Reluctance motors are singly-excited. The stator side of the motor is excited by an AC source. Due to excitation, a revolving magnetic field is created and the rotor rotates with a synchronous speed.
2. Reluctance motor can operate on DC supply.
A. True
B. False
Answer: A
The reluctance motor can operate on AC supply. It cannot operate on DC supply. Reluctance motors are singly-excited.
3. An infinite bus-bar has _____________
A. Constant voltage
B. Constant frequency
C. Constant voltage and frequency
D. constant speed
Answer: C
An infinite bus bar has constant voltage and constant speed. It has many types like single bus-bar, and double bus-bar. It is connected to the grid.
4. Calculate the value of the Input power factor for a 3-Φ Fully controlled rectifier if the firing angle value is 90°.
A. 0
B. .2
C. .3
D. .4
Answer: A
The value of the input power factor for a 3-Φ Fully controlled rectifier is
.95cos(90°)=0.
The input power factor is a product of the distortion factor and displacement factor.
5. Which of the following motor is used in recording instruments?
A. Reluctance motor
B. Induction motor
C. Synchronous motor
D. DC motor
Answer: A
The reluctance motor is used in recording instruments. It is used for many constant speed applications. The starting torque of the motor depends upon the rotor position.
6. Increasing the stator poles ___________ the speed of Reluctance motor.
A. Decreases
B. Increases
C. Remains the same
D. Negative
Answer: A
Increasing the stator poles decreases the speed of the Reluctance motor. The rotating magnetic field is developed which rotates at the speed of Ns. Ns∝(1÷P).
7. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 27° and the supply frequency is 50 Hz.
A. 11.8 msec
B. 14.4 msec
C. 16.1 msec
D. 16.5 msec
Answer: A
The circuit turn-off time for 3-Φ Fully controlled rectifier is (240°-α)÷ω. The value of circuit turn-off time for ∝ < 60° is
(240°-27°)÷6.28×50=11.8 msec.
8. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 120° and the supply frequency is 60 Hz.
A. 1.8 msec
B. 3.2 msec
C. 6.3 msec
D. 2.7 msec
Answer: D
The circuit turn-off time for 3-Φ Fully controlled rectifier is (180°-α)÷ω. The value of circuit turn-off time for ∝ ≥ 60° is
(180°-120°)÷6.28×60=2.7 msec.
9. Calculate peak-peak voltage if Vmax=180 V and Vmin=60 V.
A. 120 V
B. 150 V
C. 170 V
D. 110 V
Answer: A
Peak-Peak voltage is equal to the difference between the maximum and minimum voltage. It is mathematically represented as
Vp-p=Vmax-Vmin=180-60=120 V.
10. Calculate the value of the Crest factor if Vpeak=798 V and Vr.m.s=489 V.
A. 1.63
B. 1.54
C. 1.59
D. 1.26
Answer: A
The value of the crest factor is
Vpeak÷Vr.m.s=798÷489=1.63 V.
It signifies the peak value is 1.63 times the r.m.s value.
