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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.