by no means an expert, it doesn't work quite the way you say. Rather, mine doesn't. When you say static phase converters use a buck transformer, not all do. Some are nothing more than capacitors and can be bought for under a hundred bucks, weigh less than a pound and are about the size of a DMM powering up to about 5 hp, I believe. Phase-a-matic being one of the more common. Essentially, this is what I have, but on a much grander scale. While the phase-a-matic is designed to get the motor started, it doesn't use a contactor or start capacitor. No idler motor either. Though, I suppose you could use an idler to help smooth it out. The one downside with these is that there is no 3rd leg. Not really. Sure, the motor is generating a 3rd leg, but it isn't contributing to the operation of the motor. Can't get something for nothing. In essence, your 3 phase motor is running on only two legs. Translation... you get less than 2/3rds the power from your motor.
The idea, as I understand it is this, once started, the motor is spinning. You have your squirrel cage, to use a simplified model, turning inside 3 sets of windings 120 degrees apart. There is no way, that is going to do anything BUT produce 3 signals 120 degrees apart, in theory. In practice, of course, there are other dynamics going on, but basically, the motor is producing the power you want. My 25 HP motor, in theory will produce about 2/3'rds the power, so around 16 hp, 3 ph.
As for start capacitors staying engaged too long... as someone pointed out earlier, wouldn't the capacitors likely explode before the motor? Be that as it may, a NO push button switch is the starter. Never engaged for more than a second. Could some malfunction keep the caps engaged even after releasing the button? Without blowing up the capacitors? Hypothetically, could the start capacitors be sized so incorrectly to burn out the windings?
What is "5 core wire." "Loop area of the wire" also throws me. I've been around electricians my whole life. Actually even sat for and passed the journeyman's exam. But, I'm unfamiliar with those terms.
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There are two kinds of rotary phase converter. A full motor-generator, more expensive kind, and a "poor man's" motor-doing-double-duty-as-a-generator kind.
The motor generator kind has a driving motor taking mains input power coupled on a shaft to a three phase generator. The generator produces three equal, fully symmetrical phases that can be used to power arbitrary three phase loads.
The "poor man's" kind uses a three phase motor running off a single phase supply, and uses the extra windings on that motor to "fill in" the missing two phases by treating the motor partly as a generator. This is the kind you apparently have.
I think though, that the inexpensive kind of RPC that uses a motor doing double duty as a generator is not a full substitute for a full three phase supply. There will be limitations on what kinds of load it can supply.
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I have a 1200 sqft shop in my residence. Even if the power company would bring in 3 phase, they'd want in the neighborhood of $25k. Look, I may not be the smartest person in this thread, but I'm not exactly stupid. Like most people here, I'm a problem solver. I doubt anyone here is going to make decisions without carefully weighing the more obvious choices. How about giving me the benefit of the doubt? As for the 40 HP installation? I know it's on some kind of farm to operate irrigation pumps. I'm assuming if a farmer chooses to go with an RPC, they must have good reason. I don't need to know the reason, I can imagine many different possibilities and I'd probably still not guess the right one.
I had a half dozen different options to solve my 3 phase needs. I picked one. A reasonable one. One that thousands choose. I don't know why that's so hard to accept. And, you're right... it is off topic.
Thats a nice sized shop, it makes be curious for more information, but i'll try not get too off topic so i'll be concise.
I wouldn't be so sure about the 25k, unless you have asked before, even if you need a new pole or two it shouldn't be THAT much, of course with a 1200 sqft shop you probably do need more than a few poles for the assumed distance.
On the weighing of choices, yeah, I assume that, sometimes though, people get an idea in their head and they shove it until it fits kinda thing and they just need it breaking down and suggesting ti may be simpler in the end, i'm sure you've met people like that before, I just wanted to put the idea out there.
For the 40hp RPC for irrigation pumps, that would be acceptable for up to say... 25hp, beyond that, it would be too inefficient it wouldn't take long to pay for getting 3ph connected, or using an electronically controlled stationary motor+impeller. I know, my family owns a few farms, one of which is a very large irrigated orchard. That said, knowing farmers, what might seem right to you and me might not seem right to them :P
Yeah, RPC makes perfect sense in a lot of situations, I can understand why you chose it.
@peter... I agree with your point about the motor frying without a load, in theory. I'm inclined to it, in fact. But, I think it' safe to say there is a spike with all motors on start up? Isn't it a possibility, even if less likely, that harmonics or some other cause could incrementally breakdown the winding insulation, weakening it? Then, the moment it starts, its the straw that breaks the camels back?
As for the wiring. It's top notch. Cutler Hammer 50A, copper bus. Less than 12 years old. 10 feet away, #4 cu. if memory serves. Breaker did trip when the motor fried.
Yes, there is a spike on startup with all motors, however, that should end as soon as the motor is up to speed. If the motor turns over from stationary to matched speed in a very short period of time (the 1.5-3 seconds you suggest) then the heating would be incredibly minor from that.
I doubt it's harmonics or something like that. K.I.S.S.. If you think that over time XYZ electrical factor had broken down the winding insulation, its 10 times more likely that over time, dust had been abrasive and broken down the insulation. As for the brand new unit? Probably a accidental stuff up on installation or it was dropped in transport and the winding moved a bit and rubbed against something or it was brand spankin new and never used and it was a production fault or during shipping a small rock or something got in there, so much more statistically viable simple reasons.
Yeah, the wiring would have to be good to handle the current, i mean, 240v, 10hp load via a RPC, you're looking at 35A+.
