Three-Phase Induction Motor - Voltage Across Slip Rings

Would you explain why you wanted to do so?

If you keep supply in the stator and try to rotate the rotor in opposite direction, It would brake the motor. It would behave similar to locked rotor condition, slip =1, 50 Hz frequency

No, definitely not.

I believe 100Hz emf (of unknown magnitude) would be induced in the rotor windings if it turned in the opposite to the stator field's direction at 3000rpm. At 1000rpm, probably, 66,6Hz?

Sorry, did not notice it had 6 poles, need to think more about it.

To say honestly have not even seen in "live" a slip ring induction motor. Emf frequency induced in the rotor should be also dependent on the number of rotor poles.

If the stator was using DC power the rotor would work like a three phase 50 HZ alternator or generator. However you are using AC in the stator so its going to have half of each pole sweep going with one current direction and the other half of the pole sweep going the other way.

what you will end up with is likely a wildly fluctuating output with a combination of the 50 HZ base frequency and the 100 HZ of rotation superimposed onto that.

I think. Try it and see what you get. And why would you want to do this any way?

(Spellcheck is down again)

He said INDUCTION motor, not synchronous.

he said

If the stator was using DC power the rotor would work like a three phase 50 HZ alternator or generator. However you are using AC in the stator.

The frequency on the slip rings will be 100 Hz. About at 1943-44 there was a Hungarian engine which used that effect to control the speed.

http://erojr.home.cern.ch/erojr/content/models/Kando/KAN_HIS3.HTM#2

World6,

I agree with 100Hz. An electric motor can be described as a transformer in which the primary winding is in the stator and the secondary winding is on a rotating shaft within the stator. To have the best possible magnetic field coupling between the "primary" and "secondary" windings, you must keep the gap between the rotor and stator as small as practicable. In a "squirrel-cage" motor, the secondary winding is the bus bars and end rings commonly cast in place (aluminum) or brazed in place (copper). With a wound-rotor secondary, there is a set of windings that are brought out to the three slip rings (for a 3-phase motor). I believe that your 100V is representative of the turns ratio between the stator and rotor windings. Speed control of such a motor is by the amount of resistance you insert between the slip rings. Highest speed is with the slip rings shorted. I believe that with them open-circuited, you will have no torque and therefore the rotor will not attempt to turn (I don't know what this would do regarding current flow in the stator and heating of it). I've worked with these motors in sawmills (fixed resistor banks and multi-step drum switches) and sewage pump stations (liquid resistors). They have significant maintenance requirements for the resistors, switches, brushes, etc; but they are otherwise simple, very quiet, and have lower inrush currents than most other reduced-voltage starting methods.

Interesting question and posting. --JMM

The matter is simple: the frequency depends on the slip.

f_cage = s.f_pri

Slip, s = (n_synch - n_rotor)/n_synch

when n_rotor = - n_synch, s = 2

so

f_cage = 2f_pri

From where appears the number - 100Hz? Forgive me my gaps in electrotechnics studies but now I think there would flow no current (had the rings been loaded) because emf in the rotor would work precisely against the emf in stator - it would be 0V , 0Hz, (the torque needed to turn the rotor would be very high by that).

Consider relativity.

You normally under blocked rotor condition have rotor stationary, the stator field rotating at 50Hz- generating the same frequency emf in the rotor.

Now the rotor is rotated in opposite direction by another motor/ prime mover.

The relative rotation of the magnetic field with reference to the rotor will be 2f in this case (otherwise f_synch+f_rotor)

And automatically a relatively rotating field at thet frequency will create an emf of that frequency.

And then to think about it, the OP is not asking the frequency of the induced emf. he wants Voltage across slip rings.

Why not 66.66Hz (1.33f)? 100Hz (2f) should be if the rotor was rotating 3000rpm (in the direction opposite to the direction of the field).

6 pole machine- Synch speed is 1000 RPM