rsdoran
Lifetime Supporting Member
With most of what we have been saying so far, the basic justification has been 'Well, the mtor model says...'. Well the motor model also says that the stator resistance goes negative. I don't know about you but my meter doesn't go that way when I switch it to Ohms.
I see why now. I can not explain it in detail but maybe able to offer some ideas that will make it easier for YOU TO EXPLAIN.
I think the main problem may be terminology being used, technically resistance can not change but "impedance" (total resistance of an AC ckt) can, so I always thought of it with that in mind.
The rotor when it over_hauls changes it frequency which in turn changes its "impedance" which in turn should change its voltage, the greater voltage/frequency will be the one that supplys power...I know its more complicated but it should basically be a differential computation...ie the stator is at 480vac but the rotor is at 485vac and the impedance factor of the rotor is 5 ohms so the difference is 5v divided by 5 ohms which will put 1 amp of current from the rotor into the stator.
A magnetic field does not instataneously collapse so there should be residual magnetism which will be used for "excitation". It would be very complicated to "exactly" match the stator to 1800 rpm for a period long enough to collapse the magnetic field(s).
I know it is not as simple as I have stated but at this time I am not capable of nor qualified to explain it using all the equations. I do not claim my math above to be fully accurate, it is shown to give you an idea.
For years I worked with generators that were paralleled, to accomplish this you had to sychronize (match voltage and frequency) to couple them. Once synchronized then you would increase or decrease the speed (slightly) to develop the current supplied to/from the other set(s). The faster running genset would be at a slightly higher frequency and voltage. I guess I always used this as a basis for understanding re-generation in motors.
Again the genset info is in basic terms but maybe it offers some ideas so you can understand then maybe you will write a technical paper on the subject so more of us can understand it in "better" detail.