Speed control of Induction motor

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Discussion Overview

The discussion revolves around the speed control of induction motors, specifically focusing on the reasons for employing E/f or V/f control methods over variable frequency control. Participants express varying levels of understanding and seek clarification on the topic.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why E/f or V/f control is preferred over variable frequency control, indicating a lack of clarity on the topic despite having read several books.
  • Another participant references a previous thread that contained a significant amount of disagreement regarding the topic, suggesting that the issue is complex and not universally agreed upon.
  • A third participant provides a link to the mentioned thread, implying that it may contain relevant discussions or insights.
  • A participant explains that at frequencies below 50 Hz, constant torque output is achievable, while above this frequency, torque decreases with speed reduction. They emphasize the importance of maintaining a constant V/f ratio to keep the air-gap flux density constant for consistent torque output.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the preferred method of speed control for induction motors, with multiple competing views and some references to previous disagreements on the topic.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about torque output and the conditions under which V/f control is effective. The relationship between frequency, voltage, and torque is presented, but the discussion does not resolve the complexities involved.

rohit63
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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.
 
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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.
 
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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