How Does Reducing Excitation Affect a Synchronous Induction Motor's Performance?

[Laa]

Homework Statement

(i) A star-connected, three-phase synchronous induction motor takes a current of 10 amps from a 415 volt supply at unity power factor when supplying a steady load. If the synchronous reactance is 5 ohms/phase and the resistance is negligible, draw the phasor diagram and calculate the value of the generated emf and the load angle.

(ii) If the excitation is reduced by 10% and, as a consequence the generated e.m.f. is reduced by 10%, calculate the new value of the generated e.m.f E, the stator current, the power factor and the load angle, assuming that the load does not change[/B]

Homework Equations

The Attempt at a Solution

For part (i) I have the emf calculated as 245V and the load angle as 11.6 degrees

I am struggling with part (i). I have the new emf at 220.5V but don't know where to go from there.

If I do the same equation for load angle as in part one it doesn't work.

Any help appreciated.

 

[cnh1995]

Hi Laa, welcome to PF!

I am struggling with part (i). I have the new emf at 220.5V but don't know where to go from there.

Since the load torque doesn't change and it's a 'synchronous' motor, what does that tell you about the active power drawn by the motor?

 

[Laa]

Hi Laa, welcome to PF!Since the load torque doesn't change and it's a 'synchronous' motor, what does that tell you about the active power drawn by the motor?

Hi, thanks for the quick reply.

Not quite sure what you are getting at there but as the excitation has been reduced, power factor will be leading and the load angle will increase. The value of the stator current should remain the same

 

[anorlunda]

synchronous induction motor

By the way, that is wrong. Synchronous motors and induction motors are two kinds of motors. There is no such thing as synchronous induction motor. The problem is for a synchronous motor.

Since the load torque doesn't change and it's a 'synchronous' motor, what does that tell you about the active power drawn by the motor?

Following up on that clue. How does the speed of a synchronous motor change with excitation change?

 

[cnh1995]

The value of the stator current should remain the same

Why?