4Kw dummy load

[fcb]

Got an interesting one here.

I've got to develop a sensorless drive for some 4KW@1000rpm brushless motors, I need to create a piece of of kit that can act as a variable load, sadly not much budget, but I do have a 68VDC 4KW@1000rpm brushed motor (the motor that the new unit will replace).

My current idea is to build a big boost converter and drive a couple of paralleled 2KW/240V electric heaters - probably 4 boost converters in some sort of dsPIC controlled poly-phase mashup.

I did look at using 30x 2KW/240V electric heaters and a few relays to build a load, but this would be quite expensive and take up a silly amount of space.  I also thought of buying some thick Nichrome wire and a 25L metal tub of oil and making a cantenna.

Any ideas welcome, I've got some-time to do this.

[Jon86]

Go for some kind of heater/wire. You're not going to have much luck trying to do it electronically unless you've either got a ton of money or loads of experience with power electronics.
You're going to need something mechanical, so have a search around until you find something suitable, good luck! 

[fcb]

We did look at something 100% mechanical (hydraulic motor forcing oil though a small orifice), but it's difficult to make variable AND repeatable.

Just to explain a bit better, the brushless motor shaft will be coupled to the 68VDC motor shaft directly, so the DC motor is effectively a generator.

One other thought - perhaps the motor could be shorted out with a big FET or two, the motor has watercooling connections - apply variable PWM to the FET's.

[penfold]

If your brushed motors have field windings accessible you could implement something akin to a ward-leonard set to produce a variable load.

[dr.diesel]

 - How much does it have to vary?
 - Do you have to lug it down still the motor physically stops?
 - Do you have to automate it?
 - Is this a one time deal or will you be designing this and handing it off to production?
 - What is your actual budget?

[G7PSK]

Look up Froude dynamometers, they made and still do I believe hydraulic dynamometers. Just looked them up they are now Froude Hofmann.

http://www.froudehofmann.com/index.php/products/engine-dynamometers/engine-based/6-high-speed-dynamometers

Basicly they are a torque converter with a load cell to measure the torque.
Cant be very hard to build a small version.

[johansen]

your sensorless motor drive should be able to work backwards and use the motor as the boost inductor, within limits.
braking torque is proportional to rpm, if you want more you have to burn watts in the motor to produce reverse torque.

so you can electrically load test the motor by using its own rotational inertia.. this is done in the industry for load testing really big motors.

in any case, my recommendation for a dump load is 8 resistors, sized to draw ,16,32,64,128,256,512,1024,2048 watts from your generator. you can control which resistors are turned on with a uproc or an old 8 bit ADC directly. use mosfets of course for switching the resistors in and out.

[TopLoser]

I've got a couple of dozen 300W 1R metal clad resistors if they are any use?

These:
http://uk.farnell.com/welwyn/wh300-1r0ji/resistor-300w-5-1r/dp/1768251

Plus a truck load of very nice heatsinks 

[fcb]

If your brushed motors have field windings accessible you could implement something akin to a ward-leonard set to produce a variable load.

Just the two terminals on the motor.

- How much does it have to vary?
 - Do you have to lug it down still the motor physically stops?
 - Do you have to automate it?
 - Is this a one time deal or will you be designing this and handing it off to production?
 - What is your actual budget?

- The motor/drive under development is being used to drive a propeller, so the load torque will increase roughly with the square of the speed.
- No
- No, but a repeatable hand-control would be preferential.
- One time deal, well over a couple of months.
- Ha, in the hundreds not thousands for the load.

Look up Froude dynamometers, they made and still do I believe hydraulic dynamometers. Just looked them up they are now Froude Hofmann.

http://www.froudehofmann.com/index.php/products/engine-dynamometers/engine-based/6-high-speed-dynamometers

Basicly they are a torque converter with a load cell to measure the torque.
Cant be very hard to build a small version.

