B.E (ELECTRICAL& ELECTRONICS ENGINEERING)CLASS TEST- I5thSEMESTER, SESSION 2014-15 Electrical Machines-IIMax Marks: 40 Times: 2Hours---------------------------------------------------------------------------------------Note: - Part (a) is compulsory of each unit. Attempt any two from remaining three parts from(b),(c) and (d)

Q.1 (a) Why the speed of the 3phase induction motor cannot be equal to synchronous speed? (2)

(b) A 3 phase, 6 poles, 50Hz induction motor takes a power of 32KW at its full load speed of 950 rpm. The total stator losses are 2.5KW and friction and windage losses are 1 KW. Calculate (i) slip (ii) rotor Cu loss (iii) Shaft power (iv) shaft torque (v) efficiency. (6)

(c) Why single phase Induction Machine is not self-starting ?What are the different types of single phase induction motor on basis of starting methods? Explain any one of them. (6)

(d) Draw the Torque/Slip curve of induction motor at different values of rotor resistances for s=0 to 2.Indicate the stable operating zone. (6) Q.2 (a) What is winding factor? (2)

(b) A 3phase Induction motor at rated voltage and frequency has starting torque of 150% and maximum torque of 220% of full load torque. Neglect stator resistance and rotational losses. Calculate:

(i) Slip at full load (ii) Slip at maximum torque (iii) The rotor current at starting in terms of full load rotor current. (6)

(c) A three phase cage induction motor has a short circuit current equal to 5 times the full load current. Find the starting torque as a percentage of full load torque as a percentage of full load torque if the motor is started by:i. Direct switching to the supply ii. Star delta starteriii. Auto transformer starteriv. A resistance in the stator circuit The starting current in case (iii) and (iv) is limited to 2.5 times the full load current and full load slip is 4%. (6)

(d) Explain the different methods of speed control of speed control of induction motor. (6)

Q.3 (a) Explain double cage induction motors induction motor. (2) (b) Show that a rotating magnetic field moving at constant speed is produced when 3 phase balance supply is given to three phase balance winding. What is speed of field? (5) (c) Explains cogging and crawling of three-phase induction motor. (5) (d) Find the number of series turns required for each phase of a 3 phase, 50Hz, 10 pole alternator with 90 slots. The winding is to be star connected to give a line voltage of 11KV. The flux per pole is 0.16 wb. (5)