Author Topic: 3-phase motor drawing high current  (Read 15785 times)

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Offline iXod

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3-phase motor drawing high current
« on: August 12, 2016, 04:12:24 am »
0.75kw motor drives the hydraulic pump in a horizontal band saw. Label FLA (nominal 220v 60hz) is 2.9 A. Measured is 4.17. Motor overload trips after 30 seconds or so.

When the motor is running the saw operates normally... for 30 seconds.

What causes a motor to draw more current? Pressures are unchanged so I am thinking that the pump isn't loading the motor more than usual.

Bearings? No unusual noises are heard and the motor doesn't seem unusually warm.

The mains voltage is 205, not yet 10 percent off the rated 220, so that wouldn't cause this amount of current draw, would it?

I have read that an unbalanced phase (or 2?) of more than 5 percent can cause higher current draw. Is this true? I think the phases are 205, 205, 210.

Being a 35-year-old machine I'm not confident that a new motor/pump can be had, and if so, would probably make the repair not economical, so re-winding/re-bearing is the only option I think.

The electric utility changed the transformer supplying this building last weekend and I think this is when the trouble started. (There are no electronic components in this machine.) I don't know what the voltage was prior to the transformer change-out so I can't say if there is now a change in the mains voltage.

I think I've covered all the bases I have considered. Which are most likely and what haven't I thought of?   

Since I'm the one who will be making the call of whether or not to get the motor rebuilt, I'd like to be right.  ;D

Thanks.
« Last Edit: August 12, 2016, 04:26:37 am by iXod »
 

Offline noidea

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Re: 3-phase motor drawing high current
« Reply #1 on: August 12, 2016, 04:52:20 am »
From the photo the rated voltage on the motor is 230VAC not 220, at 220V it would draw more than the 2.9A rated and if you are down around 205 then even more.
If the utility has just replaced the transformer and you are only getting 205-210VAC on each phase then you should get them to adjust the taps to at least give you your nominal supply voltage.
Have you checked the winding resistance between phases? what is the percent difference between them as I doubt the actual resistance specifications would be easily available. Also what is the insulation resistance to earth using a insulation resistance tester (megger)
When you say the saw runs for 30 seconds is that under load cutting or just free running? Is there any way to run the motor without any load?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #2 on: August 12, 2016, 06:03:34 am »
Oops... 230.

Will measure winding resistances. Have no megger (meaning to get one...).

The pump provides hydraulic pressure for clamping & maneuvering work on the saw. The blade is run by second motor. While the cut is happening the pump is providing a static pressure which keeps the work clamped in the vise.

The only way I know to run the motor without load is A) disconnect the hoses; B) unbolt the pump from the motor. Both not desirable... )c; Will this give a meaningful measurement, with no load at all? It should drop to FLA value if the problem is the pump? Is that what I should presume?

The voltage measured at the LV outlets is 115-117. Wouldn't changing taps push this over the max? Because of this I'm not sure the utility would consider changing taps, but haven't asked them.

Thanks.
« Last Edit: August 12, 2016, 02:07:46 pm by iXod »
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #3 on: August 13, 2016, 02:20:48 am »
Low voltage on 3-phase will definitely cause increased amperage draw, especially under heavier loads. For the same task (i.e. motor torque/horsepower draw) and lower volts, higher amperage is drawn. The same amount of work in watts is being done, so the energy has to come from somewhere.
 

Offline SeanB

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Re: 3-phase motor drawing high current
« Reply #4 on: August 13, 2016, 03:25:17 pm »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

To test the motor under no load without too much mess, just run the pressure output from the pump back to the oil reservoir, which will drop the pressure as low as it can possibly be, with maximum flow and almost zero head.
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #5 on: August 13, 2016, 04:21:15 pm »
My guess is your motor was overloaded to begin with but only slightly, and when the the voltage was dropped from 240vac to 208 vac, that's when the current climbed from about 3.5 amps to the 4.2 amps you measured..


however, if your service was 240vac before, then you would expect to see about a 15% increase in motor amps when dropping the volts to 208. this helps explain how a slightly overloaded motor didn't trip before, because the 10% increase in motor current when dropping the volts from 230 to 208, is not enough to trip the thermal breaker.. usually.


if the voltage was 208 before then something is wrong with the motor or you are simply overloading it.
shorted turns are almost always catastrophic failures.


if you lost a phase the motor should not start.
« Last Edit: August 13, 2016, 04:35:46 pm by johansen »
 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #6 on: August 13, 2016, 05:03:04 pm »
Is the motor really 60Hz?  The pump load would greatly increase with a switch from 50.  It just might handle that, but not with a drop in voltage.  Still, I'm thinking shorted turn in the winding.  A check of each phase current would confirm that.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #7 on: August 14, 2016, 05:32:09 am »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

Currents equal (4.45A). Voltages: 205, 205, 210 (if I remember...)

