4 ms·

The animation seems to show that the rotor has a DC (ie. constant) field. That is easy to do with slip-rings and a DC current source, but with any induction m

The animation seems to show that the rotor has a DC (ie. constant) field. That is easy to do with slip-rings and a DC current source, but with any induction mechanism, there must be a rectification stage on the rotor. They don't show any of that.

MayeulC 3y ago
I'm wondering if performance could be improved by controlling the polarity of the rotor coil fields, to push against the stator-induced ones. Since some semiconductor rectifiers are necessary in the stator, it doesn't sound much more complex to control the polarity. Part of the control logic could be handled before the inductive power transfer mechanism, by modulating the frequency, though it gets complex as you probably want to tune the resonant frequencies of the sender and receiver.
- Animats 3y ago
  Wind turbines work that way. Early wind turbines had to mechanically rotate at a fixed speed to match the grid. With modern ones, the field is sometimes electrically rotated to adjust for rotor RPM. This can be done quickly as the wind changes. The range of control is not large, though. This is a quick adjustment made electronically until the blade pitch can slowly be changed by motors to re-tune the wind turbine for the current wind speed. Look up "doubly fed induction generator".
  - MayeulC 3y ago
    Interesting to know, thank you. They seem to mostly use this to adjust the frequency of the induced current in the stator, as they control the rotation speed of a "virtual rotor" (rotation of the rotor + rotation of the magnetic field around the rotor). That's indeed similar, but in the wind turbine use-case, the stator is passive. A direct application could be to tune the torque/speed response, or improve motor weight by reducing the number of coils on one of the elements.
Animats 3y ago
Hm. Do they have diodes in the rotor? It's common for large, power station sized, generators to have that. The field windings are on the rotor, and the heavy power comes out from the stationary armature windings. To power the field windings, there's a smaller generator as part of the machine, but it's the other way round - stationary field, rotating armature producing power. The output of the smaller generator is rectified by diodes on the rotor to provide DC power for the main rotating field. No commutators or slip rings. No wear, no arcing. Did someone just scale this down for motors?

MayeulC 3y ago

I'm wondering if performance could be improved by controlling the polarity of the rotor coil fields, to push against the stator-induced ones. Since some semiconductor rectifiers are necessary in the stator, it doesn't sound much more complex to control the polarity. Part of the control logic could be handled before the inductive power transfer mechanism, by modulating the frequency, though it gets complex as you probably want to tune the resonant frequencies of the sender and receiver.

Animats 3y ago
Wind turbines work that way. Early wind turbines had to mechanically rotate at a fixed speed to match the grid. With modern ones, the field is sometimes electrically rotated to adjust for rotor RPM. This can be done quickly as the wind changes. The range of control is not large, though. This is a quick adjustment made electronically until the blade pitch can slowly be changed by motors to re-tune the wind turbine for the current wind speed. Look up "doubly fed induction generator".
- MayeulC 3y ago
  Interesting to know, thank you. They seem to mostly use this to adjust the frequency of the induced current in the stator, as they control the rotation speed of a "virtual rotor" (rotation of the rotor + rotation of the magnetic field around the rotor). That's indeed similar, but in the wind turbine use-case, the stator is passive. A direct application could be to tune the torque/speed response, or improve motor weight by reducing the number of coils on one of the elements.

Animats 3y ago

Hm. Do they have diodes in the rotor? It's common for large, power station sized, generators to have that. The field windings are on the rotor, and the heavy power comes out from the stationary armature windings. To power the field windings, there's a smaller generator as part of the machine, but it's the other way round - stationary field, rotating armature producing power. The output of the smaller generator is rectified by diodes on the rotor to provide DC power for the main rotating field. No commutators or slip rings. No wear, no arcing. Did someone just scale this down for motors?