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

[EVriderDK]

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 :)

 

[Averagesupernova]

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.

 

[EVriderDK]

I cannot find anything about this, sorry.

 

[Enthalpy]

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.

 

[EVriderDK]

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?