Electrical Machines - Online Test

Q1.
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

Answer : Option C
Explaination / Solution:
No Explaination.


Q2. 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
Answer : Option A
Explaination / Solution:
No Explaination.


Q3.
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,

Answer : Option C
Explaination / Solution:
No Explaination.


Q4.
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.
Answer : Option B
Explaination / Solution:
No Explaination.


Q5.
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
Answer : Option B
Explaination / Solution:
No Explaination.


Q6. 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
Answer : Option D
Explaination / Solution:



Q7.
The single phase, 50Hz, iron core transformer in the circuit has both the vertical arms of cross sectional area 20cm2 and both the horizontal arms of cross sectional area 10cm2. If the two windings shown were wound instead on opposite horizontal arms, the mutual inductance will

Answer : Option A
Explaination / Solution:
No Explaination.


Q8.
A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical load. The torque-speed characteristics of the motor (solid curve) and of the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B?

Answer : Option A
Explaination / Solution:
No Explaination.