Nice piece of kit, thanks for the link.

your sensorless motor drive should be able to work backwards and use the motor as the boost inductor, within limits.
braking torque is proportional to rpm, if you want more you have to burn watts in the motor to produce reverse torque.

so you can electrically load test the motor by using its own rotational inertia.. this is done in the industry for load testing really big motors.

in any case, my recommendation for a dump load is 8 resistors, sized to draw ,16,32,64,128,256,512,1024,2048 watts from your generator. you can control which resistors are turned on with a uproc or an old 8 bit ADC directly. use mosfets of course for switching the resistors in and out.

Thanks.  Food for thought, I hadn't thought about using it's self to act as a brake. Will look-up how this is done!

I've got a couple of dozen 300W 1R metal clad resistors if they are any use?

These:
http://uk.farnell.com/welwyn/wh300-1r0ji/resistor-300w-5-1r/dp/1768251

Plus a truck load of very nice heatsinks 

PM sent.

[NiHaoMike]

What about just use the propeller spinning in open air? You could use a smaller propeller to simulate thinner air.

[fcb]

What about just use the propeller spinning in open air? You could use a smaller propeller to simulate thinner air.

Not really feasable as the 4KW is at 900RPM - it would have to be a huge propeller for air.  We debated putting the thruster in a tank, but 4KW moves ALOT of water.

I've been looking at these earlier tonight http://www.ebay.co.uk/itm/251254071726, anyone have experience with the exotically named "Kanthal Nikrothal 802"?

[nictinkers]

I'd go down to the local import kitchen goods place and look for toasters / waffle makers / hairdryers etc. You can likely pick up something with a 1-2kW heating element in it for under $10. Spend $200 and you'll have enough to dissipate the load from the motor. You may be able to find something with a switch you can also use to switch in and out heating coils.

I'm sure there's a better source of cheap heaters in the UK, but this is £10 at Tescos:
http://www.tesco.com/direct/texet-fan-heater-hh-488n/546-8625.prd?pageLevel=&skuId=546-8625

Edit: though obviously in some cases you'd probably need an externally powered fan to blow air over heating elements.

[Harvs]

I believe an old-school way of loading generators was a 44gal drum of salt water and pair of metal plates (however I've never done it).  Move the plates closer to together for higher load.  Obviously done outside to let the hydrogen dissipate to free air.

[NiHaoMike]

Not really feasable as the 4KW is at 900RPM - it would have to be a huge propeller for air.  We debated putting the thruster in a tank, but 4KW moves ALOT of water.

If there's enough room, maybe build a large fountain? (It would sure look cool!) Or maybe run an air compressor and adjust the load by changing the outlet pressure?

[Psi]

Cheapest solution is some sort of thick multi-tap resistor/inductor suspended in a large drum/tank of water.

Eventually the water will boil but 4kW isn't that much. A small electric kitchen jug is 2kW and takes a minute or so to boil that small amount of water. A large drum will take ages to boil.

Edit: Actually a hot water cylinder is more than 4kW and takes all day to warm up from cold

If you do have heat problems you can grab a car radiator and water pump from an auto wrecker to recycle the water and cool it back down.

Ideally you want the resistor wire wrapped around some sort of core so it has inductance that matches the motor.
That way the back EMF is simulated as well.

[Paul Moir]

I'm with Psi.  Easy job.  Use hot water tank heaters in parallel, or just buy a 4kw one.  With water you'll get lots of time.

However, if you want to run the motors, google "test props".  Outboard motors are often tested in fairly small tanks with these, and they're optimized for churning (heating) water rather than flinging it.  And an outboard's gear ratio is typically ~ 2:1 so you'll only need one 3 times larger to run at 1000RPM.  4kw is only 5hp so you're not talking about a big one here; 15-25hp sort of thing.  Cheap alternative is to get a propeller and cut off the tips thus rendering them terribly inefficient at moving water.

Need crude variation?  Increase the water depth over the test prop so it ventilates less and less.
 

[fcb]

Thanks for the suggestions.  I like the idea of a fountain.  I did think about running a smaller prop in treacle to.

The problem with heater elements, is that they are designed for 240V, so a 2KW element would actually (@68V) dissipate 160W.  I did look at 110V heaters, but alot more expensive in the UK.

