AC motor control

jomo99 · 12-07-2010 05:22PM #1

I have an idea, but this is not my field of expertise. I have a wanting for an engine dyno for many years, but the cost for a small business like mine is too much.

If I have an engine driving an AC (or DC) motor, how do I control the speed of the engine, via the motor. Power output of the engine would be in the 450kw/500nm area. Any ideas

John

steifanc · 12-07-2010 05:46PM

eh ? what are u trying to do , control the engine speed in relation to the moter speed ?

DublinDilbert · 12-07-2010 07:13PM

It's not really clear what you want to do, I'm guessing you want to load the engine, using the electrical motor as a generator.

There's a few different ways it can be done, typically you would put a large electrical load onto the generator, which would in turn load the engine. The load on the generator will give off lots of heat, you could use heating elements for this...

jomo99 · 13-07-2010 08:44AM

Basically an engine dyno. I want to control engine speed by loading an electrical motor. I need to be able to load and engine produceing approx 450kw of power and 500nm of torque, and up to 10K rpm.

John

steifanc · 13-07-2010 01:01PM

you could try monitoring either voltage or current of the generator , and feed that reading back to the Governor or the engine

DublinDilbert · 13-07-2010 05:02PM

jomo99 wrote: »

450kw of power

450Kw is a lot of power to dissipate, serous amount of heat there. Say the generator gives off 220V

I = P/V = 450,000/220 = 2045A !

To put it another way you'll have 450 1 bar electric fires :eek:

jomo99 · 14-07-2010 03:51PM

That would be maximum power output of the engine, and I wouldn't spend a lot of time there.

Imagine an engine is driving the electric motor at 6000 rpm, and I use the Generator/Motor to slow the engine to 4500, by feeding power (3 phase 415 V ) to try and spin the motor in the opposite direction, and hold it at that speed.

John

DublinDilbert · 14-07-2010 07:09PM

No you don't need to use power from the mains to load the engine. If the motor is operating as a generator you connect its output to an electrical load (resistors).

A large separately excited dc motor from a fork lift would probably do a good job at this. You would need a number of banks of resistors that can be switched in/out to vary the load.

jomo99 · 14-07-2010 11:20PM

I need the load to be variable, and controlled by some kind of feedback loop. As I change fuel and spark parameters, the engine will increase or decrease on power, but I need the engine speed to be constant.

John

DublinDilbert · 15-07-2010 08:28AM

jomo99 wrote: »

I need the load to be variable, and controlled by some kind of feedback loop. As I change fuel and spark parameters, the engine will increase or decrease on power, but I need the engine speed to be constant.

John

Like I said you can use banks of resistors, that are turned on / off using contactors. 8 Banks of resistors would probably do it, if they are sized correctly (eg each one is double the next) that would give you 256 different levels of loading. A PLC or a PC with a speed input form a pulse sensor input would be able to control the 8 outputs to the contactors. You could use a NI6008 input device and write the code in lab-view. It wouldn't be the quickest way of varying the load but would probably do in this case.

Or if your using a sep-ex DC motor, as a DC generator you could put a fixed load on the "armature" winding, then vary the "field" current using a chopper. This will give a variable load which can be changed quickly. You'll only need to be able to control about 0 -> 5A in the field to get full power out of the armature.

AC motor control

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