A missing phase would mean no motor movement (it's rotating full speed) and nearly-immediate OL pop (the OL takes up to 30 seconds to pop).
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #8 on: August 14, 2016, 05:37:23 am »
Is the motor really 60Hz?  The pump load would greatly increase with a switch from 50.  It just might handle that, but not with a drop in voltage.  Still, I'm thinking shorted turn in the winding.  A check of each phase current would confirm that.

Currents equal (4.45A). Resistances equal (10.9 ohm -- sorry, no greater resolution available). Will check insulation resistance (megg) and report.

The motor nameplate says motor is rated at several voltages and both 50 and 60 hz.

OL started tripping months ago (saw is 30 years old), so something has changed... Motor and pump are original part numbers.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #9 on: August 14, 2016, 05:50:56 am »
My guess is your motor was overloaded to begin with but only slightly, and when the the voltage was dropped from 240vac to 208 vac, that's when the current climbed from about 3.5 amps to the 4.2 amps you measured..

however, if your service was 240vac before, then you would expect to see about a 15% increase in motor amps when dropping the volts to 208. this helps explain how a slightly overloaded motor didn't trip before, because the 10% increase in motor current when dropping the volts from 230 to 208, is not enough to trip the thermal breaker.. usually.

if the voltage was 208 before then something is wrong with the motor or you are simply overloading it.
shorted turns are almost always catastrophic failures.

Report of 240v mains was in error. Mains was and is now 208 (common in industrial buildings in USA).

So pump has always run on 208v. Pump OL has been tripping for almost a year (operators thought it was a safety time-out thing and just reset it and kept sawing). The recent utility's upgrade of transformer in the neighborhood seems to have nothing to do with this failure. ("Red herring".)

Note that the pump motor is rated not at 208 but 230v/60hz (2.9A). Recalculating for measured 205 gives 3.2A, not near the measured 4.45A.  :-//
 

Offline pbendel

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Re: 3-phase motor drawing high current
« Reply #10 on: August 14, 2016, 05:53:06 am »
Did you current test the ground wire?  Possible motor leakage?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #11 on: August 14, 2016, 06:30:34 am »
Did you current test the ground wire?  Possible motor leakage?

No, I didn't. Good idea. I'll do that next (after megger).

Thanks!
 

Offline SeanB

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Re: 3-phase motor drawing high current
« Reply #12 on: August 14, 2016, 12:43:36 pm »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

Currents equal (4.45A). Voltages: 205, 205, 210 (if I remember...)

A missing phase would mean no motor movement (it's rotating full speed) and nearly-immediate OL pop (the OL takes up to 30 seconds to pop).


Not always, if there is a failing contactor, that drops the phase after it initially makes, because the inertia is enough to get the contact past the sticking point, then it will start, especially with a pump load that is low starting torque. Was fun dropping that 130kg motor down from it's mounting where the shaft had galled to the gearbox, but as a bonus the replacement was only 83 kg to winch up with the chain block. Then I put in all new contactors and a soft starter, in a new IP44 box.
 

Offline Curtis

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Re: 3-phase motor drawing high current
« Reply #13 on: August 14, 2016, 02:10:15 pm »
Check the voltage drop from the mains to the load. If the pump was recently moved to a new location wire size plays a huge issue.
 

Offline EPTech

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Re: 3-phase motor drawing high current
« Reply #14 on: August 14, 2016, 02:10:23 pm »
Hi there,

I have had a case like Sean describes a few times, where one phase was (partially) interrupted at the fuse, contactor and motor OL trip. In such a case, the easiest way to spot the problem is to measure phase voltages under load, from the motor terminals upward, towards the supply.