[eurofox]

Available at low cost in UK

http://www.ebay.co.uk/itm/elite-110v-32-amp-fan-heater-and-transformer-/131038102410

and this one in parallel

http://www.ebay.co.uk/itm/HPL-SFH3-portable-fan-heater-3kw-110v-workshop-garage-unit-workspace-Vat-incl-/261317644482


eurofox

[poorchava]

I'd go with water heating system too. To dissipate that much power in electronic device, it would have to be huge and expensive as hell. Bigass TO-247 transistors have like 110W power dissipation rating assuming ideal infinite heatsink. And you'd need like 40 of them in parallel, balanced very well and bolted to a huge heatsink (or more likely water cooler).

[Rigby]

If you put a non-ferrous metal disc on the output shaft of the thing actually creating the rotation, you can put some electromagnets near the rotating disc and create a Lenz-effect brake.  The electromagnets will create a magnetic field in the metal disk, which (because it is moving) will turn that field into an electromagnet that will be of equal strength and opposite polarity of your electromagnet, and will resist rotation.  If you increase the current to your electromagnet, the induced magnetic field will be stronger, which will load your motor down more.  The more current you apply, the more load you create.  You can never stop the load completely, but you can get it going so slow that movement is imperceptible.  Then your drive motor kerplodes.

This technique is used in production dynamometers and uses very few moving parts and is highly repeatable.  Some F1 racecars also use this system to create brakes that physically cannot lock up and do not generate heat due to friction.  They do generate heat, though.

A homebrew version won't be calibrated, but you can get a feel for what it takes to load the motor down very quickly if you can create/source the proper electromagnets.

[ConKbot]

If you put a non-ferrous metal disc on the output shaft of the thing actually creating the rotation, you can put some electromagnets near the rotating disc and create a Lenz-effect brake.  The electromagnets will create a magnetic field in the metal disk, which (because it is moving) will turn that field into an electromagnet that will be of equal strength and opposite polarity of your electromagnet, and will resist rotation.  If you increase the current to your electromagnet, the induced magnetic field will be stronger, which will load your motor down more.  The more current you apply, the more load you create.  You can never stop the load completely, but you can get it going so slow that movement is imperceptible.  Then your drive motor kerplodes.

This technique is used in production dynamometers and uses very few moving parts and is highly repeatable.  Some F1 racecars also use this system to create brakes that physically cannot lock up and do not generate heat due to friction.  They do generate heat, though.

A homebrew version won't be calibrated, but you can get a feel for what it takes to load the motor down very quickly if you can create/source the proper electromagnets.

The net heat is the same, its just that the heat is generated in the conducting (copper, aluminum) disc via ohmic losses rather than friction.  If could work for a shaft drive in the water though, propeller on the shaft moving water over the disc to keep it cool, and a jack screw to move the magnets further/closer, and it would vary with speed similar to how a blower/fan/pump would.

[mazurov]

For brushless motor controller you need to generate back EMF somehow, otherwise your controller won't commutate. The easiest way to do it is to load a motor with another motor, like this -> https://www.circuitsathome.com/measurements/dummy-load-for-brushless-motor-controller  Mine was made to stress test the controller, you may want to add resistors between windings to decrease the load.

[calexanian]

How about just a simple electric trailer break. They are easy to modulate the braking force via a cheap 12volt brake controller, easy to mount, and run at the proper RPM. They will handle 4KW for quite a length of time before getting too hot.

[Rigby]

The net heat is the same, its just that the heat is generated in the conducting (copper, aluminum) disc via ohmic losses rather than friction.  If could work for a shaft drive in the water though, propeller on the shaft moving water over the disc to keep it cool, and a jack screw to move the magnets further/closer, and it would vary with speed similar to how a blower/fan/pump would.

With electromagnets, you don't vary the gap between the electromagnets and the metal plate, you simply increase the current through the magnets to increase physical resistance in the rotating disk.  You want the gap as small as possible to minimize loss.  Permanent magnets, though, would vary their distance to the disk.

One would need sufficient cooling, yes.  commercially available eddy current dynamometers are usually encased in oil which is cooled via a heat exchanger to a cold water supply.

