JNTU, B.Tech I Semester ELECTRO MECHANICS-I,November 2008

(Electrical & Electronic Engineering)

SET-3

1. (a) Show how mechanical energy output can be determined in the doubly excited

system in the linear magnetic system.

(b) A doubly excited magnetic field system has coil self and mutual inductances of L11 = L22 = 2, L12 = L21 = cos2?, where ? is the angle between the axes of the coils. If coil 2 is shorted while coil 1 carries a current of i = Im sin wt, derive expressions for instantaneous and time average torques. Find time average torque when ? = ?/4 and Im = p2 sin 314t.

2. The armature core of a 4-pole d. c. machine has 31 slots each designed to accommodate 4 coil sides of a simplex wave winding. The winding has total of 496

conductors. Find

(a) Total number of coils

(b) Turns per coil

(c) Commutator pitch

(d) Back, front and total pitches and

(e) Number of commutator segments.

3. (a) When are dummy coils used and which type of D.C. armature winding these

will occur?

(b) Calculate the ampere turns for each commutating pole of an 8-pole generator

with 107 slots, each containing 1000 ampere conductors. The interpole air-gap is 1.2 cm. The flux density in the air gap is to be 0.32 T. Neglect iron parts and leakage.

4. (a) How do you classify the compound generators.

(b) What are the various characteristics of compound generators. Explain them briefly?

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) What are the different methods of speed control of DC motor? Explain.

(b) A 220V DC shunt motor draws a no load armature current of 2.5A when running at 1400rpm. Determine its speed when taking an armature current of 60A if armature reaction weakens the flux by 3%. (Assume Ra = 0.2)

7. (a) Explain how hysteresis loss and eddy current loss that occur in a DC machine.

State the factors on which these losses depend?

(b) A full load brake test on a small DC shunt motor, gave the following data.

Spring balance readings : 25 Kg and 9 Kg

Outside pulley diameter : 19.5 cm

Belt thickness : 0.5 cm

Motor speed : 1500 rpm

Applied voltage : 230 V

Line current : 12.5 A

Calculate the shaft torque, shaft power and the motor efficiency at rated load.

8. (a) How the rotational losses can be estimated by retardation test?

(b) A retardation test is made on a separately excited DC machine as a motor.The induced voltage falls from 220V to 190V in 30 seconds on opening the armature circuits and 5 seconds on suddenly changing the armature connection from supply to a load resistance taking 12 A (average). Find the efficiency of the machine when running as a motor and taking a current of 22A on a supply of 250V. The resistance of its armature is 0.5 and that of its field winding is 250 ohms.