II B.Tech I Semester Supplimentary Examinations, February 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering
1. (a) Explain the doubly excited system? Explain the various losses in it.
(b) Find an expression for the force per unit area between the plates of a parallel plate condenser in terms of the electric field intensity. Find the value of the force per unit area when E = 3 * 106Volts/ meter, the break down strength of air.
2. (a) What is an equalizer connection? What is necessity of equalizer connection?
(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connected in lap with 960 conductors. Calculate the generated EMF on open circuit when it runs at 400 rpm. If the armature is wave wound at what speed must the machine be driven to generate the same voltage.
3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizing Ampere Turn per pole.
(b) What is the purpose of compensating winding? Explain in detail.
4. What is the significance of residual magnetism? Explain the experimental determination of OCC in laboratory?
5. (a) Explain the procedure of withdrawing a DC generator that is already in parallel with others and supplying the load.
(b) Eleven DC Shunt Generators are running in parallel. Each generator supplies a load current o f 400A at 210V. The shunt field current of each generator is 4A and the armature resistance of each generator is 0.04 ohm. If one generator is suddenly switched off, determine the % change in terminal voltage, the total load current being kept unchanged.
6. (a) Discuss Armature reaction and commutation in DC motors. Explain their effects on the performance of the motor and give remedies to their effects.
(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slot containing 6 conductors. The flux per pole is 20 m.wb and the armature has a resistance of 0.13 ohm when the motor is connected to 240V supply and the armature draws a current of 80A driving a load of 15KW. Calculate
ii. Armature Torque and
iii. shaft Torque.
7. (a) What are the different speed control methods of DC Shunt motor? Explain each method and enumerate advantages and disadvantages.
(b) A DC series motor is driving a fan load, whose torque varies a cube of speed. The total armature and series field resistance is 1
. It takes 10 Amp from 200 V mains and runs at 1000 RPM. Find the resistance to be connected in series with the motor to make it run at 800 RPM.
8. (a) Explain the various losses taking place in DC machines. With the help of these losses draw the power flow diagram for a DC Motor.
(b) A 440 V DC shunt motor takes a current of 3 A. at no-load. The armature resistance including brushes is 0.3 and the field current is 1 A. Calculate the output and efficiency when the input current is 20 A.