Well this might be a little hard to explain in plain words.
But is important that the rotor is free to move-on after an attraction to a given pole.
Now the problem is that the fingers of the stator will not ‘instantly’ demagnetize. If you look at if closely, the finger after the electro magnets has been turned off (or any electro magnet steel for that matter) needs some time to become ‘magnetically neutral’ again. This ‘demagnetization speed’ depends on the steel type of the core material and more specific on the hysteresis curve.
Now back to the attraction motor.
The problem exists when the fingers of the stator are as wide as the fingers on the rotor. You see, you want to maximize the ‘attraction interaction’ between the stator and rotor, so you keep the attraction pulse going until top dead center – in other words when the stator and rotor fingers are truly aligned. Now you need to shut off input power, so the rotor is not attracted anymore and is able to continue rotation to the next attraction cycle.
BUT IF you shut off the power at top dead center you have not yet fully demagnetized the poles. Since like I explained above, there is a relaxation time involved which takes time to fully become neutral. So in other words if you shut off power at top dead center you have not fully ‘released’ the rotor from the stators ‘magnetic grip’. And since it is at top dead center it wants, and must rotate onwards. But is ‘cannot’ since the steel is still in its natural process of becoming magnetically neutral, and thus is still somewhat magnetized. And so the free rotation after the attraction cycle becomes retarded to an extent.
See this animated picture:
Movie
Now this is where my suggested ‘wider rotor fingers’ come in.
The coil will shut off when the surface of the rotor fully registers with the surface of the stator. Now with the attraction phase over, the core will start to demagnetize. In this case that is fine since the rotor still has some way to go before it starts to leave the stator core. So by the time the rotor starts to leave the stator finger area, the demagnetization should be complete and thus the rotor can rotate freely and is not retarded. I make it sound like this ‘steel demagnetization’ takes ages, which is not so of course, but it will hinder the free rotation of the rotor if the rotor- and stator fingers are of equal width. This btw is the reason why commercial variable reluctance drives have a short ‘demagnetization pulse’ (just a short opposite-polarity pulse to speed up the core demagnetization)
Hope you can understand what I try to communicate here.
If not, let me know.
And please let me know if this
‘alteration of a normal induction motor setup' will result in a No Back-emf motor, because I have a bit of trouble getting the interaction between the various poles that are being generated.
Kind regards,
Steven