JNTU, B.Tech I Semester ELECTRO MECHANICS-I,November 2008
(Electrical & Electronic Engineering)
1. (a) Show how mechanical energy output can be determined in the multiply excited
(b) Two coils have self- and mutual- inductances of L11 = L22 = 2/(1+2x) = 2L12. Calculate time-average force and coil currents at x = 0.5 m if both coils are connected in parallel across a voltage source v = 100 cos 314t.
2. (a) Explain the principle of action of commutator in a d. c. generator.
i. front pitch
ii. back pitch
iii. coil span and
iv. commutator pitch as applied to d. c. armature windings and indicate the above on the diagram of a wave winding.
3. (a) With relevant diagrams, explain the phenomena of commutations in a d.c. machine.
(b) Explain what is meant by ‘straight line commutation,’ ‘Accelerated’ & ‘Retarded commutation’. Discuss the role of interpoles in improving commutation.
4. (a) What is critical field resistance? How do you calculate the critical field resistance practically.
(b) When a shunt generator fails to build up voltage, then how to rectify the fault.
5. (a) Develop the general expression for the speed of a motor in terms of supply
voltage, armature resistance and flux per pole.
(b) Discuss the applications of series motors and compound motors.
6. (a) A 6 pole, 500 V, wave connected shunt motor has 1200 armature conductors
and useful flux/pole of 20 mwb. The armature and field resistances are 0.5 and 250 respectively. What will be the speed and torque developed by the motor when it draws 20 A from the supply mains?
Neglect armature reaction. If magnetic and mechanical losses amount to 900
i. useful torque
ii. output in KW &
iii. efficiency at this load.
(b) A 230V, 10 H.P. shunt motor takes a full load line current of 40A. The armature and the field resistances are 0.25 and 230 respectively. The total brush drop is 2V, and the core and friction losses are 380W, Calculate the efficiency of the motor. Assume that the stray load loss is 1% of the rated output.
7. (a) What is the function of no volt release and overload release in a starter?
Discuss the operation of these two features in a shunt motor starter.
(b) Explain with neat diagrams construction and working of a series motor starter.
8. (a) Outline the steps to estimate the efficiency of given two d.c. machines by conducting Hopkinson’s test. Draw schematic diagram to illustrate the method.
(b) In a Hopkinson’s test on a pair of 500V, 100KW, shunt generators, the following data was obtained. Auxiliary supply 30A at 500V
Generator output Current 200 A
Field currents 3.5 A and 1.8A
Armature circuit resistances of each machine 0.075
Voltage drop at brushes (each machine) : 2 V
Calculate the efficiency of each machine acting as a generator