Asynchronous Machines 4th Sem BTEE-401 B.Tech EE Nov 2016 PTU Paper

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B.Tech. (EE-2011 Batch)

ASYNCHRONOUS MACHINES

Subject Code : BTEE-401          Paper ID : [ A1208]

Time : 3 Hrs.                          Max. Marks : 60

INSTRUCTIONS TO CANDIDATES :
1. SECTION-A is COMPULSORY consisting of TEN questions carrying TWO marks each.
2. SECTION-B contains FIVE questions carrying FIVE marks each and students have to attempt any FOUR questions.
3. SECTION-C contains THREE questions carrying TEN marks each and students have to attempt any TWO questions

Asynchronous Machines 4th Sem Batch 2011 Onwards B.Tech PTU Nov 2016 Question Paper

SECTION-A

Q1. Write Short Notes on :

a. If the rotor power factor of a 3-phase induction motor is 0.866, then find the spatial displacement between the stator magnetic field and rotor magnetic field.

b. The stator of a 6-pole, 3-phase induction motor is fed from a 3-phase, 50 Hz supply which contains a pronounced 5th time harmonic. What will be the speed of the 5th space harmonic field produced by the 5th time harmonic in the stator supply?

c. Why does a 3-phase induction motor always run at a speed less than the synchronous speed?

d. How can the direction of rotation of the 3-phase induction motor be reversed?

e. A voltmeter gives 120 oscillations per minute when connected to the rotor of an induction motor. Find the slip of the motor. The stator frequency is 50 Hz.

f. In case of 3-phase induction motor, shaft power is 2700 W and mechanical losses are 180 W at a slip of 4%, find the rotor ohmic losses.

g. The rotor power output of 3-phase induction motor is 15 kW and the corresponding slip is 4%, find the rotor ohmic loss.

 

h. Compare single-phase induction motor with a polyphase induction motor having the same size and speed.

i. A 6-pole, 50 Hz single-phase induction motor runs at a speed of 900 rpm. Calculate the frequency of current in the cage rotor.

j. In a 3-phase induction motor, maximum torque of 100 Nm occurs at a slip of 10%. Calculate the torque at a slip of 5%.

   

                                      SECTION-B

Q2. Attempt any FOUR questions.                                                                    5×4=20

 2. Explain the production of torque in a 3-phase slip-ring induction motor when the rotor is running with a slips. Hence introduce the concept of load angle.

3. Discuss the following for a 3-phase induction generator :

a) It operates always at a leading power factor.

b) Application of squirrel-cage induction motor in the generating mode.

4. What are linear electrical machines? Give the principle of operation of LIM.

5. Derive the equivalent circuit of a single-phase induction motor with the help of double revolving field theory.

6. Explain how the speed of a polyphase induction motor can be controlled by injecting a voltage in the rotor circuit of a polyphase induction motor.

                                     SECTION-C

Q3. Attempt any TWO questions.                              2×10=20

 7. A 4-pole induction motor delivers 37 h.p. at the shaft at a speed of 1425 r.p.m. on 500-V, 50 Hz supply. The mechanical losses total 3 h.p. and the power factor is 0.9.Calculate for this load : (i) The slip   (ii) The rotor copper loss

(iii) The total power input if the stator losses are 2500 W  (iv) The efficiency
(v) The line current

8. (vi)The The number of complete cycles per minute of the rotor e.m.f.

test on a 3-phase star connected 400V induction motor gave the following test
results :

No load test : 400V, 20A, 5000W and -3200W Blocked rotor test : 50V, 60A, 2300W and 750W
D .C. test : applied voltage (across two windings in series) 18V, 60A Find
a) the equivalent circuit of motor,

b) slip for pull out torque and the magnitude of pull out torque.

The synchronous speed of the motor is 25 rev/sec.

9. In double-cage polyphase induction motors, explain how the desirable features of high- starting torque and low-operating slip are attained.

When the rotor power factor of a 3-phase induction motor is 0.866, then find the spatial displacement between the stator magnetic field and rotor magnetic field.

 

Stator of a 6-pole, 3-phase induction motor is fed from a 3-phase, 50 Hz supply which contains a pronounced 5th time harmonic. What will be the speed of the 5th space harmonic field produced by the 5th time harmonic in the stator supply.

 

Explain Why does a 3-phase induction motor always run at a speed less than the synchronous speed.

 

Explain How can the direction of rotation of the 3-phase induction motor be reversed.

 

Voltmeter gives 120 oscillations per minute when connected to the rotor of an induction motor. Find the slip of the motor. The stator frequency is 50 Hz.

 

3-phase induction motor, shaft power is 2700 W and mechanical losses are 180 W at a slip of 4%, find the rotor ohmic losses.

 

Rotor power output of 3-phase induction motor is 15 kW and the corresponding slip is 4%, find the rotor ohmic loss.

Differentiate single-phase induction motor with a polyphase induction motor having the same size and speed.

 

6-pole, 50 Hz single-phase induction motor runs at a speed of 900 rpm. Calculate the frequency of current in the cage rotor.

 

A 3-phase induction motor, maximum torque of 100 Nm occurs at a slip of 10%. Calculate the torque at a slip of 5%.

Asynchronous Machines 4th Sem BTEE-401 B.Tech EE PTU Nov 2016 Paper Click Here

Production of torque in a 3-phase slip-ring induction motor when the rotor is running with a slips. Hence introduce the concept of load angle.

 

A 3-phase induction generator :

a) It operates always at a leading power factor.

b) Application of squirrel-cage induction motor in the generating mode.

 

Linear electrical machines

Principle of operation of LIM.

 

Equivalent circuit of a single-phase induction motor with the help of double revolving field theory.

 

How the speed of a polyphase induction motor can be controlled by injecting a voltage in the rotor circuit of a polyphase induction motor.

Asynchronous Machines 4th Sem BTEE-401 B.Tech EE PTU Nov 2016 Paper Click Here

Asynchro nous Machines 4th Sem BTEE-401 B.Tech EE PTU Nov 2016 Paper

4-pole induction motor delivers 37 h.p. at the shaft at a speed of 1425 r.p.m. on 500-V, 50 Hz supply. The mechanical losses total 3 h.p. and the power factor is 0.9.

Calculate for this load :

(i) The slip

(ii) The rotor copper loss

(iii) The total power input if the stator losses are 2500 W

(iv) The efficiency
(v) The line current

 

Number of complete cycles per minute of the rotor e.m.f.

test on a 3-phase star connected 400V induction motor gave the following test
results :

No load test : 400V, 20A, 5000W and -3200W Blocked rotor test : 50V, 60A, 2300W and 750W
D .C. test : applied voltage (across two windings in series) 18V, 60A Find
a) the equivalent circuit of motor,

b) slip for pull out torque and the magnitude of pull out torque.

The synchronous speed of the motor is 25 rev/sec.

 

Double-cage polyphase induction motors, explain how the desirable features of high- starting torque and low-operating slip are attained.

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