A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

Which of the following statements are true? a. As load increases armature current decreases, regardless of excitation. b. For under and over excited motors, power factor tends to approach zero with increase in load c. With normal excitation when load is increased, change in armature current is greater than in power factor which tends to become increasingly leading. d. Both with under and over excitation, change in power factor is greater than in armature current with increase in load.

A

a, b and c are true

B

Only c is true

C

Only d is true

D

None of the above

Consider the following statements: In the above figure, the curves pertain to the DC motor A. Speed vs Armature current characteristic of a DC shunt motor B. Torque vs Armature current characteristic of a DC shunt motor C. Speed vs Armature current characteristic of a DC series motor D. Torque vs Armature current characteristic of a DC series motor

A

A - 4, B - 1, C - 3, D - 2

B

A - 2, B - 1, C - 3, D - 4

C

A - 4, B - 3, C - 1, D - 2

D

A - 2, B - 3, C - 1, D - 4

When the load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because:

A

back emf becomes less than applied voltage V

B

net resultant voltage in the amature is increased

C

power factor is decreased

D

motor speed is reduced

When excitation of synchronous motor is increased upto normal excitation from under excitation, armature current

A

increases

B

decreases

C

remains constant

D

None of the above

In case of synchronous motor, which of the following are true? a. Synchronous motor is not self-starting b. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, motor vibrates. c. Variable frequency supply is used to start synchronous motor. d. By using frequency converter, a synchronous motor can be brought from standstill to its desired speed.

A

Only a and b are true

B

Only b and c are true

C

All are true

D

None of the above

A synchronous motor on load draw a current at a leading power factor angle ϕ. If the internal power factor angle which is the phase angle between the excitation emf and the current in the time phasor diagram is ψ. Then the air gap excitation mmf lags the armature mmf by

A

Ψ

B

π/2 + ψ

C

π/2 – ψ

D

Ψ+ ϕ

A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

A

power factor as well as armature current will decrease

B

power factor as well as armature current will increase

C

power factor will increase but armature current will decrease

D

power factor will decrease and armature current will increase

A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have

A

no change of power factor

B

lagging power factor with over-excitation

C

leading power factor with under-excitation

D

leading power factor with over-excitation

Consider the following statements regarding synchronous machines: 1. When a synchronous motor is over-excited, its back emf is greater than the supply voltage 2. When a synchronous motor is over-excited, its power factor is leading 3. Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impractical Which of these statements are correct?

A

1 and 2

B

1 and 3

C

2 and 3

D

1, 2 and 3

A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will

A

remain same

B

go down

C

improve

D

none of the above

A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

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