Required alternator torque

[Forcetrek]

As a mechanical engineer I assume that the torque required to rotate an alternator would be simular to the torque produced by a motor, only that it works in reverse.

It's never a good idea to assume, that's usually where the problems begin! That's why I have to ask this question.

I know that any alternator when started does not produce any power, therefore the starting torque to rotate the alternator would be negligable. The torque required would be needed when the alternator starts producing the power at a certain speed. Can anyone confirm my thoughts and show me the formula(s) used to calculate the torque required so that I can finally put my mind at rest.

Thanks in advance

 

[LukeSkywalker]

Hey

I don't know the exact formula for that, but I do know that some brand of alternator do start to produce current at idle.

If you're not in a hurry I might have some info at home.

Luke

 

[GusD]

Hi Forcetrek

The Alternator would require some power to spin it even without a load depending on the size of the rotating mass. The bigger the mass the more power he would need,which goes without saying.When at full load the rule of thumb is "The Motive Power should be 20 to 30% larger then the Driven Load"
An example:
We have an 80 MW Gas Turbine Generator that to start requires us to use an 800 HP motor just to spin it to approx.30% speed ,at which time the Turbine kcks in.
This 800 hp motor runs for about 20 Min totalin 10 Min intervals,the motor load is about 150% ,reason for the cool down during 20 min start up.
GusD

 

[DougMSOE]

The typical approach is;
Power(watts)= Torque(Newton*meters)*w(omega radians/sec) + windage and friction(losses)

Yes, any alternator can produce current at low revs, depending on the magnetic field of the rotor. Further, an alternator with proper field circuit control will have a regulated/controlled voltage output.

 

[Forcetrek]

Thanks DougMSOE,

You basically confirmed my thoughts all except the windage loss. I never considered it Maybe because I'm not familiar with it, unless you mean the windings moment of inertia!?