Hi
i have seen a few motors that has (impedance protected) written on it. What does that mean ?

 

It means that even if the rotor is stalled, the motor current will not rise enough to overheat the windings.

This is done by having a poor magnetic coupling with the rotor. Actually a high mutual inductance. An induction motor is actually a kind of transformer with the secondary on the rotor (usually a "squirrel cage" - a shorted winding).

Only done on small, low efficiency motors (fans, etc.).

 

If the motor because any reason caused it didn't running, and the power supply still continuing then the motor will increasing the impedance to reducing the current and protecting the motor.
What is impedance protection?

 

It means that even if the rotor is stalled, the motor current will not rise enough to overheat the windings.

This is done by having a poor magnetic coupling with the rotor. Actually a high mutual inductance. An induction motor is actually a kind of transformer with the secondary on the rotor (usually a "squirrel cage" - a shorted winding).

Only done on small, low efficiency motors (fans, etc.).

Wouldn't a thermistor connected series with the winding do some thing similar ?

 

and add heat. ( the thermistor would reduce resistance when hot.)
there are impedance protected transformers too, most wall warts are impedance protected instead of fused.