Electrical Machines - Online Test

Q1. Leakage flux in an induction motor is
Answer : Option D
Explaination / Solution:
No Explaination.


Q2. The angle δ in the swing equation of a synchronous generator is the
Answer : Option D
Explaination / Solution:
No Explaination.


Q3. A single-phase transformer has no-load loss of 64 W, as obtained from an open circuit test. When a short-circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV windings, he measured loss is 81 W. The transformer has maximum efficiency when operated at
Answer : Option C
Explaination / Solution:
No Explaination.


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


Q5. 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,

Answer : Option C
Explaination / Solution:
No Explaination.


Q6. 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
 
Answer : Option D
Explaination / Solution:
No Explaination.


Q7. 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 
Answer : Option C
Explaination / Solution:



Q8. In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f. Which of the following statements relating to low frequency operation of the motor is TRUE?
Answer : Option B
Explaination / Solution:

During constant V/f control, at low frequency, the voltage also applied to the induction motor is low. Hence the stator flux also decreases from its rated value.

Q9.
A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.

If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is f1 and frequency of voltage across commutator brushes is f2. The values of f1 and f2 respectively are 
Answer : Option A
Explaination / Solution:



Q10. A 20-pole alternators is having 180 identical stator slots with 6 conductors in each slot. All the coils of a phase are in series. If the coils are connected to realize single-phase winding, the generated voltage is V1. If the coils are reconnected to realize three phase star connected winding, the generated phase voltage is V2. Assuming full pitch, single-layer winding, the ratio V1/V2is
Answer : Option D
Explaination / Solution: