**JNTU B.Tech November 2008**

**ELECTRICAL MACHINES-I**

**(Electrical & Electronic Engineering)**

**SET-1**

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1. In a rectangular electro magnetic relay, he exciting coil has 1200 turns. Cross sectional are of the core is A = 6 cm × 6 cm. neglect the reluctance of he magnetic circuit and fringing effects. With coil current kept constant at 2A, derive expression for force on armature as a function of air gap of length x. Find the work done by the magnetic field when x decreases from 1 cm to 0.5 cm by integrating the force.

2. (a) Deduce an expression for the voltage induced in a DC generator.

(b) The lap wound armature of a 4 – pole generator armature has 51 slots. Each slot contains 20 conductors. What will be the e.m.f generated in machine when driven at 1500 r.p.m. If useful flux per pole is 0.01Wb?

3. (a) Explain the armature reaction in a D.C generator on no-load. Enumerate and explain the method to overcome the adverse effects of the armature reaction.

(b) A 4-pole 50 KW, 250v wave wound, shunt generator has 400 armature conductors. Brushes are given a lead of 4 commutator segments. Calculate the demagnetizing ampere turns/pole if shunt field resistance is 50 ohms. Also calculate extra shunt filed turns/pole to neutralize the demagnetization.

4. (a) How do you determine the magnetization characteristics of a DC generator.

(b) A separately excited generator when running at 1200 rpm supplies a current of 200A at 125 V to circuit of constant resistance. What will be the current when the speed drops to 1000 rpm if the field current is unaltered? Armature resistance is 0.04 ohm and the total voltage drop at the brushes is 2V. Ignore the change in armature reaction.

5. (a) Explain the drooping charectoristics of a D.C shunt generator

(b) A d.c series generator having an reternal charectristics which is a straight line through zero to 50V at 200A is connected as a booster between a station busbar and a seeder of 0.3 resistance. Calculate the voltage different between the station busbar and the far end the seeder at a curent of

i. 1200A and

ii. 50A.

6. A 20 KW,250 V dc shunt motor has a full-load armature current of 85 A at 1100

rpm.The armature resistance is 0.18 ohm.

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(a) the initial torque developed;

(b) the internal torque of the field current is suddenly reduced to 80% of its originalvalue;

(c) The steady motor speed in part

i. assuming the load torque to have remained constant.

7. The speed of a 50 h.p series motor working on 500V supply is 750 rpm at full load

and at 90 % efficiency. If the load torque is made 350Nm and a 5 resistance I connected in series with the machine, calculate the speed at which the machine will run. Assume the magnetic circuit to be unsaturated and the armature and field resistance to be 0.5.

8. The Hopkinson’s test on two similar machines gave the following full load results.

Line current =48A

Line voltage =110V

Motor armature current=230A

The field currents are 3A an 3.5A. Armature resistance of each machine is 0.035 ohms. Calculate the efficiency of each machine assuming a brush contact drop of 1V per brush.