The efficient Dyno

[Simon]

So, for test purses of some equipment I am going to connect two identical motors together, drive one and load the other which will be a generator once turning to mechanically load the motor being driven by the driver. These are brush-less motors so essentially the output will be a 3 Phase voltage that will never exceed the input voltage to the driver particularly at low revs. So i take it that at 1.1KW I have no hope of getting out enough voltage to put back into the input rather than just feeding it into power resistors as any form of SMPS jiggery pokery is well beyond worth while.

[max_torque]

As long as the dyno motor inverter is synchronously rectified, then the inductance of the dyno motor itself will drive current back into the supply when a lower average voltage is applied (lower than the Bemf at that speed).  There are certain mimimums depending on the current, due to resistive voltage losses & switch losses etc.


TBH, at just 1.1Kw it's generally not worth going regenerative as the cost of the inverter outweighs any energy saving (unless the test is run for hours and hours)

A cheap option is to use a large automotive alternator as the load, simply connect across some big water cooled power resistors, removed the std voltage regulator and PWM control the rotor excitation current to control the load.

[Simon]

I was just trying to find an alternative to 1KW of heat, once you actually work it out that is a lot of resistors, heatsinks and fans that make noise almost as annoying as driving the the original load. The thing with the motors is that they are repeatable, ideally we need to make 2 or a second one long before the first one fails so that we can transfer the "calibration/characteristic" to the second set or we defeat the object due to manufacturing tolerances.

Dumping the heat into water is probably the best idea.

The consensus with the motor manufacturer was 3 phase rectifier - smoothing cap and PWM the resistors so that the mechanical load can vary from 0-100% of that speed. I was hoping to do something like we discussed with recovering the power but no chance really I guess, bucket of water it is.

[C]

The consensus with the motor manufacturer was 3 phase rectifier - smoothing cap and PWM the resistors so that the mechanical load can vary from 0-100% of that speed. I was hoping to do something like we discussed with recovering the power but no chance really I guess, bucket of water it is.

This is a buck power supply with resistor load.
An option is
 a boost power supply with motor drive DC bus load.

[NiHaoMike]

What about some incandescent light bulbs to dissipate the power?

[jbb]

So i take it that at 1.1KW I have no hope of getting out enough voltage to put back into the input rather than just feeding it into power resistors as any form of SMPS jiggery pokery is well beyond worth while.

The SMPS jiggery-pokery may not be as hard as you think. Can you sketch your system? What voltage level do you feed into the motor driver? Can you feed DC power into the motor driver? How tightly do you need to control the load torque? Do you need constant torque during rapid changes in speed? Does your generator have Hall effect sensors fitted?

Maybe an ODrive in regenerative braking mode would be sufficient? They only go up to 48V DC bus, though.

[BrianHG]

I use 12 50watt 12v halogen spot bulbs.  (I only need to drain 500 watts max, wired in series + parallel to get the voltage I wanted), + I have a PIC's PWM driving a N-Channel mosfet to set a programmable load VS generator RPM.  I use the RTC input pin with series cap and resistor to on one of the generator's phases to acquire the RPM.

I use 6 schottky diodes + CAP to create the DC.  Use a super huge cap value if you want to electronically simulate a huge fly-wheel.  This may be useful for wind and water power generation as long as you load your power from the DC cap.

Note that without a socket, these bulbs are a hassle to solder wire to.
100w versions exist.

[BrianHG]

A bunch of these would work, but, they will also make a nice space heater & though they don't need it, I would still recommend a fan:
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180618170743&SearchText=300+watt+high+power+ceramic+tube+resistor
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180618170842&SearchText=500+watt+high+power+ceramic+tube+resistor

If you use the 6 diode bridge to get DC with cap, you can use the PWM mosfet trick with a smaller resistor than you need, you can simulate different larger resistor values.

[BrianHG]

As for dumping the heat into water, you can get a 120v kettle and use it as a 10 ohm resistor, though, once the water boils away, your in trouble.  The 1-2$ halogen lamps are the cheapest solution, but, if you leave them on at full power each continuously, they will burn out after around 3000 hours of use.  The big green resistors from aliexpress, or, equivalents from digikey are designed to radiate a ton of heat and last much longer. 

Here are Virshay 300 watt and 1Kw resistors from Digikey.
https://www.digikey.com/products/en/resistors/chassis-mount-resistors/54?k=resistor&k=&pkeyword=resistor&v=274&FV=8041f%2C80439%2C80486%2C80081%2C8050b%2C80082%2C8082c%2C8082d%2C8082e%2C8082f%2C80830%2C80831%2C80832%2C80833%2C80834%2C80835%2C80836%2C80837%2C80838%2C80839%2C8083a%2C8083b%2C8083c%2C8083d%2C8083e%2C8083f%2C80840%2C80841%2C80842%2C80843%2C800d7%2C80022%2C80023%2C80214%2C80215%2C8023d%2C8027f%2C80291%2C802bd%2C80315%2C8033f%2C803d4%2Cffe00036&quantity=0&ColumnSort=1000011&page=1&stock=1&nstock=1&pageSize=25

Note that for 1Kw, 75$ is not bad, however, the resistor is designed to go beyond 350 degrees Celsius!  You need to mount it in the clear.
With a fan, you can go a bit beyond 1kw.

[Amper]

This kind of setup is very possible and used when testing large bldc systems. I have worked on a 250kW system, where it was done like this, total loss was just about 5kW at 150kW mechanical. Though a very good control software is needed not to get everything to oscillate with loops working against each other.

If you want to just dump it into a load there is large 1.5kW power resistors that cost just around 50 bucks new and dont need heat sinking, just maybe a small fan for comfort.

