Ok, but WHY does the asynchronous AC induction motor have an unstable zone?

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Discussion Overview

The discussion revolves around the stability of asynchronous AC induction motors, particularly focusing on the conditions that lead to instability and stalling when overloaded. Participants explore the mechanics behind torque behavior at different rotor speeds and the implications of motor design on stability.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that an asynchronous induction motor can stall if overloaded, causing the rotor speed to drop below the maximum torque point, leading to instability.
  • Another participant suggests that the motor becomes unstable when the decrease in torque at lower speeds outpaces the load's torque decrease, indicating a relationship between load characteristics and motor behavior.
  • A claim is made that squirrel cage motors can be designed to avoid stalling when unloaded, contrasting with traditional designs that may not have this capability.
  • One participant expresses confusion about why a motor can stall despite the stator's magnetic field still being active, questioning whether rotor over-magnetization is a factor.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the conditions that lead to motor stalling, with some agreeing on the mechanics of torque behavior while others raise questions about specific scenarios and motor designs. The discussion remains unresolved regarding the exact reasons for stalling and the implications of different motor types.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about motor design and load characteristics, as well as the lack of detailed exploration into the mathematical relationships governing torque and speed.

EVriderDK
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I have been tought several times that if an asynchronous induction motor is overloaded, so that the rotor speed goes below where the maximum torque point is, the motor will become unstable, and simply stall, even if you unload it.

Why does it do that?

fx. here: http://youtu.be/ze8LY4yq9Wk?t=38m20s

Thanks in advance :)
 
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Do a search on this forum about induction motors. There have been a number of threads that discuss the operation in depth. I think it would help you.
 
I cannot find anything about this, sorry.
 
It is unstable if the motor's torque decreases with the lower rotation speed faster than the load's torque decreases.

A load with constant torque can let the motor stall, an fan for instance never does.

Squirrel cage motors born after you and I won't stay stalled if unloaded. Better, they can be built so that their torque doesn't decrease at low speed.

These days, squirrel cage motors tend to have a driving electronics which changes everything. Bare squirrel cages still exist for fans. And since driving electronics are common, squirrel cage motors replace most other motors.
 
Yes, i get that, but my teacher said, that if it stall, there is nothing else to do, than turning it off and on again, also if it gets unloaded. I cannot understand how it can stall with the stators magnetic field still fluxing around?!

Is it because the rotor gets over magnetized or something like that?
 
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