JNTU B Tech Ist Semester Electrical Technology, November 2008

JNTU  PAPERS

JNTU B.Tech I Semester Electrical Technology, November 2008

(Electronics & Instrumentation Engineering, Bio-Medical Engineering and Electronics & Control Engineering)

SET-1

1. (a) State the reasons for a self exited DC shunt generator to fail to build up

voltage, and suggest the necessary modifications.

(b) Explain the importance of critical field resistance and how it can be determined.

(c) A 4-pole generator having wave-wound armature winding has 48 slots each slot

contains 20 conductors. What will be the voltage generated in the machine

when driven at 1,500rpm assuming the flux per pole to be 7.0 mWb? [7+3+6]

 

2. (a) Compare the different methods of speed control of a D.C. Shunt Motor?

(b) What is the application of divertors provided D.C. Motors?

(c) A 250V shunt motor has an armature current of 20A when running at 1000rpm

against full-load torque. The armature resistance is 0.5. What resistance

must be inserted in series with the armature to reduce the speed to 500rpm at the same torque, and what will be the speed if the load torque is halved with this resistance in the circuit. Assume the flux to remain constant throughout and neglect brush contact drop. [4+4+8]

 

3. (a) Explain the principle of operation of transformer. Derive its e. m. f. equation.

(b) A 1-phase transformer has 180 turns respectively in its secondary and primary

windings. The respective resistances are 0.233 and 0.067.

Calculate the equivalent resistance of

i. the primary in terms of the secondary winding,

ii. the secondary in terms of the primary winding, and

iii. the total resistance of the transformer in terms of the primary. [8+8]

 

4. Describe the exact and approximate equivalent circuit of a single-phase transformer.

Also describe experiments to obtain the parameters of the equivalent circuits. [16]

 

5. (a) Explain the rotor resistance starter for an induction motor.

(b) A 3-phase, 6 pole, 400 V, 50 Hz induction motor. takes a power input of 35

kW at its full-load speed of 890 r.p.m. The total stator losses are 1 kW and

the friction and windage losses are 1.5 kW.

Calculate

i. slip

ii. rotor ohmic losses

iii. shaft power

iv. shaft torque and

v. efficiency. [6+10]

 

6. (a) Derive e.m.f equation for an alternator and explain distribution factor and

pitch factor used in e.m.f. Equation.

(b) Write the expression showing the relationship between speed frequency and

no. of poles of a synchronous machine. The speed of rotation of the turbine driving an alternator is 166.7 r.p.m. What should be the no. of poles of the alternator if it is to generate voltage 50HZ. [10+6]

 

7. (a) A 500V, 50 HZ, single-phase synchronous motor take 50 A current at a power

factor of 0.8 lagging. The motor has a synchronous reactance of 2 ohm and neglible resistance. The armature has 120 full pitch coils in series, with a distribution factor of 0.95. Assuming a sinusoidal variation flux in the air gap.

Calculate the flux per pole.

(b) Draw the phasor diagram of a synchronous motor. [8+8]

 

8. (a) Draw a diagram showing the construction of a stepper motor and discuss its

operation.

(b) Discuss the various applications of stepper motor. [10+6]

 

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