EDIT: here is one for example https://www.mouser.de/ProductDetail/TE-Connectivity-CGS/TE1500B1R0J?qs=sGAEpiMZZMtbXrIkmrvidIFFm%252butUMKCrJSE%2fDccfJU%3d

[Simon]

Putting it into water will mean not heating the room. We usually work in small enclosed rooms and currently testing turns those into hot hell holes very quickly.

[Benta]

Incandescent (or halogen) bulbs are not really optimal for this, as they are very non-linear. Of course, if you monitor output voltage and current it could work. At least it's cheap.

[Simon]

I don't want to heat the air in the room.

[DTJ]

I use 12 50watt 12v halogen spot bulbs.  te the DC.  Use a super huge cap value if you want to electronically simulate a huge fly-wheel.  This may be useful for wind and water power generation as long as you load your power from the DC cap.

Note that without a socket, these bulbs are a hassle to solder wire to.
100w versions exist.

I've used that style lamp in the past. Initially to connect we used some ceramic screw terminal blocks, later we bought the proper push on connectors - probably from RS or Farnell.

[BrianHG]

I don't want to heat the air in the room.

If your water is flowing continuously from a cold outside source, then, it wont heat the room, otherwise, you will just boil a liter of water in less than 10 minutes and you will be steam heating the room.  Without external water-flow, the water will eventually just begin to heat the room.  The heat you are generating doesn't vanish just because you placed your resistive element in water, it just takes a little more time to heat up the physical matter which is the water.  Once the water is hot enough, it will heat your room.  Once you turn off the power, that hot water with all that stored heat will stay hot continuously heating the room until the water cools down.  You haven't solved your heating problem with water cooling, just delayed it.  And with 1.5kw, you can boil 10L of water in around an hour and create a steam bath unless you have a continuous source of cool water coming in.  A metal heatsinked resistor with tubed air vent to outside of the building may be cheaper to deal with.

Note that some of the glass ceramic style 300 watt large virshay/dale resistors can be left in water to keep them cool, however, I'm not sure about contact corrosion.

[Ian.M]

A 20L electric tea urn would give you an 1H 35M run time from 25°C to boiling.

A 230V 2.2KW element will have a resistance of about 24 ohms, so your rectified generator output needs to be over 165V on load to get 1.1KW dissipation.

If you don't have that  DC bus voltage, then you'll need to parallel 120V 1.5KW immersion heater elements at about 9 ohms each.  Three would get you down to 60V.  Folded ones are about a foot long, so could easily be mounted through the base of a 20L unheated urn.

With a low level switch above the elements, and a thermistor probe above the fill level to detect steam it should be fairly idiot proof.

Lag it with an insulated jacket so it doesn't heat your room too much.
After an hour and a half of testing, its tea break time!   Don't forget to dump the hot water what's not used for tea down the drain.

[Simon]

We would not be testing for extensive periods so water is a good compromise. With the amount we use changing it half way through the day in not unthinkable or we can run a pipe outside to a radiator capable of dissipating many KW so it might not even need a fan.

The bog standard aluminium power resistors should take water ok. Aluminium case, filled with resin at the ends and tinned terminals.

[CopperCone]

Can you redirect theheat into another room or enemy bathroom etc using a duct?

The right way to do this would be to get a evaporator / cooling tower, put it outside and build a house water cooling system. If its infrequency you just need rubber hose, big pump and a cooling tower. Some have build in reservoirs so you dont need a seperate tank and stuff. I think 2000$ would cover the power levels you have used, less if you have pumps.

Doing anything else sounds like a troll or one time gimic. I cant imagine a corporation in their right mindmaking some giant red hot wall, leave those things for satan.

https://m.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m4084.l1313.TR12.TRC2.A0.H0.Xcooling+tow.TRS0&_nkw=cooling+towerP

The only thing is that you probobly want to, if in frequent use, add some kind of blowers or directors or place it away from the building to prevent humidity increase, the large one will billow but i have seen the small ones used in something that could practically be considered residental.. I dont think there is a problem putting it against the side of a building so long its not running alot.. And maybe drain after use

Otherwise cook turkeys at work to make a bit of side money or feed the homeless?

[Simon]

erm i think we just need 20-50L of water with the load in it. a 1/2 inch hosepipe, self priming diaphram pump and a reject/returned radiator that does not leak, we supply them for BIG machines so will have something who's thermal mass alone will be a benefit before it needs to loose some heat.

[NiHaoMike]

I think 2000$ would cover the power levels you have used, less if you have pumps.

With that sort of budget, it should be pretty easy to get a grid tie inverter to use as a regenerative load.

[Ian.M]

The bog standard aluminium power resistors should take water ok. Aluminium case, filled with resin at the ends and tinned terminals.

Don't bet on that.  It only takes a tiny seepage through the seal as they cool down and next time you switch on it will puke its guts as the droplet of water that got inside turns to steam. 

If you *MUST* use aluminium case resistors,   the sane thing to do would be to mount all the resistors to a heavy slab of aluminum as a heat spreader and interface that to the bottom or side of a rectangular stainless tank (think catering equipment suppliers) with thermal grease, stainless bolts fitted from inside the tank with gasket paper washers coated with hermatite red (or potable water jointing compound if you want to make tea!) under their heads, nuts on the outside directly to the tank, then oversize holes in the aluminum slab, big, lightly greased penny washers to bear on the aluminum, then a stack of Belleville washers and a locknut to provide contact pressure without rigidly fixing the aluminium so its free to slide slightly against the stainless to allow for its differential expansion and contraction.

However if you can import 120V water heater elements within your project timescale, you can almost certainly do it more easily with less machine shop work.  They just need a deburred hole in the tank bottom or flat side + as they are usually intended to be mounted in a NPT threaded boss, you'll need some stainless nuts with the right thread to go on the inside. 

