Behavior of Synchronous and Induction Machines Question

[DarkLegion PE]

Hey all, 

I'm trying to conceptually understand Synchronous and Induction machine behavior a bit more. Is my current thinking correct?

If you're adding a bigger load to a motor;

• An induction motor's speed and frequency is lowered due to need for a larger torque via the torque relationships
  
• A synchronous machine will maintain constant speed and frequency but the torque angle will increase and will call for the need of more current drawn. 
  

 

[akyip]

Here is how I understand it.

For induction motors, I agree with you. Induction motors need slip (difference in speed between synch speed ns and rotor speed n) to generate torque for the load. The more a rotor's speed n decreases away from synch speed ns, the more slip and thus the more torque produced. Real power P is directly proportional to torque.

Tm = 9.55 * Pr / ns = 9.55 * Pout / n (neglecting windage loss Pv)

Tm = 5252 * HP / rpm

For synchronous motors, I also agree with you. All synchronous machines (synch gens and synch motors) always run at synchronous speed ns, and the only way to change ns is by changing frequency f (a given synch machine's number of poles p is fixed/constant). The equation for real power for a synchronous machine (neglecting resistance) is:

P = (Ea * Vt / Xs) * sin(theta)

Usually theta = Angle of Ea - Angle of Vt. For a synch motor, Ea lags Vt, so theta would be negative... But I think you're just concerned with the magnitude of P. I guess for synch motor, maybe you can just use angle of Vt - angle of Ea for theta?

I think the terminal voltage Vt and synchronous reactance Xs are usually fixed/constant for a synchronous machine connected to an electrical system. So the only ways left to increase P is by increasing Ea (increasing the excitation) or by increasing theta (mainly by changing the angle of Ea, since I think the angle of Vt is usually fixed/constant).

 

[Byk]

I was actually curious about effect of load on synchronous machine couple weeks ago.

One thing that I remember is that armature current increases as load increases, current will become less leading  and more lagging as the load increases.