Did the breaker occasionally trip during motor start up? I sort of hope it would; it would be pushing it pretty hard...
Also, yeah, good to know it did trip when the motor cooked, at least you know you won't have any fires from that.
Now it is obvious that you can't run a transformer powered CNC off of a static converter, but you can from a rotary, because of the phase and pf stabilisation done by the idler motor. The transformer will almost certainly power DC servo motors, and the VFD will similarly run a DC bus with the variable frequency switching. What is forgotten here is that both are assuming a solid 3ph supply. There will be a bridge rectifier using six diodes, and the overlapping phases will result in a considerably easier smoothing task than from a single phase supply. The snag is the three phases are not the same. One phase will come from the transformer, the other two from the capacitor, singular. Note that there is one good solid phase and two created phases. The diodes will switch in the few microseconds, and the current they switch, as they start to charge the smoothing capacitors, comes from the idler motor. You guess as to what happens then is a as good as mine. It seems reasonable that the transformer phase will end up supplying most of the real power to run the CNC and VFD. This sounds like harmonics.
This is why I don't think running a CNC/VFD from a RPC is a good idea. Certainly the well over rated idler motor will help to maintain phase balance, but as you say, burning out out seems impossible.
None of this matters because the RPC cooked before the CNC/VDF had been on for ANY measurable amount of time - it wouldn't have mattered if it was the CNC or if it was a knife switch short he switched on and it would have cooked in the same amount of time.
To quote;
It's never started under load. The last motor started fine the last time it ran. 30 seconds after starting, before we even turned on a machine, the motor smoked. No load. Nothing powered.
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Do you know what the startup circuit in the rpc is? Do the start caps automatically cut out or is it a manual button? I'm still leaning toward some fault in startup since that's when you would see the largest current flows. I also think it would be a good idea to get some cheap panel meters and monitor the 3 phases.
Poke around the web a bit; there are lots of (shudder) Arduino based power monitors. Might be worth throwing together something to watch the power. This one's a start with some building blocks:
http://openenergymonitor.org/emon/buildingblocks (http://openenergymonitor.org/emon/buildingblocks)
and here's another one:
http://boredomprojects.net/index.php/projects/home-energy-monitor (http://boredomprojects.net/index.php/projects/home-energy-monitor)
You could set up logging to watch the current draw at startup and run. Could be a fun project. Real data is always useful. You would be able to identify what's going on before burning up another idler. Right now we have one fact (a burned up idler) and a lot of speculation.
You may want to check around for industrial surplus as well. I found a 25 HP motor for $50 at a "garage sale" from a local company. You might check with HGR Inc out of Cleveland. They have 20-40 HP motors for around $400 and they are often willing to deal if something has been sitting in inventory for a while. There may be other industrial surplus near to you.
Re: 3 ph power in the US. If you don't have 3ph already, the charges to bring in 3ph power are absurd. Also, you can be in a rural area with a 2 wire single phase loop (like me) and it is absolutely unavailable, regardless. If you're in a residential area, might as well forget it as well. Finally, the pricing structure will ream you a new one making something like an RPC very attractive. If you have a lot of money, you can get a Phase Perfect system which is essentially a big 3ph inverter and synthesize your power. The rest of us are stuck with RPCs since the money is better spent on CNC tooling.
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Re: 3 ph power in the US. If you don't have 3ph already, the charges to bring in 3ph power are absurd.
I wonder if electric vehicles will change that?
The lack of availability does seem strange from a European perspective where 3ph power is everywhere and runs down every street.
(https://www.eevblog.com/forum/projects/rotary-phase-converter-burning-up/?action=dlattach;attach=84368;image)
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3 phase is run down most every street in the usa, and they charge you to send a truck out there with a brand new pole transformer.
if you've already got a 50KW transformer they will probably only have to add one TX..
if the pole has a 25KW unit feeding two or more homes they might bring two more transformers out.
they don't exactly bill you for the transformer but it sure seems like they do.
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Rural areas will often have a SWER power supply, as this cuts the cost of the overhead wire considerably. Adding the extra 2 wires for 3 phase is very expensive for only a small number of consumers. A pole mounted transformer to give split phase power works well with SWER, but for 3 phase power you will need a much bigger 3 phase unit and will need to have the same at each dropper point as well, not a standard yard item but rather a special voltage unit. At the switch yard they will have 3 phase power, but will split it off into SWER 11kV lines in groups of 3 to balance the supply side somewhat.
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Rural areas will often have a SWER power supply, as this cuts the cost of the overhead wire considerably. Adding the extra 2 wires for 3 phase is very expensive for only a small number of consumers. A pole mounted transformer to give split phase power works well with SWER, but for 3 phase power you will need a much bigger 3 phase unit and will need to have the same at each dropper point as well, not a standard yard item but rather a special voltage unit. At the switch yard they will have 3 phase power, but will split it off into SWER 11kV lines in groups of 3 to balance the supply side somewhat.
Around here, rural areas have 3 phase 11/22/33kv, massive cool-rooms, irrigation systems, massive blower/dryers ect encourage power companies to go the extra mile as far as distribution goes.
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Around here, rural areas have 3 phase 11/22/33kv, massive cool-rooms, irrigation systems, massive blower/dryers ect encourage power companies to go the extra mile as far as distribution goes.
True, but you can fit NZ into South Africa 4 times, and over 40 times into the USA, there is a little difference in scale. Cabling is a lot more expensive if you have, like here, 100km between consumers.
