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

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Discussion Overview

The discussion revolves around the performance of a synchronous induction motor when excitation is reduced. Participants are addressing a homework problem that involves calculating the generated emf, load angle, and other parameters under varying excitation conditions, focusing on theoretical and mathematical aspects of synchronous motors.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant calculated the generated emf as 245V and the load angle as 11.6 degrees for the initial conditions.
  • Another participant noted that with a 10% reduction in excitation, the new emf is calculated to be 220.5V but expressed uncertainty about subsequent calculations.
  • Some participants suggest that since the load torque remains unchanged, the active power drawn by the motor should also remain constant.
  • There is a claim that the power factor will become leading due to reduced excitation, and the load angle is expected to increase.
  • One participant pointed out a conceptual error regarding the terminology of "synchronous induction motor," clarifying that synchronous motors and induction motors are distinct types.
  • Questions were raised about how the speed of a synchronous motor is affected by changes in excitation.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the implications of unchanged load torque on active power but disagree on the terminology and the effects of reduced excitation on current and load angle. The discussion remains unresolved regarding the precise impact on the stator current and the load angle.

Contextual Notes

There are unresolved assumptions regarding the relationship between excitation, load angle, and power factor, as well as the definitions used for different types of motors. The calculations presented may depend on specific definitions and conditions not fully detailed in the discussion.

Laa
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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.
 
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Hi Laa, welcome to PF!

Laa said:
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?
 
cnh1995 said:
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
 
Laa said:
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.
cnh1995 said:
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?
 
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Laa said:
The value of the stator current should remain the same
Why?
 

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