JNTU II B.Tech I Semester Supplimentary Examinations, November 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering)
1. With the help of neat diagram obtain the expression for the energy stored in a magnetic system for a simple attracted armature type relay. Explain the operation of system.
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 the effects of armature reaction in a d. c. generator.
(b) Determine per pole the number
i. of cross-magnetizing ampere-turns
ii. of back ampere-turns and
iii. of series turns to balance the back ampere-turns in the case of a.d.c. generator having the following data 500 conductors, total current 100A, 4 poles, 2-circuit wave winding, angle of lead = 100, leakage coefficient = 1.3.
4. (a) Explain experimental determination of critical field resistance for a self excited
(b) What are the causes and indication of over load generator.
5. Six DC generators are running in parallel, each having an armature resistance of
0.15 ohm, running at the same speed and excited to give equal induce e.m.f(s). All generators share load equally at a terminal voltage of 500V. The total load is 360KW. If the field current of one generator is raised by 5% and the speed remains constant, Calculate
(a) New Terminal Voltage
(b) Output of each machine.
6. (a) Distinguish between motor and generator action. Derive the equation for the back e.m.f induced in a DC motor.
(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 16KW. Calculate
ii. Armature Torque and
iii. shaft Torque.
7. (a) With Neat diagram Explain the construction and working of 3-point starter
(b) A 2 ? pole lap wound DC shunt motor with 360 conductors operates at a constant flux level of 50mWb. The motor armature has a resistance of 0.12 and is designed to operate at 240 V, taking a current of 60 A at full load.
i. Determine the value of external resistance to be inserted in the armature
circuit so that armature current does not exceed twice its full load value at starting.
ii. The external resistance is completely cut out when the motor reaches its finale speed, with the armature current at the full load value. Calculate the motor speed under these conditions.
8. (a) Compare Swinburne’s test and Hopkinson’s test conducted on DC machines.
List the advantages and limitations of both the methods.
(b) The Hopkinson’s test on two DC machines gave the following results for full
load: line voltage 250 V, line current = 45 A excluding field currents, motor armature current = 385 A, field currents 5 A and 4 A. Calculate the efficiency of each machine. Armature resistance of each machine is 0.015 .