[JackOfVA]

In the radio transmitter world, a 50 KW dummy load is normally water cooled. The actual resistive element is rather small.  Often the water is operated "open loop" with a flow meter to monitor flow (and a safety switch to the transmitter enable circuit so that if water flow is below the minimum permitted level, the transmitter cannot be keyed or if operating, it will drop off) and discharged into the normal sewer system after one pass through the dummy load.  The load I'm most familiar with was permanently plumbed into the transmitter building drinking water system with a solenoid valve to turn water on/off.

I've also seen more elaborate versions with circulating oil and a fan cooled radiator - more parts, more sensors, but self-contained and can be operated without access to water and sewer.

Don't know if I've seen a detailed specification on how much power the 50KW load could dissipate without water flow, but from the physical size of it, I would guess that the water flow increased the permitted dissipation by 25:1 or more.

[cosmos]

Assuming your new motor is also about 68V DC,  what stops you from using the old motor as a generator and DCDC convert its output back into the new motors supply? (apart from cost and time to make it )
This way you can circulate the power and lots of heat problems should go away.
Regulating the load should be possible but I have no known working solution to that, maybe simply increase the ouput voltage slightly over the normal power source until you get the wanted load?

In industry they do this, sometimes feeding the load back into the mains (not recomended unless you know exactly what you do)
Watched a looped setup like this (power converters not motors but same principle), it was drawing about 1kW from the a 400V DC power but the circulating power inside the setup was somewhere around 15kW (nice efficiency too, 1kW loss for two 15kW DCDCs in series => 97% or so in each).

[mtdoc]

One inexpensive option might be using diversion dummy loads built for wind turbines using banks of large power resistors -->LINK

[G7PSK]

You could use a car or a bus/truck alternator mount it so that it swivels on two mountings and use spring balance on the belt tension mount. short the output and have a separately controlled field, then take readings from the spring balance this will give you your torque, basicaly you will be making an electro dynamometer.

[fcb]

There are some great comments here.

I like the idea of the non-ferrous disc - although I guess it would get hot and need to be quite large (2ft?).

Load-dump resistors, I also didn't think of that huge resistors where available in Farnell/RS, should have looked! - Thanks for the links.

The 110V/32A heaters look great, although would need at least 3 of them.

[Paul Moir]

How about just a simple electric trailer break. They are easy to modulate the braking force via a cheap 12volt brake controller, easy to mount, and run at the proper RPM. They will handle 4KW for quite a length of time before getting too hot.

That's a much better idea than mine for dynoing the motors.  Far less messy and fiddly.

[poorchava]

I guess you could try to find a retarder from a small truck on a junkyard. If you manage to get it up and running you could have a really nice  variable mechanical load.

[amyk]

The specific heat of water is almost exactly so that 4kW will raise the temperature of 1L of it by ~1C/s, or 10L by ~0.1C/s.

(Maybe you could also tap off some of the power to drive a refrigerator to cool off the water it heats...)

[mjrandle]

So long as you are operating over a well defined speed range, I would use DC motor to drive a hydraulic pump and adjust load on motor by adjusting the relief valve.  The hydraulic pump should be piston type to maximise volumetric efficiency, an oil cooler would definitely be required an the relief valve could even by controlled electrically with a pressure transducer scaled to read torque.  The only problem is that hydraulics are bloody expensive.


Regards,

Mike

[NiHaoMike]

So long as you are operating over a well defined speed range, I would use DC motor to drive a hydraulic pump and adjust load on motor by adjusting the relief valve.  The hydraulic pump should be piston type to maximise volumetric efficiency, an oil cooler would definitely be required an the relief valve could even by controlled electrically with a pressure transducer scaled to read torque.  The only problem is that hydraulics are bloody expensive.

The air compressor idea might actually work really well. You could even pipe the hot air outdoors.

[calexanian]

I am liking my trailer brake idea more and more. You can connect it to the load via a pulley in place of the propeller and run a belt around a trailer wheel without a tire. this is like a $250 solution. A little welding of a small steel frame to hold it all and you are in business loading down the actual product, not just a electrically simulated load.

[C]

I've got to develop a sensorless drive for some 4KW@1000rpm brushless motors,

- The motor/drive under development is being used to drive a propeller, so the load torque will increase roughly with the square of the speed.

