DIssapating 50HP (~38kW) of DC 2

[p27182]

Hey experts,

I want to ask for some advice.
(skip to the bottom to jump to the point)

I'm currently tasked with finding a load bank for a dynamometer whose mechanical load is a 50hp DC generator. (The dyno is to be used testing big VFDs and whatnot).

On a dyno we've already got hooked up and running, we have been adjusting the field voltage on the generator while it's being driven by a motor hooked up to a VFD in order to provide a variable mechanical load. The output of the generator is currently hooked up to a big air cooled resistor bank to dissapate the heat.

Since we don't have another big bank like this, I've been asked to find out how to acquire another one, build one, smaller if possible (water cooled?).

So long story short:
What would you guys reccomend for dissapating about 38kW of DC power for a lengthy amount of time (hours)?
and where would I look for resistor banks capable of dissapating this much juice? I'm not getting nearly as many reasonable google results as I expected.

thanks for any info/suggestions!

-pat

 

[waross]

What Gunnar says. But the terminal voltage of a DC machine is almost proportional to the speed. You don't have any problems braking with a resistor but the number of heaters may have to be increased for full current at lower speeds. Regeneration becomes more difficult as the voltage drops.

These issues depend on the use of the dyno. It will be pretty easy to test line powered induction motors, but it may be challenging testing VFD driven motors at slower speeds.

For examples sake, assume that you are testing a motor and at 100% speed the generator is producing 100 Amps at 100 Volts. You are dissipating this in a load bank with 1 Ohm resistance. Your electrical load is 10 kW.
Now at 50% speed you want to maintain 100 Amps but your voltage is now about 50 Volts. You must now drop the resistance to 0.5 Ohms to draw 100 Amps at 50 Volts.
At 10% speed you may need 10 times the number of heating elements to draw 100 Amps.

One good alternative may be the liquid rheostat. The resistance may be varied by moving the electrodes closer together. One type of liquid rheostat used to start large wound rotor motors uses two circular plates in a vertical plastic (non-conducting) tank. Initially the electrodes may be about two feet apart but as the motor is accelerating the upper electrode is being dropped until it is just a few inches from the lower electrode. The current carrying capacity is mostly determined by the area of the plates and the resistance is easily varied.

Another option may be an induction motor and a regenerating VFD.

The greater the speed range that you want to test over the more challenging your application becomes.

Gunnar pretty well answered your last question, but may I add that at higher speeds power may be returned to the supply but as the voltage drops there comes a point where the returned energy must be wasted in resistors, internal or external.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

As to the construction of a liquid rheostat;
For temporary use such as one time load testing a generator for a week, I have used plastic barrels with discarded leaf springs from a truck for electrodes.
For a permanent installation you will probably want to use stainless steel and consider a circulating pump.
Thanks to Gunnar and itsmoked for the liquid rheostat suggestions.

Bill
--------------------
"Why not the best?"
Jimmy Carter