Another idea - if the frequency range is reasonable, boost the voltage with transformers then use a three phase Variac for power control and run a cable to bathroom heaters mounted on the exterior of the building, with a small roof over them to shield them from direct weather.

[Simon]

I think 2000$ would cover the power levels you have used, less if you have pumps.

With that sort of budget, it should be pretty easy to get a grid tie inverter to use as a regenerative load.

Possible but we do want to vary the load at various outputs o would become complicated unless we are simply torture testing.

[BrianHG]

erm i think we just need 20-50L of water with the load in it. a 1/2 inch hosepipe, self priming diaphram pump and a reject/returned radiator that does not leak, we supply them for BIG machines so will have something who's thermal mass alone will be a benefit before it needs to loose some heat.

The radiator still moves all the heat, you just have the option of placing the radiator portion outdoors.  Anyways, with a 2k$ budget, just use one of these water cooled high power resistors.  The center is just a water pipe and it's approved & designed for water cooling:
The resistor will be a fraction of your budget.
Google 'Water Cooled Braking Resistors'.  These guys go up to the megawatts in some cases, and they have 3 phase resistor contacts designs with wiring and tubing built in for the 3 resistors in some designs.

[Gregg]

A tank less electric water heater has most of the necessary plumbing done for a water cooled resistance load bank.  It should be fairly easy to wire directly to the heating elements and connect water with garden hose.  Just make sure it stays full of water and you don’t have any air pockets around the heating elements when powered and adjust the flow so that it doesn’t boil.

[BrianHG]

A tank less electric water heater has most of the necessary plumbing done for a water cooled resistance load bank.  It should be fairly easy to wire directly to the heating elements and connect water with garden hose.  Just make sure it stays full of water and you don’t have any air pockets around the heating elements when powered and adjust the flow so that it doesn’t boil.

They have automatic over-heat cutoff and you cant purchase the desired resistance you require, but, I guess it's possible.

[CopperCone]

Well if you wanna do it right the best way would be to attach a heated aluminum block to a cold plate. Then you bore holes in the block and usse swaged self sealing cartridge heaters to act as resistors if your frequencies are low. Then you can gang em using a switch or relay bank.

A cheap way to do it is instead of the machinework to take an aluminum cooling block and stick heating strips of correct impedance on both sides then heavily seal it using sticky sided polyurathane foam. Add some epoxy atound the lead wires, cover it in paper and dip coat for water proofing.. You can make a 1kw heater this way in a few hours.. Key is to degrease the shit out of everything so the strips can adhere good and to uze gloves and a really clean work surface when using the stuff.

If you need better frequency responce then i gues you need to use electronics grade resistors.

Thehrating strips are actually decent because they are thin, a power resistor is something like 3 mm of cement, an aluminum housing and then your grease and then your block. The resistance material is in a helix coil in a resistor wheras in a strip its a flat zigzag maze like structure

A heating strip is flat, so you have say 50w distributed over 5 square inches seperated from the aluminum by something like 0.4mm of heat conductive rubber, so the thermal transfer characteristics are not too bad.

The swaged heating cartridge is the best because it will make a tight seal with the block by way of thermal expansion, it butterflies out like an arrow heat. If you calculate its expansion factor and bore performance you may get nice results but idk if you can heat up water cooled metal enough to properly make the seal.. But anyway if designed right no grease and easy replacement at room temp. And its something like 0.4mm or so of steel and the thermal transfer medium is highly compressed magnesium oxide so it will be decent despite its thickness.

From experiance something like a 12x6 inch heat plate covered with those strip heaters and foam can easily dissipate 600w with like 2-3 gpm flow of water while only rising in temperature by a little bit. This was with a half decent aluminum cold plate though that had various features for increased thermal transfer.. I think a 12x12x1.5 inch cold plate covered with heating strips would do real nice industrially even if cheap plates were used. Just beware of clogging and put thermostats on it to disable if overheating.

And ground it

[CopperCone]

The biggest problem is clogging.

You can make a fully servicable heat exchanger using swagelok parts if you are good at machining. You make a bunch large diameter pipes connected by t junctions. Then you basically make em into a zigzag.

 So you have a lenght of pipe connected to a t junction. On the perpendicular connection you put a 45 degree bent pipe that basically reverses the direction of water flow.

On the opposite end, you stick in a piece of coated polished star shape milled tube stock with a cartridge heater in it.

So the water flows in the tube that has a star shaped heat exchanger that water flows past, and changes direction to another section, then you make a ladder network.

Very expensive but no welds or threads in this, it can be fully swaged and disassembled by the simplest tools and all components besides the heat exchangers can be quickly obtained from manufacturers. You can do a periodic full clean or even internal polish and your heat exchangers can be removed for recoating or replacement. Fu boroscope inspection using only 1 and a quarter turns.

I will be god damned if anyone builds it though.

And it would also work at ridiculous pressures.

If you trust tig welding and can do the inspection you can bend a stainless snake out of tubing and drill holes on the side then weld on sections of pipe (maybe pure silver or gold braze would b good) to eliminate the need for all the t junctions. You would wanna line all the shit up in a vacuum kiln though with the correct spacing for brazing, i dont know if welding a section of bent pipe with a tig is a good idea, imo it creates a good place for corrosion to occur. But it will have sharp internal angles that are difficult to polish and inspect and clean. T junctions would be more expensive and less reliable but cleaner and more servicable


Now i guess you can also cut a heat exchanger in half and rely on a gasket but low pressure and periodic maitence is really necessary. And you could not use machine tools to repolish and inspection is difficult compared to removable heat exchangers

Also you would need to be wary of pressure drop, high pressuremeans more errosion but cost will go nuts with large diameters.