When you say propeller, I first think of attached to boat. When you command the motor to slow or reverse, my thinking is that that  propeller will turn that motor in to a generator.
You also say MOTORS.

With proper commands an H-bridge can also break the motor resulting in the H-bridge supply voltage increasing.

Just build two motor drives and connect their H-bridge supply voltages together. With the two motors shafts connected, while one motor is acting as a motor, the connected motor could be breaking (acting as a generator). The motor is your load to the generator. As the electronics are not 100% efficient, you would still need some additional current to the  H-bridge supply. So no big load resistors would be needed.

C
 

[RobB]

I believe an old-school way of loading generators was a 44gal drum of salt water and pair of metal plates (however I've never done it).  Move the plates closer to together for higher load.  Obviously done outside to let the hydrogen dissipate to free air.

Almost 100% correct. The only thing is one the plates needs to be submerged by different depths to vary the area in contact with the saline water.

If there is any doubt about the power capacity of this sort of thing many years ago I worked at a mine which had an old DC drive winder (a two drum winch) which was used to raise ore as well as men. The salt bath resistor was used to vary the armature current to allow speed control. The motor would have been at least 300kW.
This is almost certainly the simplest and cheapest solution.

[johansen]

With proper commands an H-bridge can also break the motor resulting in the H-bridge supply voltage increasing.

Just build two motor drives and connect their H-bridge supply voltages together. With the two motors shafts connected, while one motor is acting as a motor, the connected motor could be breaking (acting as a generator). The motor is your load to the generator. As the electronics are not 100% efficient, you would still need some additional current to the  H-bridge supply. So no big load resistors would be needed.

C

yes but that is easier said than done.

if you know of any open source three phase synchronous motor controllers that cost less than 200 times the parts involved (not including the switching elements...(igbt/mosfets, drivers, and the capacitor, heatsink, bus structure)) then the opensource electric vehicle community would be all over you.. i'm talking about the park transformation, not the common trapezoidal back emf "sensorless" commutation.

[SeanB]

The salt tank with electrodes works well, and handles an impressive power. We used them to test 115VAC 400Hz alternators and drove 100A through them. For the 28V side there was an old bed with a slot cut in the short sides and a bakelite block providing the insulation with a set of heavy coil springs glowing bright cherry at our test current of 8kA.

[fcb]

I've got to develop a sensorless drive for some 4KW@1000rpm brushless motors,

- The motor/drive under development is being used to drive a propeller, so the load torque will increase roughly with the square of the speed.

When you say propeller, I first think of attached to boat. When you command the motor to slow or reverse, my thinking is that that  propeller will turn that motor in to a generator.
You also say MOTORS.

With proper commands an H-bridge can also break the motor resulting in the H-bridge supply voltage increasing.

Just build two motor drives and connect their H-bridge supply voltages together. With the two motors shafts connected, while one motor is acting as a motor, the connected motor could be breaking (acting as a generator). The motor is your load to the generator. As the electronics are not 100% efficient, you would still need some additional current to the  H-bridge supply. So no big load resistors would be needed.

C

So, we have developed this "thruster", it consists of a large DC brushless motor (4KW@900RPM), for various reasons we cannot use hall-commutation, so we have to develop a sensorless amplifier for this motor. I reckon I'll need to be able to mechanically load the motor during the development of the drive.

Before I became involved in the project, there was a previous similar brushed DC thruster developed (4KW@900RPM) that ran on 68V. They tested this by driving a hydraulic pump (pumped oil through a small orifice into a tank), this is still an option (although the rig will need to be rebuilt), I also have one the DC brushed motors available.

My thought was simply to add a huge (<1ohm) resistor (via a big MOSFET, PWM controlled) to the output of the 68VDC motor.

[C]

Special test equipment is just that, special. Due to the limited use, I have often seen where someone spent hours testing/ trouble shooting a problem in a piece of equipment only to find out that that seldom used special test equipment was at fault. Your previous hydraulic test set is an example of what happens. Note that the old hydraulic test set would not test the "thruster" control where the "thruster" is acting like an alternator ( the boat moving and turning the prop).

