Induction Motor - Rotor Slip

Can someone explain both conceptually and mathematically why the rotor in an induction motor will never catch up to the revolving stator field? I understand that the motor would not work if it did, but I can't figure out WHY it settles at a speed just under synchronous speed (at no load).

Thanks.

Well, think of Faraday's law

E=-dΨ/dt

-Thus, for voltage, to be induced in the rotor a changing magnetic flux has to link the rotor.
-Imagine that a force rotates the rotor at the same speed as the magnetic field (synchronous speed), then the magnetic flux linking the rotor will be constant. -Thus, no voltage will be induced in the rotor, hence no current will be produced.
-Therefore, the torque will be zero.
-If the force rotating the rotor at synchronous speed disappears, the rotor will start slowing down.
-Changing magnetic flux will link the rotor, voltage will be induced, current will flow and torque will be produced.
-As the rotor speed decreases more and more flux links it, thus more current (torque) is produced.
-The rotor will keep slowing down until the produced torque is sufficient (overcome frictions or whatever load is on the shaft) to keep it in motion.
-At this point, the rotor speed will stabilize and will be less than the synchronous speed by a factor determined by the slip.


I hope this is helpful.

Thanks, that does clear things up a bit. But is there an equation that describes where that equilibrium is reached? What I'm thinking is, what if the rotor were very close to synchronous speed and not yet at an equilibrium position. When the rotor slows down so that the voltage increases and so does the current, what is to prevent the torque from being so large that it pulls the rotor up to synchronous speed?

This is quite a comprehensive link with equations etc.

http://www.ece.vill.edu/~singh/ece8830/88303a.ppt

Looks like I've got some studying to do...thanks for the link.

Is it even possible for the rotor to catch up to the field in the induction motor? Does that ever happen in real life?

shespuzzling said:

Looks like I've got some studying to do...thanks for the link.

Is it even possible for the rotor to catch up to the field in the induction motor? Does that ever happen in real life?

Click to expand...

Only in the ideal case where there is no drag on the rotor.

You could simulate this in real life by driving the rotor with precisely enough torque to overcome the drag on it at synchronous RPM. Might make a decent physics lab experiment.

Edit: Or you could reduce the frequency of the stator field at precisely the same rate as the rotor would slow down at if the field was removed. There should be essentially no difference in the slowing rate. This would mean that the stator field was not contributing any energy at all to the rotor and therefore their fields must be exactly in phase.

Thanks for your help!