[max_torque]

If you're going to passively rectify the AC "dyno" motor output to DC, buffer into some caps (small capacity just for voltage spike smoothing), and then PWM into a "load"  you may as well BOOST through an inductor and back into the supply (and prob some more caps) feeding the test motor drive.  Hardly any extra effort (needs 1 more load switch) and an inductor, but as space isn't an issue that's an easy thing to make (just hand wrap copper wire around an old large transformer core.  You don't really care that much about efficiency, a slong as most of the power (lets say 75%) goes back to the supply, the heat released is fairly small and probably a small fan blowing on the induct or and power switch.

Voltages are all reasonably small so no exotic power components needed

[Simon]

OK well there is no budget here and i aw having to convince them to let me do this so it's a zero budget.

A water cooled resistor would be erm.... cool.... . It would make it more movable as the system would be closed but would require a header tank. Yes I'd stick the radiator outside that is the whole point to move the heat out of the room.

A cheapo diagram pump will move 8l/m and have a few bars of pressure so it won't go down much from peak flow.

[Simon]

If you're going to passively rectify the AC "dyno" motor output to DC, buffer into some caps (small capacity just for voltage spike smoothing), and then PWM into a "load"  you may as well BOOST through an inductor and back into the supply (and prob some more caps) feeding the test motor drive.  Hardly any extra effort (needs 1 more load switch) and an inductor, but as space isn't an issue that's an easy thing to make (just hand wrap copper wire around an old large transformer core.  You don't really care that much about efficiency, a slong as most of the power (lets say 75%) goes back to the supply, the heat released is fairly small and probably a small fan blowing on the induct or and power switch.

Voltages are all reasonably small so no exotic power components needed

Yea i guess why not, like you say anything is better than nothing. I'm no SMPS expert I'm just worried that we can control the load from low rev's when the voltage out is quite low. the input is always 20-30V if we are running slow and can't make enough of a boost voltage to match the input voltage we have no load. Obviously i have no idea what the output voltage will be, I guess it is linear with speed ? 20% is a reasonable minimum speed we would run at and I guess a 5X boost is not unreasonable.

[IanB]

A water cooled resistor would be erm.... cool.... . It would make it more movable as the system would be closed but would require a header tank. Yes I'd stick the radiator outside that is the whole point to move the heat out of the room.

There are electric immersion heaters for domestic hot water tanks. Could you construct a fixture with one or more of these installed in a big water tank? You could even plumb in the circulation to the outside radiator to keep the system fully sustainable?

[max_torque]

For "zero" budget, i'd still go with a s/h car alternator, some kettle elements for the load (can be slung straight across the 3ph output of the alternator, removing std rectifier pack), and a cheap ebay H bridge to pwm control the rotor excitation current.  Simple magnetic Hall sensor to measure rotor speed and a current sensor around the phase output to measure "torque".  Std alternators go to around 18krpm, so plenty of headroom, or the ability to run a small belt drive if you want to leverage some gearing (ie generate more dyno torque)

I think you could have it up and running on the bench in a day, for under £200

[Simon]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it? I was thinking that with 2 identical shafts it will be easier to join them together and we already know they are matched, baring motor/motor inefficiencies we get the same out as we put in. What sort of speed can an alternator do? if we start doing pulleys and stuff it will never happen which is why i wanted to keep it really simple, 12mm tube heat-shrunk on or drill a hole in each shaft and pun a pin in. Once it is rectified we can decide what to do with the power. making a crude boost converter would be nice as it makes the whole thing more compact and portable and minimal heat output.

[cellularmitosis]

On the topic of dummy loads, Mike goes over a few of his, including just a bunch of magnet wire in a bucket of water.

https://youtu.be/WECW88rJYrE?t=1m53s

[CopperCone]

oh yea with my idea, so long you can make heat exchange rods,, there is no reason you can't make it out of shitty PVC pipe if you want to keep down costs, the exterior surface does not need to be metal for any reason then pressure, but it can melt and be a heat hazard, so i would recommend some kind of scaffolding made of metal on the outside.

then you have something along the lines of a adjustable power load if your voltage is constant, otherwise you would need a dc dc converter before it

[IanB]

One "thinking outside the box" possibility, if you want to avoid dumping heat into the room, is to consider a power storage scheme. For example, use an alternator to charge up a battery bank (through a rectifier and charge controller). You can then use a power inverter to recover the stored energy and make use of it later.

I know this is big, expensive, and complicated, and maybe not reasonable in this case, but it is one way to avoid generating lots of waste heat that you have to dispose of.

[Ian.M]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Coppercone's heat exchanger ideas sound like a *LOT* of work.  If you've got access to a milling machine, it would be a lot simpler to just mill two mating serpentine half-channels in two slabs of aluminum, then bolt them together all round the edges with a gasket, drill and tap the edge faces where the two channel ends emerge to bolt on flanged pipe couplings again with gaskets, drill and tap blind holes in the faces for the metal cased resistors and run ordinary automotive coolant through it and the radiator to control corrosion.

[BrianHG]

12mm tube heat-shrunk on or drill a hole in each shaft and pun a pin in.

?  Are you sure you know how much torque is involved in braking 1kw?
Car alternators usually need a 12v excitation voltage applied to it's magnet generating coil.

Just adding a metal or copper disc, or pulley/flywheel on your motor shaft + some neodymium magnets mounted near that flywheel and you can generate a nice smooth braking force, no circuitry, no wires, no boost switchers.  But holding back the torque of an excess of 1 horse power (figure 2hp to be safe since you are going over 1kw and you might have an engage jerk reaction to deal with) will require sturdy mounting whether you use a generator or simple magnetic braking.

Your generator 1kw alternator, especially a car alternator, will probably make more heat than your resistors which will be liquid cooled.

