Electrical Machines

Electrical Machines

Electrical Machines
| Electrical Machines |
Q.1
A 4 – point starter is used to start and control the speed of a
A. dc shunt motor with armature resistance control
B. dc shunt motor with field weakening control
C. dc series motor
D. dc compound motor
Answer : Option A
Explaination / Solution:
No Explaination.


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Q.2
A three-phase, salient pole synchronous motor is connected to an infinite bus. Ig is operated at no load a normal excitation. The field excitation of the motor is first reduced to zero and then increased in reverse direction gradually. Then the armature current
A. Increases continuously
B. First increases and then decreases steeply
C. First decreases and then increases steeply
D. Remains constant
Answer : Option B
Explaination / Solution:
No Explaination.


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Q.3
A single phase air core transformer, fed from a rated sinusoidal supply, is operating at no load. The steady state magnetizing current drawn by the transformer from the supply will have the waveform
A.
B.
C.
D.
Answer : Option C
Explaination / Solution:

It is an air core transformer. So, there is no saturation effect.

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Q.4
A 220 V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1.0 Ω and armature current is 10A. of the excitation of the machine is reduced by 10%, the extra resistance to be put in the armature circuit to maintain the same speed and torque will be 
A. 1.79 Ω
B. 2.1 Ω
C. 18.9 Ω
D. 3.1 Ω
Answer : Option A
Explaination / Solution:



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Q.5
In a constant V/f induction motor drive, the slip at the maximum torque
A. is directly proportional to the synchronous speed.
B. remains constant with respect to the synchronous speed.
C. has an inverse relation with the synchronous speed.
D. has no relation with the synchronous speed.
Answer : Option C
Explaination / Solution:



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Q.6
Consider a system consisting of a synchronous generator working at a lagging power factor, a synchronous motor working at an overexcited condition and a directly grid-connected induction generator. Consider capacitive VAr to be a source and inductive VAr to be a sink of reactive power. Which one of the following statements is TRUE?
A. Synchronous motor and synchronous generator are sources and induction generator is a sink of reactive power.
B. Synchronous motor and induction generator are sources and synchronous generator is a sink of reactive power.
C. Synchronous motor is a source and induction generator and synchronous generator are sinks of reactive power.
D. All are sources of reactive power.
Answer : Option A
Explaination / Solution:



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Q.7
A 4-pole, lap-connected, separately excited dc motor is drawing a steady current of 40 A while running at 600 rpm. A good approximation for the waveshape of the current in an armature conductor of the motor is given by
A.
B.
C.
D.
Answer : Option C
Explaination / Solution:



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Q.8
If an ideal transformer has an inductive load element at port 2 as shown in the figure below, the equivalent inductance at port 1 is

A. nL
B. n2L
C. n/L
D. n2/L
Answer : Option B
Explaination / Solution:



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Q.9
A three –phase 440V, 6 pole, 50Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect to stator magnetic field are
A. zero, - 5 rpm
B. zero, 955 rpm
C. 1000rpm, -5rpm
D. 1000rpm, 955rpm
Answer : Option A
Explaination / Solution:



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Q.10
The direct axis and quadrature axis reactance’s of a salient pole alternator are 1.2p.u and 1.0p.u respectively. The armature resistance is negligible. If this alternator is delivering rated kVA at upf and at rated voltage then its power angle is
A.
B.
C.
D.
Answer : Option B
Explaination / Solution:



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