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SWER power distribution was developed in NZ by Lloyd Mandeno in the mid 1920's.
If one must use a single to three phase converter it is best to use a rotary converter rather than any form of static converter and I am including units with an induction motor in that group, wherever the load varies you will have persistent trouble with static units, get a rotary converter or motor gen set as they tend to be these days. I have been supplying diesel gensets to farmers for 30 years and in that time the easiest sell I have is to customers who have already tried phase converters and come to grief.
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Ah, I must admit I never considered that the RPC didn't have a boost transformer from the incoming 230V to the motor 400V. As I said, running motors on converters in delta is seriously not a good idea, circulating currents will cause immense losses and heating, motors must be wye connected.
Terminology, you don't get a spike on startup, you get a surge. The instantaneous start current is about 10x full load, and this is what limits the maximum size motor you can direct on line start. This raises the question of what type your 25hp idler motor is, it is far beyond DoL starting so is probably 690/400V star delta rated?
The little converters are nothing more than a capacitor and a varistor. They will happily start the correct size motor without problems, off load. But again you have the problems of delta connection and circulating currents. The only converter worth spending money on uses a boost transformer.
If you want to do the maths you can also work out that the capacitor based converter, on its own, will provide 100% of the current as from a normal 3ph supply. To do this you will have to tweak the capacitors from 90-100% of full load, and usually simply isn't worth it. The only motors this might fail is the very cheap crappy ones, or the extremely high quality ones where the running power factors are either too bad or too good. But, again, a problem for only over 90% of full load.
Will the capacitors explode? No, not really. Consider that when up and running the phase current for one whole phase is flowing through the capacitor. If full load current is say 5A then about 3.5A is flowing through the capacitor. This is happening all the time the converter is on. Modern polypropelene or similar capacitors have incredibly low losses, much less than 1%. So the current surge on start is nothing to them. But the very low impedance of the start capacitors will allow the capacitor phase to easily hit a 50% over voltage situation, this is what destroys the motor winding insulation.
Wiring can be either using multi-core, all in one overall insulation and usually with braid protection, or individual wires. In trunking the use of individual wires is common, also that as you get beyond 2.5 sq mm area the multi-core cable starts to resemble wresting an anaconda. The important point I was making is that AC current is affected by impedance, not resistance. Impedance is the AC resistance of the circuit, and one serious problem is the loop area of the circuit. ALL AC wires must run as close as is possible to the other wires in the circuit. This is automatically done in multi-core cable of course, but not necessarily so with individual wires. Think a spot welder, as the length of the arms increases the current capability quickly drops.
My suggestions, as are from others, for the way forward are:
Get some voltmeters to measure the three phases.
Get some clamp ammeters to measure the currents.
Go and measure the volts and amps!
Check how the start timing is done, no more than a second or two.
The instruments are your window onto the circuits, use them. If you are really keen pick up an old four channel chart recorder off ebay for nothing, obviously need some paper and pens. Then use some 400V to 6V DC transformers to drive the recorder, three phases plus the mains in. Some current transformers would also be nice. Just set the chart at no more than 10-20" per min and just switch it on and off, then look at the plots. One thing to be careful of is that the voltage transformers will probably saturate, some thing with a 600V primary would be better.
I still think your problems are in the start circuit, over voltage is burning out the idler motor.
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I still think your problems are in the start circuit, over voltage is burning out the idler motor.
... and there it is. I'm inclined to agree with you on this point.
Upon further reflection, I realized something I said in a previous post in this thread that was incorrect and need to take back. I stated that I had inadvertently forgotten to turn off the power to the CNC mill prior to shutting down the RPC, burning out the varistors. My memory isn't what it used to be. I'm 95% certain that what happened was: before starting the RPC, I threw the CNC mill's knife switch disconnect to on and pressed the mill's start button (NO push button). The knife switch powers the mill's internal transformer. The start button appears to only close the relays for the machine's electronics. So, upon throwing the knife switch, I heard the familiar whir of the cooling fans as usual. Pressing the start button resulted in NO familiar click. I immediately realized I'd forgotten to power the RPC, took 4 steps to the RPC control switch and turned it on. The varistors blew immediately (crowbar circuit parallel to the feed to the transformer). Seems that would support your hypothesis as well.
BTW, I'm not sure where you are, but there are clearly some terms we use differently. I don't recall ever seeing some of the ones you use in the NEC. Loop area, trunking... multi-core cable... I'm not saying they aren't there... I just don't remember them. It's been 25 years since I sat for the exam. Anyway, I think I understand what you were saying. I'm old, but not old enough to remember a time where conductors could be run anyway BUT together. What you call "multi-core" I know as "sheathed cable," colloquially known as "Romex." Braided wire is older than me. Maybe they still make it, but THHN is preferred, and most commonly available. So, it goes without saying that the conductors are "close as possible." Knob and tube wiring is also older than me! ;) Only ever saw it once. FYI... Panel to CNC mill is a dedicated, direct 3/4" conduit with 4 #8 AWG, THHN Cu, multi-strand wire. No neutral required.
Your suggestion of meters and monitors is all well and good, but I'm not willing to expose another motor to potential damage without first taking steps to at least mitigate the risk. The idler motor shaft has been cut off, so a rope starter or pony motor isn't an option for the motor I have on hand. Would a line reactor be the best protection to reduce the voltage? Would it be enough protection? Any other suggestions to reduce voltage?
