11. Calculate the active power in a 168.12 H inductor.
A. 65 W
B. 0 W
C. 68 W
D. 64 W
Answer: B
The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in the case of the inductor so the angle between V & I is 90°.
P = VIcos90° = 0 W.
12. Calculate the maximum value of slip when rotor resistance is 2 Ω and rotor reactance is 3 Ω.
A. 0.66
B. 0.33
C. 0.44
D. 0.99
Answer: A
The maximum torque occurs when the slip value is equal to R2÷X2. Maximum torque is also known as breakdown torque, stalling torque, and pull-out torque. The maximum value of slip is
R2÷X2=2/3=.66.
13. Calculate the total heat dissipated in a rotor resistor of 21 Ω when.81 A current flows through it.
A. 13.77 W
B. 12.56
C. 16.78 W
D. 13.98 W
Answer: A
The rotor resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in the case of the resistor so the angle between V & I is 90°.
P=I2R=.81×.81×21=13.77 W.
14. Calculate the active power in an 8.12 F capacitor.
A. 89 W
B. 41 W
C. 0 W
D. 48 W
Answer: C
The capacitor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in the case of the capacitor so the angle between V & I is 90°.
P=VIcos90° = 0 W.
15. The stator voltage control method is a part of the slip changing technique.
A. True
B. False
Answer: A
The stator voltage control method comes under the slip changing technique. The load torque remains constant here. SV2=constant.
16. Calculate the value of the new slip using the given data: V1=12 V, S1=.1, V2=5.
A. 0.576
B. 0.247
C. 0.487
D. 0.987
Answer: A
This question is based on the concept of the stator voltage control method. The load torque remains constant.
S1V12=S2V22=constant.
S2=.576.
17. Calculate the average value of the sinusoidal waveform y(t)=4.56cos(77.64πt+4800π÷68).
A. 41 V
B. 0 V
C. 48 V
D. 78 V
Answer: B
The sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α)
where
Vm represents peak value
ω represents angular frequency
α represents a phase difference.
The average value of a sine wave is zero because of equal and opposite lobes areas. Since sine wave is an odd function then the net area of the waveform over a period is a Net area
= A+(-A) = 0.
The average value is Net area÷Time=0.
18. R.M.S value of the periodic square waveform of amplitude 20 V is _______
A. 20 V
B. 18 V
C. 17 V
D. 13 V
Answer: A
R.M.S value of the periodic square waveform is Vm and r.m.s value of the trapezoidal waveform is Vm÷3½.
The peak value of the periodic square waveform is VM. Vm=20 V.
19. Calculate the time period of the waveform y(t)=sin(.1πt)+cos(.2πt).
A. 20 sec
B. 30 sec
C. 40 sec
D. 10 sec
Answer: A
The fundamental time period of the sine and cosine waves is 2π. The time period of y(t) is L.C.M {20,10}=20 sec.
The time period is independent of phase shifting and time-shifting.
20. The V/F control is a part of the synchronous speed changing technique.
A. True
B. False
Answer: B
V/F control comes under the synchronous speed changing technique. Speed above or below synchronous speed can be achieved using V/F control.
