synchronous to squirrel cage induction conversion

[ccjersey]

I saw the post about using a VFD on a WRIMotor with the rotor shorted. I wondered if one could short the armature windings (or connect a resister in the circuit)of an AC alternator and use it as an induction generator. Might be easier to switch back and forth with a brush/slip ring design armature vs a brushless design.

The engine speed would have to increase, but that would be a simple matter on most generator sets.

Maybe this would solve the problem of not having backup power available from an induction generator.

Jim

 

[UKpete]

I don't think that using the rotor from a synchronous alternator would make an efficient induction generator, even with the brushes shorted as you suggest. It would be like an induction motor with only a very small number of rotor slots albeit effectively very large ones.

 

[aolalde]

Synchronous to squirrel cage induction conversion is not recommended. Adding to the disadvantages referred by UK-Pete, since the air gap is extremely high the magnetizing current to cross it and induce in the rotor bars is to high and with high leakage of flux lines ( When it works as synchronous, the field is created in the rotor).
The magnetizing current needs to be provided by the load line. If you do not have a reference three phase system it will not produce electricity at all.

 

[ccjersey]

Thanks aolade and ukpete.
Besides inefficiency, do you think it would hurt the synchronous alternator to do this. Maybe if the armature current didn't exceed "normal" levels.

I understand the implications of induction generators vs excitation, that is what I am trying to get around by having the wound rotor and AVR for backup use.

Jim

 

[aolalde]

Working as induction machine, the weak link are the rotor cage bars, rather than the stator winding. Unless a reduced power is handled (well below the nominal current as synchronous machine), the overheating will destroy the amortisseur winding (rotor cage)in a short operating time.