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the capacitors i was talking about exploding are the 2000uF electrolytic caps that are needed to start a 3 phase motor off single phase.... and i would expect them to explode only after they are left connected 30 seconds after the motor is startedup.
not the polyproplylene run caps.
furthermore, the start currents on those motors is on the order of 200 amps when wired for 480 volts, but that's for DOL starting on a zero impedance 480v bus.
so, it will be significantly less starting on single phase with a start capacitor.
but, 200 amps through .3 ohms is 12KW, so yes the wires will heat up.
the fact however that the varisters blew imediatly is evident of some kind of voltage surge is what killed the motor, however, it would have to be a lot more than just 50% more.
the run capacitors connected across the RPC should have stopped the minimum of a kilovolt or two required to blow the insulation.
still not clear how your capacitors are hooked up in this circuit..
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Is it a concern that the neutral wire is not connected? Woodchips has said that source and load should be connected in wye, not delta, formation, which would imply a four wire L1/L2/L3/N connection needed between RPC and loads?
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Johansen, I'll take the panel cover off the RPC control tomorrow and do a thorough run down. I was operating on the assumption that the manufacturer was infallible. Mistakes can happen. I'll draw up a schematic. So, do I understand you to say the start caps could be responsible for overvoltage? If not, what do you think might cause the spike?
Another thing I found odd, maybe because I've never actually seen the insides of a motor bigger than a few hp, is the diameter of the wire. I mic'ed it and it's 18 gauge. I thought that was small. I realize it permits more winds and all, but... That's why I hadn't mentioned it before now. It's a Reliance. Wouldn't expect them to get that wrong.
Ian, I was talking about the CNC mill being fed without a neutral. Not the motor.
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Ian, I was talking about the CNC mill being fed without a neutral. Not the motor.
That's the point. You have a three phase supply going from the RPC to the CNC mill. I'm not sure it is a good idea to omit the neutral in that supply.
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Ok ???
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Are we making any progress?
There won't be a neutral coming from the RPC. There is one one the idler motor assuming it is wye connected, but would have to think about the consequences of using it as such. The three phase wires are all that are needed, in fact the neutral could start to introduce circulating currents, not good.
Are the start capacitors electrolytics? They are good enough provided the number of starts per hour is limited, say only 3-4, but I imagine you start it up and leave the idler running?
I doubt that the run capacitors would act as voltage transient suppressors. The difference in the capacitance between start and run is so great that they just get the overvoltage that blew the varistors. As a question, if easily answered, what physical size are the varistors that blew? I would expect them to be larger than a disc about 1" in diameter?
Start up surges. Going back to first principles the way the converter works is that the capacitors introduce a 60 degree phase shift between the transformer voltage and the voltage across the capacitors. This results in a phasor diagram looking like an equilateral triangle, each side of which is 400V. This is why you just need three voltmeters to get the capacitor phase correctly balanced, can of course use two voltmeters but three is belt and braces. This 60 degree phase angle is a balance between the impedance of the capacitors and the phase in the load. The capacitor will normally give a 90 degree phase shift, that is an inherent part of what they are. The larger the capacitance, the lower the impedance, and the closer to this 90 degree shift will be the voltage across the capacitor. During start the motor impedance is almost nothing, really just winding resistance and wiring impedance. So an enourmous capacitance is needed to minimise the impedance to get a reasonable phase shift to get the motor rotating. As soon as the motor starts then back emf in the windings will reduce the current draw, and also make the capacitor voltage phase angle increase towards 90 degrees. It can be seen that if this start capacitance isn't reduced quickly then there is so much capacitance in circuit that the phase angle will in fact reach nearly 90 degrees. So, instead of a nicely balanced equilateral triangle of voltages you have a right angled triangle. The transformer side is still 400V, but the capacitor voltage is now undefined and the hypotenuse will be even larger. I can't at the moment work out what the capacitor voltage will rise to, it will be at least 400V, so making the hypotenuse 570V, root 2 times 400V. I would suspect, but don't know for certain, that in reality the capacitor voltage will just carry on rising, possibly to 1000V? This sort of voltage is easily high enough to blow the insulation on the idler motor. This is why I would like some eyeballs on some analogue voltmeters during start.
Looking back at the posts, I am still a little concerned at the size of the idler motor. The bigger the better with the transformer in the CNC, but 25hp is big! Can't put my finger on the sizing at the moment, but doesn't the idler have to be about 50% of the largest motor that needs to be run? To complicate things of course have the problem of adjusting the run capacitance, a well oversize idler means this isn't really necessary.
As you say, 18 gauge wire does seem small. Can't find my book of tables but isn't that about 1.5mm diameter? I would have expected something 3-4 times the area, unless there are 3 or 4 windings in parallel for ease of manufacture.
When everything is off, and you hit the start button for the idler, what happens? Do you hear the sound of the contactor coming in followed by the start capacitor contactor and then 1 to 2 seconds later the start contactor dropping out. Does, did, the idler accelerate up to speed within those 1-2 seconds? Perhaps it is possible to pull the power fuses leaving just the control circuitry powered, so you could try operating it without the idler trying to start?
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Sorry for the silence. Got caught up in some other stuff yesterday.