Almost all hydraulic pumps are constant flow rate pumps (positive displacement). If you pinch the output, ex. by operating a valve, the pump will try to maintain it's flow rate and as a result, pressure will increase. If the pump has to work harder, it will need more power and as a result current will increase. There are a few things you can check without having to decouple the pump  from the motor.
1. As suggested before, decouple the pressure side at the valve block and put the hose in the reservoir. This will effectively have the pump run at free load. Check motor current but also check whether the oil is clear of bubbles. If the oil is foamy, that meant the pump is cavitating, not good. If this is the case, check the filter in the reservoir. If the oil is milky white, there is water in the reservoir, very bad.
If the pump works normally and the oil is OK, the hydraulic system may be dirty or the valve does not open up correctly.
Reconnect the pressure hose to the valve block. If the machine is in the unclamped position, there should be almost a much oil coming out of the return hose, as coming from the disconnected pressure hose before.
2. Check whether the correct viscosity oil has been used. A lower viscosity means the pump has to work harder to push the oil through the system, especially at low temperatures.

Good luck.
Kind greetings,

Pascal.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #15 on: August 14, 2016, 05:16:06 pm »
Check the voltage drop from the mains to the load. If the pump was recently moved to a new location wire size plays a huge issue.

Not moved recently. Only recent external event is utility replacing transformer for the building/neighborhood. Doesn't seem to have had any direct effect as saw worked after that.

But will do test tomorrow.

Thanks.
 

Offline madires

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Re: 3-phase motor drawing high current
« Reply #16 on: August 14, 2016, 05:27:21 pm »
Do you got one those simple 3-phase testers showing the phase rotation or could borrow one?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #17 on: August 14, 2016, 05:54:04 pm »
Do you got one those simple 3-phase testers showing the phase rotation or could borrow one?

No I don't. What do you think this will show?
 

Offline madires

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Re: 3-phase motor drawing high current
« Reply #18 on: August 14, 2016, 06:12:58 pm »
It would show if it's really 3-phase and the phase rotation. Some months ago I had an issue with a 3-phase motor running in the wrong direction, because someone swapped L1 and L3 in the IEC wall socket. If you think the problem is related to the change of the transformer, check if the phases (voltage & rotation) are correct. If those are fine, it's problem with your motor, which is triggered by accident after replacing the transformer. As always, check the power supply first ;)
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #19 on: August 14, 2016, 06:51:30 pm »
It would show if it's really 3-phase and the phase rotation. Some months ago I had an issue with a 3-phase motor running in the wrong direction, because someone swapped L1 and L3 in the IEC wall socket. If you think the problem is related to the change of the transformer, check if the phases (voltage & rotation) are correct. If those are fine, it's problem with your motor, which is triggered by accident after replacing the transformer. As always, check the power supply first ;)

Thanks for you reply.

Problem (pump OL tripping) preceded the transformer replacement. Operators thought it was a safety time-out thing and just reset it each time(!), not telling anyone. This has been for many months now.

So I think that either 1) short in a few windings (but all currents are the same, then unlikely shorts in all windings at the same time); 2) voltage reduced, either at mains source or higher resistance in path (v measured at motor is not low enough to cause increase of measured current (4.4A) over rated current (2.9A) -- v must drop to 140v to cause such an increase in A, if my calcs are correct); 3) pump load increased (maybe due to dirt or other restriction in hydraulic oil path)
 

Offline Jr460

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Re: 3-phase motor drawing high current
« Reply #20 on: August 14, 2016, 06:52:20 pm »
My guess would be two things.

Something, age, bearings, pump is putting on more load to the motor than over a year ago, and you are running a motor not rated for 208 on 208.

Get the right motor on the unit, or the right voltage to the existing motor before getting into all kinds of other things.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #21 on: August 14, 2016, 06:53:07 pm »
Disconnecting pump hydraulic output should result in rated motor current (2.9A max), correct? If motor is healthy, that is.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #22 on: August 14, 2016, 06:54:42 pm »
My guess would be two things.

Something, age, bearings, pump is putting on more load to the motor than over a year ago, and you are running a motor not rated for 208 on 208.

Get the right motor on the unit, or the right voltage to the existing motor before getting into all kinds of other things.

