If the rotor rotates at near the speed of the stator field, the induced magnetic poles stay near in line with the stator poles.
When the slip becomes too great, the poles are too far out of line to maintain the maximum induced field.
The poles and circulating current then have to re-align at every half cycle so the inductance of the rotor means the maximum field is never achieved.
Remember that an AC electromagnet is used to demagnetise or degauss objects.
With too great an angular change, the rotor field has to be rebuilt every half cycle rather than being boosted at only a fractionally different angle every half cycle.
That's why the starting torque is low and the starting current high.
Out of curiosity, the angular changes...
Synchronous speed 1500 RPM, Nominal speed 1440:
That's 60 rpm difference, 1 rev per second.
With three phase 50 Hz, there are six AC peaks (and so pole shifts) per cycle; 300 per second.
That puts the peak torque rotational slip or "drift" between the rotor and stator at around 1.2 degrees per half cycle.
Thanks a lot. It was really helpful and I was able to picture it better. I have repeated your words below using a single phase motor.
Suppose a single phase capacitor start induction motor
Suppose synchronous speed or rotating magnetic field speed is 3600 RPM, nominal speed or rotor speed 3540 RPM:
that's 60 RPM difference, 1 rev per second. In other words, rotor falls back 1 rev or 360 degrees per second in relation to the rotating magnetic field speed.
With a single phase 60 Hz, there are two AC peaks (and so pole shifts) per cycle; 120 peaks per second. Or, we can say that there are 120 half cycles per second.
{360 degrees per second} / {120 half cycles per second} = 3 degrees per half cycle
That puts the rotational slip or drift between the rotor and stator at around 3 degrees per half cycle.
Just to confirm:
3540/3600 = 0.98333333
0.98333333*360=354
full cycle: 360-354=6
half cycle: 6/2 = 3 degrees
120*3=360 degrees
The only way that could happen is if there were a residual permanent magnetic field in the rotor.
It's not by design in a normal induction motor and probably means it's a lower quality motor with a poor grade of iron laminations, or the motor shaft itself is magnetised.
So, under no frictional losses and no-load, the stator and rotor rotate will still have some minimum slip; in other words, the rotor would lag behind the rotating magnetic field by some least degree. I do understand that induction motors come in different sizes and different designs but is there a way to know how much slip would there exist between the stator and rotor under no frictional losses and no-load? Is it almost negligible?
The following picture shows stator windings for a single phase induction motor. We can say that during one half cycle, the upper winding would act like a north pole and in later half cycle, it become a south pole, and so on. It makes sense.
I was watching this video and how the winding is shown doesn't look correct to me. Please watch it from 1:22 to 1:41. It only shows a single loop and it looks wrong how North and South poles are shown. Do you agree? Thank you.