As promised (day late), I tore into the RPC's control box to draw up a schematic. Guess what I found. A wiring problem! I don't know what impact this might have, or if it could create a voltage spike, but 4 of 9 capacitors in the starter bank weren't connected! 3 of the other 5 were leaking, but the indicator showed they are all still working. The four caps that weren't connected, are all daisy-chained in parallel, but only one lead goes back to the terminal block. The bank of 5 is all in a vertical row. The bank of 4 are all in a horizontal row. They meet to form an angle, if you can picture it. The bank of 5 is connected to the circuit twice, once at the junction of the two banks, and once at the other end of the row. I'm guessing the connection was intended to go to the adjacent cap of the bank of 4. Not sure yet the impact. Was just to excited to not share. I'm going back now to complete the schematic. Be back later.
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Low value caps lead to non optimal start current, thus cooked windings. I would suggest you change all 5 caps in the working side, even though they still appear to work. With mains applied they will break over if they are leaking, as this means they are building up pressure inside. the standard method is to release pressure by blowing the end cap off, and hopefully this disconnects the internals, but if not they cook the motor.
Pictures please.
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Good idea... will post pictures. I called the manufacturer. He really seems like a good guy. Sent me a megger to test a motor I had on hand for replacement, a couple weeks ago. Sent at his expense. When I told him of the wiring error, he asked me to send him the control box so he can go through it and make whatever repairs necessary. I thought that was kind of silly since the expense of sending him a 50 lb control panel would be more than the cost for me to repair it. He insisted and offered to pay for shipping. I like the idea of him using one of HIS motors for testing. I've already burned up two. Don't know what else, if anything, he'll do to compensate me for it, if it turns out the box was causing the problem. All I know is it cost more to replace two motors than I paid for the RPC new.
I bought it 3 maybe 4 years ago. It's way out of warranty. Still, I'll be interested to see how he handles this.
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Here are some pictures. An overview of the control box. Pictures are at or around 300k
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Close ups...
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... and the schematic. Forgive the poor job. I'm better at reading them than making them... only by a little mind you. Notice the two areas I circled. Yes, the caps are not all connected. Also, coming off the contactor in the upper right, they join at a terminal block, not actually at the contactor. I think that was an accident at the factory.
I omitted the low voltage stuff. I don't want to disassemble everything and have to put it all back together. It's ships to the manufacturer in the morning. Safe to say that part works properly. Plus, I noticed some kind of little module, looks like a timer of some sort. I think it's connected to the start switch. That should eliminate "long starts" as the culprit.
image is 295kb
<edited image>
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not all the start caps connected isn't exactly an issue, it might even be intentional, with more connected for larger motors.
i'm more concerned about the run capacitors connected properly.
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No, it was an oversight. Each of these are made to order. Besides, why would they have bothered connecting the one lead? Is it impossible for the start circuit to be responsible for the voltage spike? Any problems I've had happened on start. Varistors blowing, motor frying... I realize that doesn't exclude the run circuitry, but it runs fine. I know it. I've checked voltages. Always good, but I never checked during start or shut down. Varistors are an indication of an overvoltage spike. They've never blown except during a start. I'm all but absolutely convinced the problem is happening with the start, not the run. Inrush current? Eh, maybe. But that doesn't explain how you get more voltage on the line than grid voltage. I don't believe in coincidence. Explain the varistors and I'll be open to other theories.
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Thanks for the photos. Half the start capacitors not connected has to be a problem.
The other immediate reaction to the photos is how small, physically, the start capacitors are, what voltage and capacity are they?
Not keen on the way the start caps are wired, in a parallel daisy chain. Means that if one connection isn't too good then everything after isn't really in circuit. Don't forget the start current is 100A. The start caps should be individually wired back to a single terminal block.
I will explain that with a digression. Many years ago, in the USA, a building was constructed with a suspended walkways one above the other across the foyer. After a few years the walkways collapsed and killed many people. The walkways were suspended from the ceiling using steel bars. What should have happened was that each walkway used a nut on the support bar to support it. How it was installed was with one support going down to the top walkway, and then a completely separate bar going down to the lower walkway. Therefore the nut supporting the top walkway was in fact supporting both. Whilst the support bar was easily strong enough in tension to support both walkways, the nuts weren't. This is exactly the same as the start capacitor wiring scheme used, the crimp terminals on the capacitors are not rated at 100A+.
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What a bonehead! I uploaded the wrong schematic! :palm:
woodchips, I'm sorry about that. The caps sizes are indicated on the schematic. They're the 270-324 MFD caps. Voltage I don't recall and it's too late to check. I think they were 480? The run caps were 250VAC IIRC. That struck me as low. Took a picture of run cap labels but not the start. Sorry.
As for the daisy chain, I didn't mention he loops them back. In other words, they're tied in at both ends for double ampacity.
I see your point about the terminals. Probably not the best way to do it. This was one of his first units. It'll be interesting to see how it comes back. After a 1000 units sold, I'm guessing he's learned a thing or two.
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Start caps are pretty small, as they are only designed to have up to 30 seconds of power applied per hour, so the high leakage and really poor performance are not too much of an issue. Those are the right size, if they were run caps they would not fit in the box at all.
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Been busy for a week or so, lost track of the thread. Was this ever fixed? What was the problem? Worth recording it for anyone in the future, these converters are very common and do have their little gremlins.
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Nothing definitive was discovered by the manufacturer, though there was one item of interest that could have been the cause if the stars had been aligned just so. Just got it back today. I'll connect it and run some tests today or tomorrow.
On another board, I got a good suggestion about testing the current signal for noise with the scope and a clamp probe. I'm anxious to see the results of that test.
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I got the converter reinstalled using the nameplate-less motor I had tested a month ago.
