problem 1: Briefly describe why an electronic soft starter is becoming much popular to limit the starting current of an induction motor.
problem 2: State ONE benefit and ONE drawback of using a soft starter with a three phase delta joined squirrel cage induction motor.
problem 3: A 440 V, 3 phases, 50 Hz, 4 – pole, Y – connected induction motor has a full load speed of 1425 rpm. The rotor has an impedance of (0.4+j4) ohms and rotor or stator turns ratio of 0.8. Compute (a) full load torque (b) rotor current and full load rotor Cu loss (c) power output if wind age and frictional losses amount to 500 W, (d) maximum torque and the speed at which it occurs (e) starting current (f) starting torque.
problem 4: State TWO benefits of using a synchronous induction motor rather than an asynchronous induction motor.
problem 5: Regarding skewing of motor bars in a squirrel-cage induction motor, (or SCIM) which statement is false?
a) It prevents cogging.
b) It raises starting torque.
c) It produces more uniform torque.
d) It reduces motor 'hum' throughout its operation.
problem 6: A three-phase, two-pole, 50-Hz induction motor is observed to be operating at a speed of 3502 revolutions per minutes with an input power of 15.7 kW and a terminal current of 22.6 A. The stator winding resistance is 0.20 ohms/phase.
Compute the I2 R power dissipated in the rotor.
problem 7: The magnetizing current drawn by transformers and induction motors are the cause of their …….power factor.
problem 8: The result of increasing the length of air-gap in an induction motor will be to rise the…………
problem 9: In a 3-phase induction motor, the relative speed of the stator flux with respect to ….. is zero.
problem 10: The eight-pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 r.p.m. through a prime mover in the opposite direction of revolving magnetic field. The frequency of the rotor current is…….
problem 11: A 3-phase, 4-pole, 50-Hz induction motor runs at a speed of 1440 r.p.m. The rotating field produced by the rotor rotates at a speed of……r.p.m. with respect to rotor.
problem 12: In a 3-φ induction motor, the rotor field rotates at synchronous speed with respect to………
problem 13: Irrespective of the supply frequency, the torque developed through a SCIM is same whenever ………is the same.
problem 14: The number of stator poles generated in the rotating magnetic field of a 3-φ induction motor having 3 slots per pole per phase is…..
problem 15: A 6-pole, 50-Hz, 3-φ induction motor is running at 950 rpm and consists of rotor Cu loss of 5 kW. Its rotor input is…..kW.
problem 16: Fill up the blanks.
When load is placed on a 3-phase induction motor, its
(a) Speed…
(b) Slip....
(c) Rotor induced emf….
(d) Rotor current…..
(e) Rotor torque….
(f) Rotor continues to rotate at that value of slip at which developed torque equals..…….torque.