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Department of Electrical & Electronics, MIT Manipal
ELE 2103: Electrical Machinery - 1
ASSIGNMENT 3 (dated 31-10-2016)
QUIZ: 10-11-2016 or later
Induction Motors: Three phase & Single phase IM
PART A
1. Explain how a rotating magnetic field is produced when a three phase balanced winding
is connected to a three phase balanced supply.
2. Why induction motors are recommended to start with reduced voltages? Sketch &
explain the working of star-delta starter.
3. Sketch and explain complete slip torque characteristics of induction machines clearing
differentiating motoring, braking and generating regions.
4. What is an induction generator? Why induction generators are not much used in electric
power generation?
5. Explain the following electrical braking schemes for induction motors (a) plugging
(b) regenerative braking (c) dynamic braking
6. Explain how induction motor speed can be controlled using (a) stator voltage control
(b) rotor resistance control
(c) stator frequency control
7. Explain the phenomenon (a) cogging and (b) crawling of induction motors?
8. Explain how improved starting torque is achieved with following rotor construction
(a) double cage (b) deep bar
9. Using double field revolving theory, explain the slip-torque characteristics of a single
phase induction motor
10. Explain briefly various types of single phase induction motors
PART B
1. A 3-Φ 220V 50Hz 4 pole induction motor has a star connected stator winding. The per phase rotor
resistance is 0.1 Ω and per phase standstill reactance is 0.9 Ω. The ratio of the stator to rotor turns
is 1.75. Full load slip is 4%. Calculate mechanical power generated and the load torque. Also find
the maximum torque and speed at maximum torque.
2. A 3-Φ 50Hz 4 pole Induction motor has a star connected rotor. The per phase rotor resistance is
0.1Ω and per phase standstill reactance is 0.9Ω. If the induced emf between the slip rings at
standstill is 100 V and full load speed is 1455 rpm, find the useful output power and express in
BHP. Frictional losses are given as 150 watts.
3. A 3-Φ, 75 kW, 3.3 kV, 6 pole , 50 Hz squirrel cage induction motor gave the following test results:
No load
3.3kV
5A
2500 W
Blocked rotor
400 V
27 A
15000 W
DC resistance/phase : 3.75Ω
Find the approximate per phase equivalent circuit parameters referred to stator side at 5% slip.
Assume delta connected stator winding.
4. A 3-Φ 50Hz 36 kW 4 pole induction motor has a full load efficiency of 82%. The friction and
windage losses are one-fourth of no load losses and rotor copper losses equal the iron loss at full
load. Determine total losses, stator core loss, rotor copper loss and friction and windage losses.
5. A 3-Φ, 440V 50Hz 6 pole star connected induction motor has the following parameters. Stator
impedance = (0.3+j0.433) Ω per phase. Rotor impedance = (0.08+j0.16) Ω per phase at standstill
condition. Stator to rotor turns ratio=1.75. Shunt resistance =54Ω per phase. Magnetizing
reactance =j8.3Ω per phase. Use approximate equivalent circuit to determine the following when
it draws a current of 65A, 0.8pf lagging at rated voltage: i) Exciting current ii) Equivalent rotor
current iii) Core loss iv) stator and rotor copper loss v) mechanical power developed vi) rotor
speed
6. The power input to a 500 V, 50 Hz, 3 phase IM running at 975 rpm is 40 kW. The stator losses are
1 kW and friction & windage losses total 2 kW. Calculate (a) slip (b) the rotor copper loss (c) BHP
and (d) efficiency
7. A squirrel cage induction motor has a slip of 4 % at full load. Its starting current is five times the
full load current. The stator impedance and the magnetizing current may be neglected. The rotor
resistance is assumed constant. Calculate the maximum torque and starting torque as a
percentage of full load torque. Also find the slip at which maximum torque occurs.
8. A 415 V, 50 Hz, 3Φ, 4 pole, Delta connected Induction Motor gave following test results:
No load
415 V
14.42 A
1800 W
Blocked rotor
190 V
32 A
4450 W
Determine using circle diagram, mechanical output, torque & speed, when the motor is
operated with maximum power factor. Assume stator & rotor resistances are equal.