First of all, the manufacturer did see something he didn't like in his wiring. As it was one of his first units, he hadn't worked out all the kinks. I failed to catch it. Basically, he was feeding the powering the contactor solenoid with a dedicated circuit. Which means that if for some reason the breaker to the motor were to trip, but not the contactor circuit, then when the breaker was reset, you would have a situation where two sets of the stator windings had voltage applied. But, unless the start switch was depressed, the rotor would remain stationary the entire time allowing heat to eventually destroy the winding insulation. I could not recall a single time where the breaker had tripped or been turned off momentarily. So, that isn't likely to have burned up my motors. I still don't know what caused this.
After reinstalling everything today, here is what I found:
In a no-load test, I found the voltages were way out of whack. L3 is the created leg. L1 and L2 was 244V. L1 and L3 was 288V. And L2 and L3 was around 160V. I called the manufacturer. He said to remove two of the run caps and try again and that the motor was likely smaller than 25HP. I did. Now, I'm reading, 244V, 255V and 272V respectively. Better, but still not very good. When I turn on the Bridgeport mill, a 2 hp single speed motor, I get a drop of about 10V on the high leg.
Again, with no-load, I read the current at the motor with a clamp. I'm getting readings of 303mV, 415mV and 640mV! I guess it's not really relevant what the mV/A is, the ratio is more than 2:1! Nevertheless, the current equals 15mV/A, so that means 42A on one leg. What might cause this disparity? Any ideas?
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Are those no-load measurements? If the voltages are out of balance, the currents will be disproportionally more out of balance. So your measurements make sense.
So, do you have the same amount of run capacitance between L1 - L3 as between L2 - L3? With those unbalanced voltages it seems there is either an unbalance in the amount of capacitance or an unbalanced load attached. Or could it be that the idler has one phase winding open, creating an open-delta (V) configuration?
With a balanced load (or on idle), optimally the amount of capacitance should be the same for the two pairs. The optimal amount depends on how much reactive power the load and the idler consume.
Capacitance can also be added between L1-L2 for power factor compensation of the utility side, but it will not affect the voltages by much if the supply is stiff.
Increasing the capacitance over a pair of legs (where one is the created leg) should increase the voltage for that pair and vice versa.
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I'm certainly no expert in this area, but one thing puzzles me:
The supply consists of one phase directly from the mains with effectively zero source impedance, and two phases generated by the converter with much higher source impedances.
In a three phase circuit, if either source or load have unbalanced impedances in different phases then I don't think we would expect balanced voltages and currents.
So why is it that we ought to expect balanced voltages here?
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The capacitors can be selected to give balanced voltages.
As the voltage over one phase increases, the capacitors generate more reactive power. At the same time, the reactive power consumed for magnetizing the induction motor(s) increases, but the increase in reactive power consumption is quicker than the increase in production in the capacitors due to saturation of the motor iron.
With a suitable amount of capacitance there will be a stable operating point at the desired voltage. :)
However, unless capacitors are switched in/out together with the load to compensate, one might have to tolerate a slight overvoltage on the generated phases on no-load.
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If you could find a "stupid" VFD, no fancy micro controllers for phase loss.
You could just feed it single phase and it would output 3 perfect 3 phase.
You would need one that is rated at 173% of what you plan to draw.
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I'm certainly no expert in this area, but one thing puzzles me:
The supply consists of one phase directly from the mains with effectively zero source impedance, and two phases generated by the converter with much higher source impedances.
In a three phase circuit, if either source or load have unbalanced impedances in different phases then I don't think we would expect balanced voltages and currents.
So why is it that we ought to expect balanced voltages here?
Expect? It's the goal. A 3 phase converter with unbalanced legs isn't very useful... especially for CNC. That is the function of the capacitors to balance the voltages within 5%, ideally. The part I don't like is that with 272V between two of the legs, it will drop to some unknown number under load, depending on the power factor and consumption, as I understand it. Since CNC won't be a constant load, I'm not sure how much voltages will fluctuate. I'm thinking more and more about springing the $1300 for the single phase transformer and just using the 3 phase for the manual equipment.
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A three phase motor-generator or diesel generator set would give you a balanced supply, but I dare say the cost would be more than $1300...
...unless you can find one surplus from somewhere?
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I just read a single post in a thread on a CNC forum. Someone claiming to have my same Make and year, slightly different model machine claims he bypassed the transformer and runs the whole machine off single phase. Since the vector drive is a VFD control for the spindle, that part might be doable. I'll have to check the voltage requirements of the servos. The electronics should be easy enough. That might be an option. Wish I had known that before!
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The part I don't like is that with 272V between two of the legs, it will drop to some unknown number under load, depending on the power factor and consumption, as I understand it.
However, that the voltage is that high and unbanlanced seems very fishy, unless you have already attached quite an unbalanced load. Were those measurements without load?
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The OP specifies which values are load and which are no-load. The 272 is no-load. With just a two horsepower spindle spinning, that voltage will drop about 10V. Keep in mind, this is a free wheeling spindle motor I'm talking about. I'll try a 6.5 HP motor today. That motor drives a gear box so I can give it a bit more load using that too.
I'm curious about the V and I signals. If I have time today, I'll take the scope out and look at those.
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Right, I went back and read your post again. Then something does indeed seem to be broken or miswired. Do you really have the same amount of capacitance across both of the generated phases?
Because the voltages mainly depend on the reactive power consumption, whether you load your spindle motor or not shouldn't matter much. A bigger motor should cause a larger voltage drop, also quite independent of whether it is loaded or not.
