# Electrical Machines (Test 3)

## Gate Exam : Ee Electrical Engineering

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Q.1
A single-phase transformer has a turns ratio of 1:2, and is connected to a purely resistive load as shown in the figure. The magnetizing current drawn is 1A, and the secondary current is 1A. If core losses and leakage reactance’s are neglected, the primary current is A. 1.41A
B. 2A
C. 2.24A
D. 3A
Explaination / Solution:
No Explaination.

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Q.2
A balanced three-phase voltage is applied to a star-connected induction motor, the phase to neutral voltage being V. The stator resistance, rotor resistance referred to the stator, stator leakage reactance, rotor leakage reactance referred to the stator, and the magnetizing reactance are denoted by rs , rr , xs , xr and Xm, respectively. The magnitude of starting current of the motor is given by
A. B. C. D. Explaination / Solution:
No Explaination.

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Q.3
A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is
A. 100
B. 98
C. 52
D. 48
Explaination / Solution:
No Explaination.

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Q.4
The following arrangement consists of an ideal transformer and an attenuator which attenuates by a factor of 0.8. An ac voltage VWX1 = 100 V is applied across WX to get an open circuit voltage across YZ. Next, an ac voltage VYZ2 =100V is applied across YZ to get an open circuit voltage VWX2 across WX. Then, are respectively, A. 125/100 and 80/100
B. 100/100 and 80/100
C. 100/100 and 100/100
D. 80/100 and 80/100
Explaination / Solution:
No Explaination.

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Q.5
The separately excited dc motor in the figure below has a rated armature current of 20 A and a rated armature voltage of 150 V. An ideal chopper switching at 5 kHz is used to control the armature voltage. If La= 0.1 mH, Ra= 1 Ω, neglecting armature reaction, the duty ratio of the chopper to obtain 50% of the rated torque at the rated speed and the rated field current is A. 0.4
B. 0.5
C. 0.6
D. 0.7
Explaination / Solution:
No Explaination.

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Q.6
A separately excited dc machine is coupled to a 50Hz, three-phase, 4-pole induction machine as shown in the figure. The dc machine is energized first and the machines rotate at 1600 rpm. Subsequently the induction machine is also connected to a 50Hz, three-phase source, the phase sequence being consistent with the direction of rotation. In steady state, A. both machines act as generators
B. the dc machine acts as a generator, and induction machine acts as a motor
C. the dc machine acts as a motor, and the induction machine acts a generator
D. both machines act as motors
Explaination / Solution:
No Explaination.

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Q.7
A separately excited DC motor runs at 1500 rpm under no-load with 200V applied to the armature. The field voltage is maintained at its rated value. The speed of the motor, when it delivers at torque of 5 Nm, is 1400 rpm as shown in the figure. The rotational losses and armature reaction are neglected. The armature resistance of the motor is.
A. 2Ω
B. 3.4Ω
C. 4.4Ω
D. 7.7Ω
Explaination / Solution:
No Explaination.

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Q.8
A separately excited DC motor runs at 1500 rpm under no-load with 200V applied to the armature. The field voltage is maintained at its rated value. The speed of the motor, when it delivers at torque of 5 Nm, is 1400 rpm as shown in the figure. The rotational losses and armature reaction are neglected. For the motor to deliver a torque of 2.5Nm at 1400 rpm, the armature voltage to be applied is
A. 125.5V
B. 193.3V
C. 200V
D. 241.7V
Explaination / Solution:
No Explaination.

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Q.9
A field excitation of 20 A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 2000V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200A is
A. 1V
B. 10V
C. 100V
D. 1000V
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Q.10
A three-phase, 4pole, self excited induction generator is feeding power to a load at a frequency f1. If the load is partially removed, the frequency becomes f2. If the speed of the generator is maintained at 1500 rpm in both the cases, then
A. B. C. D.  