4Kw dummy load

[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