But in any case, 272 V is too high to begin with. If it drops by 10 V, won't you be closer to being balanced? If you start out with a more balanced voltage to begin with, it shouldn't change as much when loads are added/removed.
I think the first thing you need to do is to find out why the voltage is so unbalanced with no load. The strong unbalance (which is especially obvious from your idler motor current readings) will give rise to excessive rotor losses in your induction machines.
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Right, I went back and read your post again. Then something does indeed seem to be broken or miswired. Do you really have the same amount of capacitance across both of the generated phases?
Both generated phases? I'm assuming you mean non-generated. Yes, the run caps are dual run caps. One side is used for one pair of legs, the other side is used for the other pair. So, they are definitely the same.
If it drops by 10 V, won't you be closer to being balanced?
Absolutely.
I think the first thing you need to do is to find out why the voltage is so unbalanced with no load. The strong unbalance (which is especially obvious from your idler motor current readings) will give rise to excessive rotor losses in your induction machines.
After posting the remark about the current readings, I spoke with the manufacturer. He explained that point from which I was taking the readings was between the motor windings and the caps. Since this is a delta-wound motor, he said that the disparity is being caused by the current chasing around the delta and that it is to be expected.
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Both generated phases? I'm assuming you mean non-generated.
This is not a mistake. The mains input is a single phase and a three phase output is being created. To convert from single phase to three phase, therefore, two extra phases must be generated.
If you label your lines as L1, L2 and L3, then if the mains appears across L1-L2, one of the generated phases appears across L2-L3, and the other across L3-L1.
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With a 6.5 HP motor load, the voltages readings are 244 (normal), 230 and 230. The last two representing significant drops from the 272V
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I'm really not comfortable with the output. To ground, the generated voltage is 190V. Since all the 2 pole voltages are close, does the 190V to ground matter? Provided of course, I don't attempt to run any non-motor loads?
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I just blew up my current probe :palm: Stupid mistake. I thought, "oh, how convenient that this double banana jack on the probe can be piggy-backed with my voltage probes... Like I said, :palm: I can't take any further current measurements until I fix it. Blew out some of the traces and completely destroyed a trip pot, and minor damage to the scale selector contacts but the coils are still intact. I should be able to repair it. |O
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Both generated phases? I'm assuming you mean non-generated. Yes, the run caps are dual run caps. One side is used for one pair of legs, the other side is used for the other pair. So, they are definitely the same.
I was going by the convention where the "phases" are the pairs of conductors to which the parts of the load/source, although I think talking about the "phase conductors" or "lines" usually makes more sense and is easier to understand. :) By the (somewhat strange) convention, the phases would be L1-L2, L2-L3 and L3-L1 in a delta system or L1-N, L2-N and L3-N in a wye system.
So then you have seemingly the same amount of capacitance connected over both generated leg pairs. But are you sure all the capacitors are working? If you measured the voltage across each bank and the current through it you could calculate their reactances if you wanted to check.
After posting the remark about the current readings, I spoke with the manufacturer. He explained that point from which I was taking the readings was between the motor windings and the caps. Since this is a delta-wound motor, he said that the disparity is being caused by the current chasing around the delta and that it is to be expected.
Unbalanced currents are to be expected with unbalanced voltages. It happens with both delta and wye connections. The currents you measured are the idler line currents I assume (the three wires connecting to the delta connected machine), with the capacitors and line connection being on the other side of the measurement point. Or are they measured "inside" the delta (sometimes called phase currents)?
If one winding or connection inside the delta were to be open it would still kind of work but with bad performance. Have you ruled out this being the case?
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I'm really not comfortable with the output. To ground, the generated voltage is 190V. Since all the 2 pole voltages are close, does the 190V to ground matter? Provided of course, I don't attempt to run any non-motor loads?
Your configuration gives you a "high-leg delta" style output, so this is as expected. Such electrical services are provided in USA and Japan among other places:
http://en.wikipedia.org/wiki/High-leg_delta (http://en.wikipedia.org/wiki/High-leg_delta)
There are lots of funny 3-phase connections used all over the world. At the same time, here in Sweden where I live, you are unlikely to find anything other than 400/230 V 3-phase Y-connected. Actually, even my apartment, which is quite small, has a 400/230 V 25 A 3-phase service (17 kVA) for some reason. :wtf: And at the same time everything in the kitchen excluding the stove is wired to a single 230 V 10 A circuit. :wtf:
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I was going by the convention where the "phases" are the pairs of conductors to which the parts of the load/source, although I think talking about the "phase conductors" or "lines" usually makes more sense and is easier to understand. :) By the (somewhat strange) convention, the phases would be L1-L2, L2-L3 and L3-L1 in a delta system or L1-N, L2-N and L3-N in a wye system.
No, you're right. I was conflating leg and phase. The voltage seen on each winding is 240 volts (supposedly). Since each winding is directly associated with each output phase, then obviously two legs are essential for each phase. My bad.
So then you have seemingly the same amount of capacitance connected over both generated leg pairs. But are you sure all the capacitors are working? If you measured the voltage across each bank and the current through it you could calculate their reactances if you wanted to check.
Yes, I'd like to do that... |O
Unbalanced currents are to be expected with unbalanced voltages. It happens with both delta and wye connections. The currents you measured are the idler line currents I assume (the three wires connecting to the delta connected machine), with the capacitors and line connection being on the other side of the measurement point. Or are they measured "inside" the delta (sometimes called phase currents)?
