AC/DC machines. Can someone help me with this? It's supposed to be easy but I can't sesem to underst

[nelly12346]

Suppose that you have a 4-pole AC machine with three-phase stator windings and threephase rotor windings. The stator terminals are connected to the 50 Hz grid and the rotor terminals are short circuited. This setup can be seen as a rotating 3-phase transformer with secondary terminals short circuited. Suppose also that at steady state, the rotor is rotating at 1400 rpm. What is the frequency of rotor currents? What would be this frequency when the motor is rotating at 1500 rpm and will this machine be able to produce net torque in that case? What type of a machine is this? How the rotor current frequency changes if you even increase rotor speed to 1600 rpm?

 

[LionelHutz]

Rotor frequency starts at line frequency at zero speed and is inversely proportional to slip.

 

[Gilgamesh07]

* 4 poles => 2 pairs
Speed of rotor Nr = 1400 rpm
Synchronous speed Ns = (50 Hz / 2 pairs) = 25 = 1500 rpm
slip = (Ns - Nr)/Ns = (1500 - 1400)/1500 = 0.067
Rotor Current Frequency = Per unit slip × Supply frequency When the rotor is stationary
Rotor Current Frequency = 0.067 x 1500 = 3.33 Hz

* When Ns = Nr = 1500rpm
The speed of the rotor is equal to the speed of the rotating magnetic flux => slip = zero => no rotor current => no net torque is produced.

* The machine is of the type asynchronous machine, induction machine.

* The last question is a bit tricky. In induction machines with short-circuited rotor windings is not possible to have rotor speed greater than synchronous speed when the machine is operated as a motor.
If an external mechanical system drives the rotor above synchronous speed, the induction machine acts as a generator.
The rotor current is generated in the opposite direction, due to the rotor conductors cutting stator magnetic field and a negative slip is obtained.
In my opinion, the Rotor Current Frequency should stay 3.33 but the direction of the current is reversed.