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.