I'm not following the question, but I think I know what you're asking... the clamp was placed on each of the 3 legs to the motor, between the motor and the caps. It's kind of confusing because the wires going to the motor are both line and load. Perhaps there's another name for them. Anyway, because of the location I was using for testing, the manufacturer said the currents were chasing around the delta. However, just prior to burning up my current probe, I measured the currents of a 6.5 HP load. This I found disturbing. L1 = 20A, L2 = 28A and L3 = 33A. V(L1-L2) = 244V, V(L1-L3) = 230V and V(L2-L3) = 230V. Also, the 190V I mentioned above was measured to ground while this load was being applied. I measured 238V to ground with NO load!! I think I'm going to call a motor shop Monday and have them do a service call. I can't afford to keep burning up equipment.
If one winding or connection inside the delta were to be open it would still kind of work but with bad performance. Have you ruled out this being the case?
Well, if you mean the idler motor, I tested it with an insulation tester (megger). I didn't disconnect all the wires to the motor as they had been crimp spliced and soldered, but just checking leg to leg (lines disconnected of course) I was getting measurements in the 3 Gohm neighborhood all around. Plus, the motor was powered up at a local motor shop for evaluation. They told me the motor was good. If you're talking about the "delta" including the run caps, then I'd have to defer to the manufacturer as the control box was tested by him a couple weeks ago.
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I'm really not comfortable with the output. To ground, the generated voltage is 190V. Since all the 2 pole voltages are close, does the 190V to ground matter? Provided of course, I don't attempt to run any non-motor loads?
Your configuration gives you a "high-leg delta" style output, so this is as expected. Such electrical services are provided in USA and Japan among other places:
http://en.wikipedia.org/wiki/High-leg_delta (http://en.wikipedia.org/wiki/High-leg_delta)
There are lots of funny 3-phase connections used all over the world. At the same time, here in Sweden where I live, you are unlikely to find anything other than 400/230 V 3-phase Y-connected. Actually, even my apartment, which is quite small, has a 400/230 V 25 A 3-phase service (17 kVA) for some reason. :wtf: And at the same time everything in the kitchen excluding the stove is wired to a single 230 V 10 A circuit. :wtf:
I'm somewhat familiar with high leg 3 phase, but is this configuration of phase converter expected to produce such a thing? I'm not using a neutral, if that matters. The ground to which I'm referring is the single phase supply.
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I'm somewhat familiar with high leg 3 phase, but is this configuration of phase converter expected to produce such a thing? I'm not using a neutral, if that matters. The ground to which I'm referring is the single phase supply.
If you measure 120 V to ground from L1 and L2 and 190 V to ground from L3, then I think this is to be expected. The ground reference is not in the center of the three phase "triangle", it is to one side. Imagine a drawing of a triangle with ground marked in the center of one side (the L1-L2 side) and the three corners marked L1, L2 and L3. The distance to ground from L1 and L2 is half the length of that side, while the distance to ground from corner L3 is much greater. If the length of a side were 240, then the distance from L3 to G would be 208, which is not far off your measured 190.
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You do end up with the same kind of configuration. Here's a nice illustration over at another forum I found:
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/three-phase-vs-single-phase-illustrated-102416/#post343400 (http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/three-phase-vs-single-phase-illustrated-102416/#post343400)
The ground reference is established on the utility side between your L1 and L3 so when you create your third leg in the phase converter it ends up at the same location in the vector diagram as in a high-leg delta service.
Too bad you blew up your current clamp, but the important thing is that you are safe! Take care!
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You do end up with the same kind of configuration. Here's a nice illustration over at another forum I found:
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/three-phase-vs-single-phase-illustrated-102416/#post343400 (http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/three-phase-vs-single-phase-illustrated-102416/#post343400)
That was very helpful. Thanks. 3 phase has always been a bit of a mystery to me. While it gives me a clearer picture, which I needed, it does not explain why I'm getting 238V to ground on the generated leg. However, I have to ask about the neutral. Is there a midpoint tap coming off of the windings of a 3 phase generator in the middle of a coil? I'm assuming there's no need for a tap on an idler motor and therefore it's omitted?
The ground reference is established on the utility side between your L1 and L3 so when you create your third leg in the phase converter it ends up at the same location in the vector diagram as in a high-leg delta service.
Okay, I see what you're saying... What you're calling L1 and L3 is indicated on the vector diagram as A and C, B being the generated leg. As IanB was saying that would be the 208V. Since I'm not getting my neutral from my idler, the neutral = ground (by virtue of bonding at the meter) = midpoint of line AC. So, I should be getting 208 where I'm getting 238. 30 volts, to me, is not close. Actually, my mains are a touch on the high side at 245. 245 x cos(30) = 212. Still not close. Under load, it's "acceptable" (190V is close to 208). And, since a certain degree of drop under load can be expected, the converter is functioning as it should? I have no worries?
If those assumptions are correct, then I am comfortable with relying on the phase converter for everything but the CNC machine, as it's power demand is about 50% more than the load giving the 190V reading to ground. OR, have I understood you to suggest that there isn't a linear relationship between hp and drop in voltage and that once the reactive power is consumed the voltage will stabilize? I hope I said that right.
Too bad you blew up your current clamp, but the important thing is that you are safe! Take care!
Thank you. When working with mains power, proper safe practice will ensure safety even when something unexpected goes wrong. For that reason, I was never in any danger. It did make me jump, though :) Also, the clamp looks repairable. I'll have to jump a trace or two that was destroyed and replace a trim pot (provided I can determine it's value before being destroyed), but it should be returned to service soon.