My 500w and 1kw neodymium low RPM 3 phase PMG generators are around 4 times bigger than a car alternator, and driven at full load for 1 hour straight, they do get toasty.  A car alternator with an excitation coil & 3 phase output will easily pass 100 degrees Celsius under the same load, even if rated at 1kw.  Unless you find an expensive PMG car alternator, you also need a 12v supply for the excitation voltage which goes up in current with load unless you tap the output voltage, capacitor filter and regulate it, and feed it back into the excitation coil like what is done in a car.

Attached low RPM, 0 cogging, 500watt/1kw neodymium PMG.
BTW, short the 3 wire on the output on these guys, and the shaft wont budge at all.

[BrianHG]

You want a cheap 3 phase PMG generator, low RPM, high voltage, if you have access to a junk yard, many use these washing machine PM BLDC motors as generators, though, they use normal magnets, but their large diameter still gives them a lot of power at low RPM.
https://www.google.ca/search?biw=1289&bih=724&tbm=isch&sa=1&ei=fospW62sBsvCjwS_h4No&q=washing+machine+motor+3+phase+BLDC+generator&oq=washing+machine+motor+3+phase+BLDC+generator&gs_l=img.3...9900.11146.0.11874.5.5.0.0.0.0.122.372.3j1.4.0....0...1c.1.64.img..1.0.0....0.23TMxcFeiCQ

[CopperCone]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Coppercone's heat exchanger ideas sound like a *LOT* of work.  If you've got access to a milling machine, it would be a lot simpler to just mill two mating serpentine half-channels in two slabs of aluminum, then bolt them together all round the edges with a gasket, drill and tap the edge faces where the two channel ends emerge to bolt on flanged pipe couplings again with gaskets, drill and tap blind holes in the faces for the metal cased resistors and run ordinary automotive coolant through it and the radiator to control corrosion.

Well if you do it just with a mill you wont have good thermal transfer because of surface area. For flats you want broached or cut grooves, you can double/triple surface area this way, which is why I said go with star

you also need coating for aluminum as mold preventers and stuff will destroy it and corrode, or impurities, especia;lly if you use an evaporator

closed loop distilled water or coolant will be alot more mild, but you won't get the concentrated cooling power of an evaporator, you need giant radiators and cooling fans to blow on them, which means more moving parts, or really really giant radiators that don't have moving parts, and you need to worry about sunlight control, etc.. hard to do outdoor radiators I think without moving parts.

A cooling tower is nice because the pump does all the work and its fairly low surface area so you don't have the sun turning it into a water heater so easily.

I mean a regular wet cooling tower that uses its own water to cool, not something more advanced, I think they are the cheapest and intermittent operation means you can get away with the downsides. I would recommend a closed loop one that uses air exchange only if its constant operation to keep down humidity, I think that at that point its worth investing in your facility to have more advanced infrastructure.

Some of the smaller ones you could also probably fit on a cart of some sort so you can keep it in a storage room when not in use to keep it clean. Bigger ones might require a fork lift or some shit.

Otherwise, you can use it, splash it down with soap afterwords using a garden hose, then clean it out ,maybe give it a gentle pressure wash if it gets filthy

[BrianHG]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

[CopperCone]

that may result in dead heading the pump which may be bad for it, the design would have to be considered carefully. You would need to consider the pump curve.

[NiHaoMike]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Why even bother with water? Compress air and release it through some sort of restriction.

[CopperCone]

I can see someone getting greedy and trying to store all that air in a cheap manner for later use. Then kaboom.

I wonder if you can make some kind of pnumatic vehicle to drive a few miles on stored air so you can run errands for the company, maybe some kind of scooter that runs on old scuba tanks or something

wow they are real
https://en.wikipedia.org/wiki/Compressed_air_car#Developers_and_manufacturers

ahhahaha

https://www.greencarreports.com/news/1096772_tata-airpod-compressed-air-car-to-launch-in-hawaii-this-year-report

imagine one of those rolling by with rap music blasting

[jbb]

As you can see, water cooling is inconvenient. And could get pricey - remember to conside your time. So don’t discount the possibility of doing a loop back.

Can you sketch your system? What voltage level do you feed into the motor driver? Can you feed DC power into the motor driver? How tightly do you need to control the load torque? Do you need constant torque during rapid changes in speed? Does your generator have Hall effect sensors fitted?

Maybe an ODrive in regenerative braking mode would be sufficient? They only go up to 48V DC bus, though.

[CopperCone]

cooling system is fairly bullet proof so long the pump is good and the plumber knew how to tighten it and did not use the wrong materials. A drive etc can break and cost alot of money, its a fairly complicated device, especially with more features like regenerative breaking, different sensors, etc.

Also since its for testing its like, how do you calibrate something strange like that?

[IanB]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Unfortunately, physics gets in the way of this. If you put sufficient load on the pump to make it draw 1 kW of electrical power, you will be dumping 1 kW of power into the circulating water. It will get hot and start boiling just as surely as if you submerged a 1 kW heater within it. The circulating "cooling" water will rapidly heat up the system and accelerate its demise...

[BrianHG]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Unfortunately, physics gets in the way of this. If you put sufficient load on the pump to make it draw 1 kW of electrical power, you will be dumping 1 kW of power into the circulating water. It will get hot and start boiling just as surely as if you submerged a 1 kW heater within it. The circulating "cooling" water will rapidly heat up the system and accelerate its demise...

You would be pumping the water through the outdoor radiator...
Simon's original design needed a generator, water submerged resistors, wiring, separate water pump and radiator.  I simply got rid of a whole lot of electrical junk in that equation.

