31. Calculate the total heat dissipated in a rotor resistor of 14.23 Ω when.65 A current flows through it.
A. 6.45 W
B. 6.01 W
C. 6.78 W
D. 6.98 W
Answer: B
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=.65×.65×14.23=6.01 W.
32. Calculate mark to space ratio if the system is on for 4.3 sec and off for 78.2 sec.
A. .054
B. .047
C. .039
D. .018
Answer: A
Mark to space is Ton÷Toff. It is the ratio of the time for which the system is active and the time for which is inactive.
M = Ton÷Toff = 4.3÷78.2 = .054.
33. In the rotor voltage injection method, when an external voltage source is in phase with the main voltage then speed will ___________
A. Increase
B. Decrease
C. Remain unchanged
D. First increases then decrease
Answer: A
In the rotor injection method, when an external voltage is in phase with the main voltage net voltage increases and the value of slip decreases, and the value of rotor speed increases.
34. A 2-pole, 3-phase, ______ Hz induction motor is operating at a speed of 550 rpm. The frequency of the rotor current of the motor in Hz is 2.
A. 9.98
B. 9.71
C. 9.12
D. 9.37
Answer: D
Given a number of poles = 2.
The rotor speed is 550 rpm.
Ns=120×f÷P=120×f÷2 = 60f rpm.
S=Ns-Nr÷Ns. F2=sf. S=F2÷f.
The supply frequency is 9.37 Hz.
35. Calculate the average value of the sinusoidal waveform x(t)=848sin(1.65πt+2π÷0.68).
A. 0
B. 78 V
C. 15 V
D. 85 V
Answer: A
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.
36. R.M.S value of the periodic square waveform of amplitude 72 V.
A. 72 V
B. 56 V
C. 12 V
D. 33 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.
37. In the rotor voltage injection method, when an external voltage source is in the opposite phase to the main voltage then speed will ___________
A. Increase
B. Decrease
C. Remain unchanged
D. First increases then decrease
Answer: B
In the rotor injection method, when an external voltage is in the opposite phase to the main voltage net voltage decreases and the value of slip increases, and the value of rotor speed decreases.
38. The rotor injection method is a part of the slip-changing technique.
A. True
B. False
Answer: A
The rotor injection method comes under the slip changing technique. It uses an external voltage source to change the slip value. The load torque remains constant here.
39. The slip recovery scheme is a part of the synchronous speed-changing technique.
A. True
B. False
Answer: A
The slip recovery scheme comes under the synchronous speed changing technique. It uses an external induction machine to change the frequency value. The load torque remains constant here.
40. The slope of the V-I curve is 9.1°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .16 Ω
B. .26 Ω
C. .25 Ω
D. .44 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 9.1° so R=tan(9.1°)=.16 Ω.
The slope of the I-V curve is reciprocal to resistance.