Autotransformer ("buck/boost") is next on the list. Options typically include boosting by 12v or 24v which would give 217/217/222 or 229/229/234, respectively, on the 3 phases. I'm opting for the higher voltages, I think.
« Last Edit: August 14, 2016, 06:58:57 pm by iXod »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #23 on: August 14, 2016, 09:07:48 pm »
I want to measure and compare resistance of each winding. I do not have a milliohm meter but do have ESR meter.

ESR meter should give accurate measurement of motor windings, yes? The 100KHz shouldn't make a difference?

If phase currents are identical, is there any reason to measure winding resistances?

Thanks.
« Last Edit: August 14, 2016, 11:52:44 pm by iXod »
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #24 on: August 15, 2016, 08:12:13 am »
It's doubtful to me that it's a winding issue. I would get that boost XFMR on first things first, then do the diagnostics the other guys recommended to check for extra load on the pump. You really shouldn't be running the motor under nameplate voltage if you want to keep it as healthy as possible for as long as possible. The motor won't draw rated amperage with no load BTW. Rated amperage is with a specified load - usually at rated torque.
« Last Edit: August 15, 2016, 08:19:50 am by eKretz »
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #25 on: August 15, 2016, 08:17:43 am »
Also, the phases are probably not hooked up wrong, as the pump is obviously functioning. If the rotation were reversed it wouldn't be working. The failing contactor is a good lead also - that can definitely cause a high load as well. Check all three legs for a voltage drop across the contactor to determine if it's OK.
« Last Edit: August 16, 2016, 01:25:33 am by eKretz »
 

Offline Halvmand

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Re: 3-phase motor drawing high current
« Reply #26 on: August 15, 2016, 09:51:25 am »
All good suggestions so far.

After you check the electricals I suggest you check the whole hydraulic system for obstructions? A filter may be plugged. And that would have a huge impact on the load of the motor.
If there is a filter in the pressure line, definitely check that.
Check for debris, sludge or foreign objects in the sump of the hydraulic tank.
Finally, if possible, separate the motor and pump and disconnect the output and se if it turns by hand easily. This is a little hit and miss since you don't know how much resistance the pump ideally should have.

 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #27 on: August 15, 2016, 11:42:43 am »
I don't think there is actually anything wrong.  This machine was likely set up for 50Hz.  At 60Hz it is pumping 6/5 more fluid even if the pressures remain the same.  It has likely been drawing more than its rated current for years, but still under the rating of the protection relay.  Now that your voltage is lower it is drawing more current.  Raise the voltage with some buck transformers and you will be fine.  Dropping the pressure a little after raising the voltage might also bring the currents more in line.  Consider yourself lucky that it has lasted this long without a problem.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #28 on: August 16, 2016, 04:50:34 pm »
It's doubtful to me that it's a winding issue. I would get that boost XFMR on first things first, then do the diagnostics the other guys recommended to check for extra load on the pump. You really shouldn't be running the motor under nameplate voltage if you want to keep it as healthy as possible for as long as possible. The motor won't draw rated amperage with no load BTW. Rated amperage is with a specified load - usually at rated torque.

So testing unloaded motor tells me... what? Nothing useful?

I can test it unloaded or loaded with the current pump load. One reading gives me much less than rated amps, the other much more than rated amps. What useful info is this in determining cause of overload?  :-//

Of course adding trannys are next the step.
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #29 on: August 16, 2016, 05:41:59 pm »
motors designed for 208 mains are usually dual voltage 200/400v motors.

either get three 32v transformers to boost your volts back up to 240, or replace the motor with a 1.5-2 hp 240v motor.

note that all the other motors in your shop are probably 240v nominal (230v nameplate) and draw 15% more current (producing 32% more heat) under nameplate "rated load" when run at 208 volts.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #30 on: August 16, 2016, 08:44:07 pm »
Yes testing unloaded is not really useful IMO. Test for voltage drop across the contractor on all three legs first, if that's good, get the boost XFMR on there and see if that does the trick.
 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #31 on: August 16, 2016, 11:09:49 pm »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #32 on: August 17, 2016, 04:46:52 am »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.