[Simon]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Yes that is true, if we can directly couple them it will make the build easier. We could machine an adaptor to screw onto the alternator having removed the pulley so that the adaptor shaft is the same as the motor. It would indeed save a lot of power electronics.

[Simon]

The other thing with two back to back motors is that we already know something about the setup. Win = Wout - inefficiencies and we know that these motors are efficient. An alternator is 50% so yea, still plenty of heat and sod knows what is going on.

Also the motors have nice flanged fronts so we just use long bolts and we have all of the fixings that take torque done easily. The idea of this getup is to measure the rotor speed and power output to measure the performance of the motor driver.

[Zero999]

I was just trying to find an alternative to 1KW of heat, once you actually work it out that is a lot of resistors, heatsinks and fans that make noise almost as annoying as driving the the original load. The thing with the motors is that they are repeatable, ideally we need to make 2 or a second one long before the first one fails so that we can transfer the "calibration/characteristic" to the second set or we defeat the object due to manufacturing tolerances.

Dumping the heat into water is probably the best idea.

The consensus with the motor manufacturer was 3 phase rectifier - smoothing cap and PWM the resistors so that the mechanical load can vary from 0-100% of that speed. I was hoping to do something like we discussed with recovering the power but no chance really I guess, bucket of water it is.

What do you want to do with the 1kW of power?

If you were able to achieve the suggestion, made in your initial post, of putting it back into the motor's input, if would make the motor look like an it had no mechanical load at all, to the driver, which I would defeat the purpose of the test.

I suppose it is realistic to put the power back into the input of the driver circuit, which should be possible with a boost converter, or even just regenerative breaking on the alternator side, which would involve using the motor's inductance as part of the boost converter.

[Simon]

Yes the power has to go back to the DC supply of the driver. If we use the generators own wingdings don't we need to do all of it 3 times and then use isolation to control all 3 phases the same? probably simpler to just rectify it and then boost it.

[Zero999]

Yes the power has to go back to the DC supply of the driver. If we use the generators own wingdings don't we need to do all of it 3 times and then use isolation to control all 3 phases the same? probably simpler to just rectify it and then boost it.

Both of those methods would work.

Using a six diode bridge to rectify the output and a separate boost converter would also work, but it would require a large inductor or transformer to perform the boost function, unless the alternator's field could be increased to raise the output voltage.

Three half bridges could be used to do regenerative braking, which would use the motor's inductance to boost the voltage. The controller would need to be more complex, but it would avoid the need for a separate inductor and is probably more efficient, although the latter is probably not the most important thing, in this case. It's quite likely an existing motor controller IC/module could be used but it would need some changes to the suggested/standard schematic.

[Simon]

The "generator" is a permanent magnet motor. Being identical to the driving motor we can anticipate the output voltage with RPM. It is a delta configuration. Unfortunately the current driver does not do regen breaking this was in fact the original suggestion max_torque made when we discussed it.

[max_torque]

As i mentioned, the car alternator idea is simple because of the following characteristics:

1) it revs to around 18krpm (easily fast enough)

2) it is easy to couple too, either directly with a bellows coupling, or via a polyV belt, meaning you can adjust the speed / torqur relationship if required

3) Load control is done simply by the rotor excitation, meaning you can control the brake torque with just a 5S bench top CC supply (get one with RS232 or GPIB etc and you can automate everything)

4) Car alternators are tough!  They run at up to 160 degC in the native application, have no permanent magnets to demag, and even come with a built in fan to help cool themsleves

5) the input shaft has very good bearings, because those bearings normally take the FEAD belt loads, so they are super robust to any input shaft loading (from out of balance DUTs etc)

6) CHEAP!  Ebay, gets you one for literally a few £, if you somehow break it, then just buy another

7) you could do with a load at all, just short the phase windings together, all current just gets turned to heat in the windings themselves.  This would need a seperately driven cooling fan to extended operation possibly, but the high thermal inertia and high operating temp mean it would almost certainly run for 5 to 10min un cooled (especially at reasonable rpm where the built in fan starts to work)

The overall set up is both simple, and can be quickly "trialed" not the case with a system that needs complex power convertors etc



Those factors make it a winner for me by a fair margin!

[Simon]

Yes as discussed the alternator is going to be easier to get going. Someone will just have to make us the metalwork. The motors run up to 12'000rpm so an 18'000rpm alternator will be fine directly coupled. We can measure the current being put back into the DC bus of which we know the voltage (and can get it from the PSU or measure it) so we know what load we are putting on taking into account the efficiency of the alternator. In this way minimal stress and variation while running and we make around half the heat. The last thing we want is to be running in the summer at 35C and be putting more heat into the environment the testers work in.

We will of course need a 24V alternator unless we are safe to take a 12V and push it up knowing the rectifier can handle it.

[C]

You might want to think about a torque arm so you can sense the mechanical torque being applied.

[Simon]

We won't go that complicated. I believe that if we know the power and the speed we can make a calculation. The main aim is to characterise a motor driver and the be able to compare it to others on the same device. Absolutes are less important than a like for like comparison.

[Zero999]

Yes as discussed the alternator is going to be easier to get going. Someone will just have to make us the metalwork. The motors run up to 12'000rpm so an 18'000rpm alternator will be fine directly coupled. We can measure the current being put back into the DC bus of which we know the voltage (and can get it from the PSU or measure it) so we know what load we are putting on taking into account the efficiency of the alternator. In this way minimal stress and variation while running and we make around half the heat. The last thing we want is to be running in the summer at 35C and be putting more heat into the environment the testers work in.

We will of course need a 24V alternator unless we are safe to take a 12V and push it up knowing the rectifier can handle it.

24V might be more efficient too, since there will be less losses in the rectifier, which I presume will be silicon. Anther thing which could cut the losses further would be to change the the silicon rectifier for a Schottky one, which would halve the rectification losses.

