Speed control of Induction motor

[rohit63]

In speed control of a induction motor, why E/f or V/f control is employed mostly rather than variable frequency control?
I've read few books on this topic but still I feel ambiguous.
Please can anyone come up with clear explanation.

 

[Averagesupernova]

If you are talking about changing the voltage as well as frequency there was a thread with a fair amount of disagreement on this. I don't remember the title of the thread but it involved speed control of induction motors. Do a search.

 

[Averagesupernova]

Here is the one I was thinking of:

https://www.physicsforums.com/showthread.php?t=140077

 

[m.s.j]

At frequencies below 50 Hz, a constant torque output from the motor is possible. At frequencies above the base frequency of 50 Hz, torque is reduced in proportion to the reduction in speed.
The output torque of the AC motor depends on the product of the air-gap flux density and the rotor current IR. So, to maintain constant motor output torque, the flux density must be kept constant which means that the ratio V/f must be kept constant, or:


V~E _induced = k.w. Φ. Cos wt,
V/f or V/w = Constant , Φ= constant
Output Torque T ∝ Φ.I R can be constant approximately.

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