# GTU Electric Machines Question Paper Dec 2010

GUJARAT TECHNOLOGICAL UNIVERSITY

B.E. Sem-III Regular / Remedial Examination December 2010

Subject code: 131701

Subject Name: Electrical Machines

Instructions:                                                                                     Total Marks: 70

1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.

Q.1 (a) Derive the e.m.f. equation of a 1-phase transformer.

The core of a 150 kVA, 11000/440 V, 50 Hz, 1-phase core type transformer has a cross section of 20*20 cm. The maximum core density is not to exceed 1. 3 wb/m2. Find (i) the number of h.v. and l.v. turns per phase.

(b) Discuss the tests to find out the iron loss and copper loss in a transformer with justifications.

Q.2 (a) Discuss open delta connections of transformers with necessary circuit and vector diagrams.

(b) Define regulation of a transformer. Compare conventional and instrument transformers.

OR

(b) Two 1-phase furnaces working at 100 V are connected to 3300 V, 3-phase supply through Scott connected transformers. Calculate the current in each line of 3-phase mains when the power taken by each furnace is 450 kW at a power factor of 0.8 lagging. Neglect the losses in transformers.

Q.3 (a) Differentiate between singly excited and multi-excited field systems. Derive the expression for the mechanical force in a current excited system.

(b) Explain the phenomena of crawling and cogging in a 3-phase induction motor with their remedies.

OR

Q.3 (a) Differentiate between cage and slip ring induction motor. Explain how the torque is developed in a 3-phase induction motor.

(b) Draw the circle diagram for a 3-phase, 29.84 kW, 415 V, 50 Hz, delta-connected induction motor from the following data:

No-load test: 415 V; 21 A; 1250 W Blocked rotor test: 100 V; 45 A; 2730 W

Estimate from the diagram for full load condition, the slip, efficiency and power factor. The rotor copper loss at standstill is half of the total copper loss.

Q.4 (a) What is slip of a 3-phase induction motor? Discuss its slip-torque characteristics.

(b) Why a 1-phase induction motor is not self starting? Explain working of split phase type 1-phase induction motor

OR

Q.4 (a) Define salient pole and non-salient pole machines. Why is armature winding of a synchronous machine stationary?

(b) What is synchronisation of alternators? Which conditions must be satisfied for proper synchronisation of 3-phase alternators?

Q.5 (a) Compare field control and armature voltage control methods of speed control of a DC motor.

(b) Explain the phenomena of armature reaction in a DC machine. State its remedies.

OR

Q.5 (a) Draw schematic diagram of a dc machine with labels. State the functions of (i) pole shoe, (ii) commutator and (iii) yoke.

(b) Differentiate between self-excited and separately-excited dc machines. Draw the load characteristic of dc shunt and series generator.