[Simon]

I'm not too bothered, we could if it makes much difference. it will be easier to have the 1KW load with 24V although if half of that goes in heat anyway then meh! but we need to match the supply voltage anyway which is 20-33V and there is nowhere else sensible to dump it.

[max_torque]

I guess a "two stage" approach is also possible with an alternator based dyno.

For the first runs, where you just want to prove the concept, just remove the rectfiers and short the phases together, and concentrate on the mechanical arrangements (mounting and shafting), and on how to drive the excitation current into the rotor (uC to measure rotational speed via a simple hall sensor and simple pwm output to drive current through the rotor, with a PI loop to do basic speed control)
 When that's all working you can add a the rectifiers back in (or replace with lower loss ones) and build the boost convertor to re-cycle the power pack into the supply.  That could be a simple voltage control loop as well, using any number of OTS boost convertor control IC's  (ie, say you target the output of the phase recifiers to be 5v, that fixes your step up ratio to  be around 5 times) Of course, that voltage setpoint also sets the minimum speed at which your system can apply load.

[Simon]

Why do we need a boost converter? if we use the alternator we can drive it to the same voltage as the supply or in effect regulate on current output. We could regulate to produce a speed or regulate to produce a desired output/load and then take the speed measurement to see what speed we get with that load.

[Ian.M]

Yes,  though you may want to add torque sensing as 'C' suggested, and also if you want to improve the low speed capability, a cooling fan forcing air into the back of the alternator + a contactor to short its phase coils before the diode plate, so at speeds too low to get the load you want at your DC bus voltage, you can essentially use it as an eddy current brake  controlled by its field coil.

Since vehicle alternator efficiency can be as low as 60%, it should have no problem running in 100% dissipation shorted brake mode with forced air cooling as the total shaft power at speeds too low to generate effectively will only be a small fraction of its rated power.l

If you are going to do any sustained full load tests, you'll need to over-size the alternator considerably to avoid overheating.   1.1KW at 28V is 40A, so I would  suggest something in the 80A to 120A range.

[C]

As lan.m stated
Slow speed is a problem.
Normal minimum speed for a vehicle alternator is engine idle. And at this speed often do not produce full output to prevent heat build-up.

lan.m's post has a lot of great points that need to be covered.

In addition to the electrical load created, the vehicle alternator will also be creating a fan load.

I would think that torque sensing would be the easy way to get the total facts that make up the actual load.

Today a lot of vehicle alternator's have their own regulator built in. If you do not want to hack this an open a can of worms that could destroy the vehicle alternator, you could connect the output to a bidirectional buck-boost converter..
Some of todays vehicle's are using a 48 volt bus with a bidirectional buck-boost converter used to supply the lower voltage loads.

C

[Simon]

I don't know what power we get from the motor and driver at low speed and if we could get the full 1KW at minimum RPM. I would not be too bothered with an output below 20% (2000rpm).

We could short the windings at low speed and switch to dumping into the supply at higher speed. I am not familiar with torque sensors but yes that makes sense as our testing rig's characteristic is more defined against something real than some condition we registered once and need to see matched.

[BrianHG]

I don't know what power we get from the motor and driver at low speed and if we could get the full 1KW at minimum RPM. I would not be too bothered with an output below 20% (2000rpm).

We could short the windings at low speed and switch to dumping into the supply at higher speed. I am not familiar with torque sensors but yes that makes sense as our testing rig's characteristic is more defined against something real than some condition we registered once and need to see matched.

Shorting the windings for 1kw load at low RPM means 1kw of heat will be coming out of the alternator itself.  The alternator will warm up your room.

To support low RPM, you would be better off using a low RPM neodymium PMG where you get over 1kw and over 200 volts, over 85% efficient already at 500rpm.  Note that units exist designed for higher RPMs compared to the type/model charts I've attached.   Note that these are direct drive wind generator PMGs.

[nctnico]

I'd mount the resistors on PC water cooling hardware and just run the water from the tap. Cheaper then buying a pump to recycle. Look at the WDBR series resistors from TE. The alumium cased resistors can't handle much power. The WDBR resistors can when mounted on a heatsink and offer better value for money. Be sure to include over temperature switches though no matter what solution you choose.

[BrianHG]

Have you considered an alternator with 2 sets of windings for each phase?  You switch to series for lower RPM to get a voltage boost and switch to parallel at higher RPM to halve the voltage.  An old fashioned mechanical relay may be used for the switch.

Also, since the output is 3 phase AC and you are already rectifying the signal, you can use a simple capacitor voltage doubler on each phase & switch to before and after the cap double for that 2x or 3x voltage boost.  (3x can be done by adding a second doubler on your negative rail.)  To get the most out of a car alternator's low voltage at low RPM, I would recommend trying the new field effect rectifiers from ST-Microelectronics. https://www.eevblog.com/forum/chat/take-a-look-at-these-new-field-effect-rectifiers-from-st-microelectronics/msg1428355/#msg1428355

[LaserSteve]

Um,   You folks know that commercial water cooled resistors are off  the shelf parts right? They are made for machine tool regenative braking systems and are common.

Steve

[CopperCone]

what are some specs of high end water cooled resistors? I am really curious.

I see like VPG has nichrome in water ones, http://www.vishay.com/docs/32548/dcrf.pdf , which may not be ideal compared to fairly well isolated ones, they don't seem to work without very pure water which can be a problem to maintain without ion exchange resin

[BrianHG]

Um,   You folks know that commercial water cooled resistors are off  the shelf parts right? They are made for machine tool regenative braking systems and are common.