A DC brushless motor is like a 3 phase alternator with a  fixed magnetic field winding when rotated.

With some limits your existing control could act like a electronic load if you replaced the motor with load resistors. Thinking of a cheap, easy and quick resistor, get 6 steal rods and some water pipe to wall mounting brackets. Two parallel rods with a shorting bar clamp is easy adjustable low ohm resistor for high power. Granted that the resistance will change some with temp but longer rods will limit this change. Your existing control may need some inductance to function with just resistance so you may need to add some wire wrapped around a steal pipe or rod for some inductance.

For quick load control software you might think of a modified 3 phase digital PFC. This would give you the 3 phase PWM.

Some power sources do not like the quick high currents and voltages that a Regenerative motor can produce. To counter the supply voltage going too high, some add a half bridge to dump the extra power in a over voltage - breaking load resistor.

I do not see the need for the fine position control in a thruster. Simple 3 phase AC will spin the motor with freq controlling the speed. The fancy software control just adds options or saves power. I would think that the existing control when faced with a loss of hall-commutation would revert to simple 3 phase AC drive. 
 
Think of a war where you have two thrusters motors with shafts connected. One half helps you test the other half and in the process makes both better. When you need to test something new, the other half is just one generation back. Field test and repair could also be a lot easer. Note that this also lets you save power while testing. Most of the time you only have to supply power to make up for the losses.
This lets you use the existing control's power circuits to help in the creation the new control while also allowing better testing.

Note that some MPU's now have a motor control capability built in making your task easer and cheaper.
http://www.st.com/web/en/resource/technical/document/application_note/CD00266366.pdf] [url]http://www.st.com/web/en/resource/technical/document/application_note/CD00266366.pdf [/url]

STM32 motor control firmware li
http://www.st.com/st-web-ui/static/active/en/resource/sales_and_marketing/presentation/product_presentation/stm32_motor_control_firmware_library_marketing_pres.pdf] [url]http://www.st.com/st-web-ui/static/active/en/resource/sales_and_marketing/presentation/product_presentation/stm32_motor_control_firmware_library_marketing_pres.pdf[/url]

an stm32F3 might be a good choice. A little extra MPU power could allow you to add some options that your current thruster does not have cheaply. The new control could also be a replacement for the control of the old motors.

TI, NXP and others also have MPU's & software like this.

Just my thoughts
C

[fcb]

Hi C,  I think I must have explained it badly or something.

What we want is:
Powersource(260VDC)>Amplifier(the bit i'm developing)>DC brushless motor>Mechanical coupling>Propeller

What I'm proposing to use as a test rig:
Powersource(260VDC)>Amplifier(the bit i'm developing)>DC brushless motor>Mechanical coupling>DC Brushed motor(68V)>Resistor(FET controlled).

Regen is not a problem in our application.  Torque (hence current) tends to go up by the square of the speed.  I'm pretty familiar with PWM/DSP, and small motor drives, it's the sensorless control I'm "looking forward" to...

[C]

First something to think about, take two DC brushless motors and connect the shafts. Connect both to the same 3 phase AC. Think of what will happen as you rotate the case of one motor with respect to the other motor with power applied. At some rotation point both will be acting as motors just over coming losses. As you rotate from this spot, one will change to a producing electric energy increasing the load on the other motor. 

What I'm proposing to use as a test rig:
Powersource(260VDC)>Amplifier(the bit i'm developing)>DC brushless motor>Mechanical coupling>DC Brushed motor(68V)>Resistor(FET controlled).

I see little to no load possible at low rpm(s), The DC Brushed motor would need power to create a load at low rpm(s).

Powersource(260VDC)>Amplifier(the bit i'm developing)>DC brushless motor>Mechanical coupling>DC brushless motor>Amplifier(the bit i'm developing)>Powersource(260VDC).
would be a better test setup.

Just suggesting you use a copy of what you are creating to handle the testing load. Assuming you are building three half bridges to control the DC brushless motor, if you must use the DC Brushed motor(68V) for the second motor just use two of the three half bridges to control it.

C