I'm working with 120/208-Y mains. I thought on Y you must have 3 trannys. Only delta can use 2?  :-//
 

Offline Circlotron

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Re: 3-phase motor drawing high current
« Reply #33 on: August 17, 2016, 05:11:55 am »
Low voltage on 3-phase will definitely cause increased amperage draw, especially under heavier loads. For the same task (i.e. motor torque/horsepower draw) and lower volts, higher amperage is drawn. The same amount of work in watts is being done, so the energy has to come from somewhere.
Or more precisely, at full load if the voltage goes down the motor speed decreases and so the frequency induced into the rotor increases and this makes the rotor reactance (same inductance and higher frequency) exceed the rotor resistance. Therefore the power factor of the induced rotor current goes very low and as a consequence the stator winding power factor also goes very low. This means it has to pull lots more amps to maintain more or less the same input power and the same mechanical output wattage.
 

Offline qno

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Re: 3-phase motor drawing high current
« Reply #34 on: August 17, 2016, 08:01:19 pm »
Have you recently changed the oil?

You may use the wrong oil.
Why spend money I don't have on things I don't need to impress people I don't like?
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #35 on: August 18, 2016, 02:06:41 am »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.

I'm working with 120/208-Y mains. I thought on Y you must have 3 trannys. Only delta can use 2?  :-//

you can use two, but you will need a transformer that can accept 208 in and then add 32 volts (or 24, and that's good enough) to it. the connections for this may be rather confusing.

or you can use three transformers that will take 120v in and add 18 volts. (or 12 volts, and call it good enough)


the two transformer method leaves the third leg a little weak. this isn't a problem for smaller, less efficient motors but for large ones it is a problem.
a 5% voltage imbalance requires a 25% derating (meaning you can only load the motor to 75% of nameplate full load), otherwise you exceed "nameplate full load temperature rise"
« Last Edit: August 18, 2016, 02:11:14 am by johansen »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #36 on: August 18, 2016, 03:46:57 am »
you can use two, but you will need a transformer that can accept 208 in and then add 32 volts (or 24, and that's good enough) to it. the connections for this may be rather confusing.

Thanks for the thorough and informative reply. (This why I love BBV forum--nothing but the greatest input.)

No reason to not go with 3 transformers. Just curious to understand the "why".

Cheers.
« Last Edit: August 18, 2016, 05:47:13 am by iXod »
 

Offline k2teknik

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Re: 3-phase motor drawing high current
« Reply #37 on: August 18, 2016, 04:47:36 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?
It can make a huge different from a normal resistance measurement when you use a megger.
The megger is used isolating measurement, many people had been in for a surprise when they discovered the different in measurement results between a normal resistance measurement and when you use a megger.
A megger is a must use instrument in a case like yours even if you think that it is unlikely that all 3 coils should have the same leakage to ground.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #38 on: August 18, 2016, 05:03:22 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?

5.5M between isolated windings, about the same to ground, using the 500v test.

Quote
It can make a huge different from a normal resistance measurement when you use a megger.
The megger is used isolating measurement, many people had been in for a surprise when they discovered the different in measurement results between a normal resistance measurement and when you use a megger.

My favorite YT video re. insulation testing -- including the surprise, as you say. (c;  ::



Quote
A megger is a must use instrument in a case like yours even if you think that it is unlikely that all 3 coils should have the same leakage to ground.

Yes. Now that I own a Fluke 1587 I can see how useful it is to eliminate these unknowns from the troubleshooting equation.

Cheers.
« Last Edit: August 24, 2016, 06:20:09 am by iXod »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #39 on: August 23, 2016, 06:09:19 am »
Installed 3 boost transformers. Input approx 205, output approx 225. Current dropped to 4.05 amps, still 50 percent above rated.

Except for the tripping overload this machine works perfectly. The pump pressure is per spec. Oil viscosity is per spec. Filters not new but look good. I'm running out of ideas. I've emailed the motor manufacturer (Yaskawa Electric USA) and will see what they say.

Cheers.
 

Offline EPTech

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Re: 3-phase motor drawing high current
« Reply #40 on: August 23, 2016, 09:19:27 am »
Hi again,

there is one more thing I can think of. Maybe the oil seals between the pump and motor have started to fail and oil is getting in the motor. If that is the case and the cable passage between the stator and the terminal box of the motor is open, you will see oil in the terminal box. If the passage is potted, you will have to separate the motor from the pump. Yaskawa support is going to ask you the same question: to  have the motor run free and check currents.