Steve

My post from page 1! https://www.eevblog.com/forum/projects/the-efficient-dyno/msg1617676/#msg1617676

[Ian.M]

There is absolutely no way the driving motor can output 1.1KW of shaft power at low speed if its rated for 1.1KW at several times that speed.

Shaft Power = 2 * Pi * n * T

where n is the speed in revs/s and T the Torque in Nm.

Neglecting frictional, windage and other losses, Torque is proportional to current in the driving motor, which has a finite upper limit above which its coils will burn up and/or its core will saturate.    Therefore if the motor is operating at 20% of the speed its rated for 1.1KW at, if its producing over 220W of shaft power its overloaded and will probably burn out if the overload is sustained.

Assuming a 24V 80A alternator, (which will actually be designed for about 28V),  its maximum output at rated speed will be about 2.2KW.  With forced air cooling, it will easily be able to handle 10% of that operating as a shorted brake, especially if the shorting contactor is directly across the stator coils so the diodes on the diode plate aren't being stressed.

Talk to a local alternator rebuild shop to find out which alternators are both readily available and  easy to mod for all phases + both field brushes brought out.   

There's little point in using external rectifiers.  Yes it may be able to improve efficiency (especially if you go to synchronous MOSFETs) but it pushes up the cost dramatically.  If the heat output from mk.1 is excessive you can always retrofit them on Mk2 without further alternator mods - just  hook up your external rectifiers to the stator phase wires you've already brought out, and move the output wire  from the terminal stud on the back of the alternator coming from the diode plate to your external three phase bridge.

I doubt you'll gain much from external braking resistors or voltage doubling - but that's easy to test - just try charging a large, 60% discharged 12V Lead Acid battery instead of regenerating into your motor drive DC bus, or patch in a bunch of wire in three buckets of water.  I suspect the speed range over which you can draw useful power at 14V but cant at 28V will be rather narrow.  Braking resistors shift much of the dissipation out of the stator, but they also push the minimum speed for a particular power upwards due to the voltage across them.

[C]

The vehicle alternators I have seen are three phase.

If you were to connect some outputs to the junction of winding to diode you could get an AC output. You could then use transformers to change voltage/current

The internal regulator will sense the DC output.
A load change to DC output will cause a mechanical  input load change.

To protect the alternator you need to keep max current equal or below rated output. A failure here is burnt windings or blown diodes.
You also need to monitor temp.

If you are building your own motor drive electronics add a half-bridge with a resistor load to keep regen from causing an over-voltage of supply bus.

C

[BrianHG]

Unless Simon is getting his 1.2kw alternator for less than 50$, it will cost less to buy a 125$ 1.2kw neodymium permanent magnet core style alternator.  He wont have to worry about the darn excitation supply and 12v through 70v versions exist, with and without output diodes and regulators, and he will get the full 1.2kw down at 1000rpm.  All he would need is a simple non-isolated switching step down regulator to recover most of the power.

example: https://www.amazon.com/Permanent-Alternator-Wind-Turbine-Generator/dp/B0755PPMPF

Full 1kw just over 1000rpm on this alternator's shaft.
Additional, be careful about car alternators, they specify their output power based on engine RPM, not alternator RPM, but 1kw 12v units exist though they aren't as efficient.

[Simon]

A large 24V alternator with modified control will do the trick. Given the not terrific efficiency a 1KW output alternator will be fine as it will be at least 1.5KW in mechanical power and the input to the motor controller is at most 1.2KW and then you have the looses in the controller and the motor.

[BrianHG]

A large 24V alternator with modified control will do the trick. Given the not terrific efficiency a 1KW output alternator will be fine as it will be at least 1.5KW in mechanical power and the input to the motor controller is at most 1.2KW and then you have the looses in the controller and the motor.

Yes, that should work.  Remember car alternators are specified at engine RPM, not their RPM.  The engine has a larger pulley than the alternators gearing up the alternators RPM compared to the engine value.

Stripping out the alternators internal electronics, or at least taping the 3 phase coils directly might help in your situation.
Modern alternators already use switching supplies to downconvert to 12v.
A 24v regulated output truck alternator, with 'self internal excitation circuitry' may be perfect for you off the shelf, no mods at all.  Just a GND and +24v output on the thing.


Something like this: https://www.amazon.com/ALTERNATOR-DELCO-ENERGIZING-HOOKUP-SE24Vv/dp/B004SOZK86/ref=pd_cp_263_1?_encoding=UTF8&pd_rd_i=B004SOZK86&pd_rd_r=JHQKM0427T3RZEWME0V5&pd_rd_w=VZSs7&pd_rd_wg=Zi6LM&psc=1&refRID=JHQKM0427T3RZEWME0V5

[Simon]

A large 24V alternator with modified control will do the trick. Given the not terrific efficiency a 1KW output alternator will be fine as it will be at least 1.5KW in mechanical power and the input to the motor controller is at most 1.2KW and then you have the looses in the controller and the motor.

Yes, that should work.  Remember car alternators are specified at engine RPM, not their RPM.  The engine has a larger pulley than the alternators gearing up the alternators RPM compared to the engine value.

The motors run up to 12'000 rpm

[joeqsmith]

Not sure I can be of much help but I did design a little toy dyno some time back to sort out the controls for a larger one.   I started out with a brush DC motor for an absorber.   I was using some old VCR recorder and a few toy hobby motors that a friend of mine had given me to simulate the DUT.   These would run over 12,000 RPM.   

The system can run constant torque while sweeping the RPM or constant RPM while sweeping the torque.   We never supported all these features with the actual dyno.  The large one is water cooled.   

Just to be clear, I don't own and have never owned any toys that use these motors.  This was just a proof of concept was all.   

Link showing the build. 
https://www.msuk-forum.co.uk/forums/topic/81512-self-built-rc-dyno/?page=6