1. If induction motor rotor power is 157.5 KW and gross developed power is 79.9 KW, then rotor ohmic loss will be _________ KW.
a) 77.5
b) 77.6
c) 76.9
d) 77.1
Explanation: Rotor ohmic losses are due to the resistance of armature windings. Net input power to the rotor is equal to the sum of rotor ohmic losses and mechanically developed power. Rotor ohmic losses=Air gap power-Mechanical developed power=157.5-79.9=77.6 KW.
2. The power factor of a squirrel cage induction motor generally is ___________
a) .6-.8
b) .1-.2
c) .2-.4
d) .5-.7
Explanation: At light loads, the current drawn is largely a magnetizing current due to the air gap and hence the power factor is low. The power factor of a squirrel cage induction motor generally is .6-.8.
3. Calculate the active power in a .89 H inductor.
a) 1.535 W
b) 0 W
c) 2.484 W
d) 1.598 W
Explanation: 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 case of the inductor so the angle between V & I is 90°. P=VIcos90° = 0 W.
4. Calculate the time period of the waveform i(t)=sin(πt+6π÷4)+sin(πt+5π÷6).
a) 2 sec
b) 4 sec
c) 5 sec
d) 3 sec
Explanation: The fundamental time period of the sine wave is 2π. The time period of i(t) is L.C.M {2,2}=2 sec. The time period is independent of phase shifting and time shifting.
5. 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
Explanation: 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 case of the resistor so the angle between V & I is 90°. P=I2R=.65×.65×14.23=6.01 W.
6. 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
Explanation: Mark to space is Ton÷Toff. It is the ratio of time for which the system is active and the time for which is inactive. M = Ton÷Toff = 4.3÷78.2 = .054.
7. 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
Explanation: 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.
8. 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
Explanation: Given a number of poles = 2. Rotor speed is 550 rpm. Ns=120×f÷P=120×f÷2 = 60f rpm. S=Ns-Nr÷Ns. F2=sf. S=F2÷f. Supply frequency is 9.37 Hz.
9. 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
Explanation: 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
10. 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
Explanation: 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.