5.5 Mohm is on the low side for an isolation test but not problematic. I have seen motors short out during operation but measure perfectly fine at stand-still. Best is to perform a megger test right after the overload condition to increase your chances that the fault is stil there.
Kind greetings,

Pascal.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #41 on: August 23, 2016, 12:04:52 pm »
No voltage drop on the contactor? I missed it if you already checked. The only thing left after that is a high mechanical load. Perhaps the pump needs a rebuild. Is it a positive displacement pump?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #42 on: August 24, 2016, 06:18:33 am »
Vdrop across contactor + overload is 0.2v on each phase.

I changed the hydraulic oil and replaced with factory-recommended type, and cleaned the pickup screen. No joy.

I'm coming to the conclusion that it's mechanical overload that is causing the problem. If I adjust the pump's pressure regulator down to near zero the current draw drops to about 2A. In my mind this means that there is no mechanical binding happening, just hydraulic load. Speaking to a pump rebuilder he said that this type of pump (trochoid or gerotor*) can lose its seal from input to output and "re-circulate" fluid which increases the load on the motor.

So that's the theory I'm working with. Presently.  ;)

* https://en.wikipedia.org/wiki/Gerotor
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #43 on: August 24, 2016, 06:28:20 am »
there was recently a thread on the practical machinist forum about those types of pumps. dammit, i might be thinking of arduino vs evil. or both, i can't recall.

not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

anyhow, as i mentioned earlier, shorted turns are almost always catastrophic failures. you can't just dump a half kilowatt of heat into a single shorted turn, to account for the half hp of missing shaft torque, and not expect the motor to warm up.
« Last Edit: August 24, 2016, 06:36:46 am by johansen »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #44 on: August 24, 2016, 06:31:26 am »
there is one more thing I can think of. Maybe the oil seals between the pump and motor have started to fail and oil is getting in the motor. If that is the case and the cable passage between the stator and the terminal box of the motor is open, you will see oil in the terminal box. If the passage is potted, you will have to separate the motor from the pump. Yaskawa support is going to ask you the same question: to  have the motor run free and check currents.

The motor is open type (not "potted" inside; can see the windings) so I rule out this possibility. Oil would be escaping in every crack of the motor housing if this were so. But a very creative deduction in any case. Thanks.

Yaskawa Electric motor tech support was not helpful. No new suggestions.

Quote
5.5 Mohm is on the low side for an isolation test but not problematic. I have seen motors short out during operation but measure perfectly fine at stand-still. Best is to perform a megger test right after the overload condition to increase your chances that the fault is stil there.

The "fault" isn't momentary or intermittent; it is constant high current. The overload takes time (it's thermal type) to go open-circuit. So I don't see any particular point in the cycle (power-on to OL trip) that would be advantageous to make insulation resistance measurement.

Cheers.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #45 on: August 24, 2016, 06:36:54 am »
there was recently a thread on the practical machinist forum about those types of pumps. not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

Yeah, these are very close-tolerance devices. I'll look for a new pump first. Thanks.

Quote
anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

No Apologies! Theorizing is part of troubleshooting. All theories (and guesses) welcome. Thanks!
« Last Edit: August 24, 2016, 06:29:18 pm by iXod »
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #46 on: August 24, 2016, 06:41:33 am »
there was recently a thread on the practical machinist forum about those types of pumps. not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

Yeah, these are very close-tolerance devices. I'll look for a new pump first. Thanks.

Quote
anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

No Apologies! Theorizing is part of troubleshooting. All theories (and guesses) welcome. Thanks!


well, at a minimum you need to disconnect the pump and measure the no load idle current of the motor. it should be around one third of the nameplate full load amps.


but i was a bit surprised that the amps didn't decrease as much as i expected. your motor may be more overloaded than i expected as well. who knows.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #47 on: August 24, 2016, 07:10:47 am »
...as i mentioned earlier, shorted turns are almost always catastrophic failures. you can't just dump a half kilowatt of heat into a single shorted turn, to account for the half hp of missing shaft torque, and not expect the motor to warm up.

Windings can short one turn at a time, decreasing resistance and increasing current fractionally. Usually from chemical or thermal breakdown of the insulation. That's what insulation resistance tests show. This is often a non-zero ohm result. Good motor maintenance uses these tests over time and can graph (and predict) the change in the breakdown of the insulation.

