Problems #6

1. How rotating magnetic field is developed in a three-phase induction machine when fed from the three-phase supply?

2. A three-phase induction motor runs at speed close to 900 rpm at no-load and at 873 rpm at full load when supplied with power from a 60 Hz three-phase source.

(i) How many poles has the motor?

(ii) What is the percent slip at full load?

(iii) What is the corresponding frequency of the rotor currents?

(iv) What is the speed of the rotor field with respect to the rotor? With respect to the stator?

(v) What speed would the rotor have at a slip of 10 percent?

3. For an induction motor show that:

Pg (Air gap power): Pm (internal mechanical power developed): Rotor copper loss=1: (1-s) : s