# Andhra University BE/B.Tech Elements of Eletrical Engineering Previous Paper 2007

First question is compulsory

Answer any FOUR from the remaining questions

All questions carry equal marks

1 (a) What are the differences between magnetic circuit and electric circuit

(b)Distinguish between dynamically induced e.m.f and statically induced e.m.f. Give one

Example of each

(c)on the same diagram ,draw the load characteristics of

(i) D.C.series generator and

(ii) D.C. shunt generator

Based on the characteristics ,give one appliocation for each generator.

(d)Name the speed control methods of D.C. shunt motor, Give one application for each

Method.

(e)3-phase ,440 volts ,50 Hz balanced supply is given to a balanced 3-phase load of(6+j8) ohms for phase connected in delta . What is the current drawn from the supply?

(f) (i)the full load copper loss in a 1-phase transformer is 400 watts. What is its value when

The transformer is delivering half full load?

(ii) Define slip of induction motor.

(g) (i)Write the emf equation of alternator when its stator winding is chorded and

Distributed.

(ii)What are the starting methods adopted for synchronous motor?

2 (a)Derive the relation between MMF,reluctance and magnetic flux.

(b) State F araday’s law of electromagnetic induction.A coil having an inductance of 60 mH is carrying a current of 60 Amps .Calculate the self induced e.m.f. in the coil .when the current is

(i) reduced to zero in 30m seconds ans

(ii)reversed in 30 milli seconds.

3 (a) Derive the e.m.f. equation of a D.C. generator

(b)A100 KW ,240 volts D.C. shunt generator has a field resistances of 55 ohms and armatuire resistances of 0.067 ohm. Find the full load generated voltage.

4 (a)it is desired to run the D.C. motor at a speed more than its related speed .suggest the suitable method of speed control and explain it with a circuit diagram.

(b)A d.c. shunt motor takes 4 amps at no load from the supply when running at 700rpm.The field resistances is 100 ohms .The resistance of armature at stand still gave a drop of 6 volts across the armature terminals,when 10 Amps werw passed through it. Calculate (i)the speed on load (ii)the torque in NW-m and (iii)efficiency when trhe full load power input of the motor is u8 KW.

5 (a)Explain the following terms applicable to a.c. circuits;

(i) Power

(ii) Power factor

(iii) Apparent power

(iv) Form factor and

(v) Inductive reactance.

(b)A coil of resistances 15 ohms and an inductance OF 0.05 Henry is connected in parallel with non –inductive resistor of 20 ohms .Find (i) the current in each branch(ii) the total current supplied and (iii)the [phase angle of the total current when a voltage at 50 Hz is applied.

6.(a)Describe with neat circuit diagrams ,the tests to be conducted to determine the parameters of the equivalent of a 1-phase transformer

(b) A 7.5 KVA,1-phase transformer with a voltage ratio of 500/250 volts has primary winding resistance of 0.2 ohm and a leakage reactance odc 0.6 ohm. The cporrespondingh values of secondary winding are 0.05 ohm and 0.15 ohm.Find the terminal voltage at full load on the secondary with 0.8 power factor lagging

7.(a)explain the operation of 3-phase induction motor and derive the expression for the torque developed under running conditions

(b)An 8-pole ,3phase,50 Hz induction motor has a rotor ompedence of(0.025+j0.1)ohm per phase at standstill.At what speed is the torque maximum?What portion of the maximum torque is the starting torqur?

8 (a)explain the operation of the synchronus motor

(b)A 3-phase ,16 pole, alternator has a star connected winding with 144 slots and 10 conductors per slot.The fluic per pole is 30 mWB.sinusoidallydistributed abnd the speed is 375 RPM. Calculate (i)Pthe frequency and(ii)the e,m,f, generated per phase and line voltage at no -load

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