A winding doesn't usually melt into a 0-ohm resistor.

Cheers.
« Last Edit: August 24, 2016, 07:17:49 am by iXod »
 

Offline Jeroen3

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Re: 3-phase motor drawing high current
« Reply #48 on: August 24, 2016, 08:57:04 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?

5.5M between isolated windings, about the same to ground, using the 500v test.
That's like only slightly above fail value... But also way below pass value (25M). Did you order a new one yet?
 

Offline k2teknik

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Re: 3-phase motor drawing high current
« Reply #49 on: August 24, 2016, 10:00:35 am »
Filters not new but look good. I'm running out of ideas.
Change that filter, it may be blocked and you may not be able to see that visually.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #50 on: August 24, 2016, 06:32:52 pm »
there was recently a thread on the practical machinist forum about those types of pumps. not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping, running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

Would you please provide a link to that discussion? I searched but couldn't see any relevant discussion in that forum. Thanks.
 

Offline ez24

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Re: 3-phase motor drawing high current
« Reply #51 on: August 24, 2016, 07:08:21 pm »
Watching for the solution  :clap:
YouTube and Website Electronic Resources ------>  https://www.eevblog.com/forum/other-blog-specific/a/msg1341166/#msg1341166
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #52 on: August 24, 2016, 07:20:41 pm »
New pump ordered. The seller (a supplier well experienced in pumps -- no, not from e-Bay) says that this is a known failure mode of this type of pump. Having exhausted every kind of electrical diagnosis I can think of, it's time for a little trust.  :-\

Will report results.

Cheers.
« Last Edit: August 24, 2016, 08:16:55 pm by iXod »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #53 on: August 25, 2016, 03:37:29 am »
Watching for the solution  :clap:

That makes 2 of us!  :)
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #54 on: August 26, 2016, 02:59:37 am »
Got to be mechanical then, IMO. You might have tried pulling the end plate off the pump first and checking for wear. Many times with these lobe type pumps the wear mechanism is mostly in side clearance of the lobes, which is pretty straightforward to fix. All you need to do is regrind the sides of the lobed gears and the end plate, leaving a small amount of clearance - usually on the order of .0005" to .001" clearance is good. Ideally you'd want as smooth a surface as you can get for longevity, so it might be advisable to "lap" the lobed gears with fine emery cloth on a surface plate after grinding. Your shop has a bandsaw - does it have a surface grinder?

I was a machinist, toolmaker and weldor for nearly the last 20 years before I hurt my back and started fiddling with electronic stuff, BTW. I've been a member at PM nearly since they started.
« Last Edit: August 26, 2016, 03:06:41 am by eKretz »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #55 on: August 26, 2016, 03:49:16 am »
Yes there's 3 or 4 surface grinders and more than that in experienced operators. But zero with knowledge of rebuilding this type of pump.

So, re. rebuilding (grinding) the pump, I've got one anonymous source of info: you. With respect, it would be trial and error to do this. More downtime for the saw.

I think they would be best off just buying a $500 pump and paying me to swap it in. No unknowns.

Cheers.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #56 on: August 27, 2016, 06:51:50 am »
Hey I hear you - time is money in a machine shop. It really isn't difficult though, no pump rebuilding experience necessary. It's just flat grinding, you don't need to regrind the lobes. If you order the new pump, they should give the job to an apprentice or something so that you have a spare on hand.
 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #57 on: August 27, 2016, 04:02:41 pm »
I have serious doubts that it will be the pump.  That is their solution for everything.  A friend had a large metal shear and the motor just hummed.  I was out of town for a month. Motor rebuilder assured him it was a bad motor. $800 later after a motor rewind it still didn't work.  Got back and 5 minutes later it was fixed. A wire had just come off a motor contactor.

I'm still betting on low voltage.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #58 on: August 28, 2016, 08:13:38 am »
He already brought the voltage up with boost XFMRs, didn't solve the issue - though I would have tried to get a bit closer to 240V. The pump could very easily be causing the problem. Gerotor (lobed) pumps rely on minimal side clearance to avoid recirculating the pumped fluid back through the pump (think blowby like with piston rings). Once this side gap wears to a larger clearance, the regulator has to be turned up to maintain the required pressure, causing a higher amperage draw on the motor.

BTW OP, how many years has the pump been in use?
 


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