21. Calculate the useful power developed by a motor using the given data: Pin = 10 W, Ia = .6 A, Ra=.2 Ω. Assume frictional losses are 2 W and windage losses are 3 W.
A. 4.928 W
B. 1.955 W
C. 1.485 W
D. 1.488 W
Answer: A
Useful power is basically the shaft power developed by the motor that can be calculated using the formula
Psh = Pdev-(rotational losses).
Pdev = Pin-Ia2Ra = 10-.62(.2)=9.92 W.
The useful power developed by the motor is
Psh = Pdev-(rotational losses) = 9.92 –(5) = 4.928 W.
22. The slope of the V-I curve is 86.5°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. 16.34 Ω
B. 15.88 Ω
C. 48.43 Ω
D. 54.57 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 16.34° so R=tan(16.34°)=16.34 Ω.
The slope of the I-V curve is reciprocal to resistance.
23. Calculate the active power in a 7481 H inductor.
A. 1562 W
B. 4651 W
C. 0 W
D. 4654 W
Answer: C
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.
Voltage leads the current in the case of the inductor.
24. Calculate the active power in a 56 F capacitor.
A. 6.45 W
B. 0 W
C. 15.45 W
D. 14.23 W
Answer: B
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.
Current leads the voltage in the case of the capacitor.
25. Calculate the active power in .an 18.064 H inductor.
A. 4.48 W
B. 17.89 W
C. 0 W
D. 25.45 W
Answer: C
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.
26. Calculate the active power in a 1.7 Ω resistor with a 1.8 A current flowing through it.
A. 5.5 W
B. 5.1 W
C. 5.4 W
D. 5.7 W
Answer: A
The 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 0°.
P=I2R=1.8×1.8×1.7=5.5 W.
27. Calculate the value of the time period if the frequency of the signal is .48 sec.
A. 2 Hz
B. 3 Hz
C. 7 Hz
D. 9 Hz
Answer: A
The time period is defined as the time after the signal repeats itself. It is expressed in second.
T = 1÷F=1÷.48=2 Hz.
28. Choose the correct in the case of V/F control.
A. Ns-Nr=constant
B. Ns*Nr=constant
C. Ns%Nr=constant
D. Ns+Nr=constant
Answer: A
In variable frequency control, Ns-Nr remains constant.
V/f control is part of the synchronous speed changing technique.
It is the most used technique in controlling the induction motor.
29. A 32-pole, 3-phase, 70 Hz induction motor is operating at a speed of 112 rpm. The frequency of the rotor current of the motor in Hz is __________
A. 40.2
B. 46.1
C. 40.1
D. 40.6
Answer: C
Given a number of poles = 32.
Supply frequency is 70 Hz.
Rotor speed is 112 rpm.
Ns = 120×f÷P
=120×70÷32 = 262.5 rpm.
S=Ns-Nr÷Ns
= 262.5-112÷262.5=.573.
F2=sf=.573×70=40.1 Hz.
30. A 20-pole, 3-phase, 90 Hz induction motor is operating at a speed of _______ rpm. The frequency of the rotor current of the motor in Hz is 20.
A. 418.56
B. 420.12
C. 421.23
D. 422.45
Answer: B
Given a number of poles = 20.
The supply frequency is 90 Hz.
