UNIT-7
A.C.Machines (3-Phase induction motor)
1. (A) Explain the principle of operation of a 3 phase induction motor.
(B) A 3 – phase, 6 – pole, 50 Hz induction motor develops 4 kW including friction and windage losses at 950 rpm. If the stator loss is 250 W. Find the slip of the induction motor.
*****2. (A) Explain with the help of diagrams how a rotating magnetic field is produced in a 3 phase induction motor. (Or) Explain why 3-phase induction motor is self stating induction motor.
(Or) A rotating magnetic field is produced in the air gap of a 3-phase induction motor when a balanced supply is given to the stator. Justify the statement with necessary mathematical equations and phasor diagrams.
(B). Three phase induction motor is wound for 4 – poles and is supplied from a 50 Hz supply. Calculate
i) The synchronous speed
ii) The speed of the motor when the slip is 3 %
iii) The rotor frequency.
4. (A) Explain why a 3-phase induction motor cannot develop torque when running at synchronous speed.
(B) Explain why induction motor is called rotating transformer
(C).A four-pole, three-phase induction motor develops 25KW, including mechanical losses of 1KW,at a speed of 1440 rpm on 440V,50Hz mains. The power factor is 0.85.for this load calculate (a) slips,(b) the rotor resistance losses. (c)the total input if the stator losses are 1.75 Kw (d) he efficiency (e) the line current and,(f) the number of complete cycles of the rotor electromotive force per minute.
5. (A) Derive the condition for maximum torque under running condition of 3-phase induction Motor
(B) A 50 Hz, 4-pole, 3-phase induction motor has a rotor current of frequency 2Hz. Determine
i. the slip and
ii. Speed of the motor.
6. (a) Explain the effect of slip on the rotor circuit.
(b) A 6-pole induction motor operates on a supply whose frequency is 50 Hz. Calculate
i. the speed at which the magnetic field of the stator is rotating,
ii. The speed of the rotor when the slip is 4%,
iii. The frequency of the rotor currents when the slip is 5%, and
iv. The frequency of the rotor currents at standstill.