1. The advantage of the double squirrel cage induction motor over a single cage rotor is that it’s _______
A. Efficiency is higher
B. Power factor is higher
C. Slip is larger
D. Starting current is lower
Answer: D
The starting current flows through the outer cage which has higher resistance and hence starting current is lower. This is one of the important advantages of the double squirrel cage induction motor over a single cage rotor.
2. A 16-pole, 3-phase, 60 Hz induction motor is operating at a speed of 150 rpm. The frequency of the rotor current of the motor in Hz is __________
A. 20
B. 40
C. 30
D. 10
Answer: B
Given a number of poles = 16.
Supply frequency is 60 Hz.
Rotor speed is 150 rpm.
Ns=120×f÷P
=120×60÷16 = 450 rpm.
S=Ns-Nr÷Ns
=450-150÷450=.666
F2=sf=.666×60=40 Hz.
3. Calculate the amplitude of the sinusoidal waveform z(t)=715sin(165πt+2π÷468).
A. 710
B. 715
C. 716
D. 718
Answer: B
The sinusoidal waveform is generally expressed in the form of
V=Vmsin(Ωt+α)
where
Vm represents the peak value
Ω represents the angular frequency
α represents a phase difference.
4. R.M.S value of the sinusoidal waveform V=48sin(6.5πt+89π÷46.8).
A. 33.94 V
B. 33.56 V
C. 33.12 V
D. 33.78 V
Answer: A
R.M.S value of the sinusoidal waveform is
Vm÷2½ = 48÷2½ = 33.94 V
r.m.s value of the trapezoidal waveform is Vm÷3½.
The peak value of the sinusoidal waveform is Vm.
5. The short circuit test on a 3-φ induction motor is conducted at a rotor speed of _______
A. Zero
B. < Ns
C. > Ns
D. Ns
Answer: A
Short-circuit test in an induction motor is also called a Blocked rotor test so it is conducted at zero speed. Net input power taken is equal to the variable losses.
6. If induction motor air gap power is 10 KW and mechanically developed power is 8 KW, then rotor ohmic loss will be _________ KW.
A. 1
B. 2
C. 3
D. 4
Answer: B
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
=10-8=2 KW.
7. A 3-phase induction machine draws active power P and reactive power Q from the grid. If it is operated as a generator, then P and Q will be _________
A. Positive and Negative
B. Negative and Positive
C. Positive and Positive
D. Negative and Negative
Answer: B
The induction generator will be delivering power to the bus to generate flux it will consume reactive power from the bus. Since active power is delivered the active power drawn will be negative but reactive power is absorbed and hence active power absorbed is positive.
8. The slope of the V-I curve is 39.1°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .81 Ω
B. .36 Ω
C. .75 Ω
D. .84 Ω
Answer: A
The slope of the V-I curve is resistance. The slope given is 39.1° so
R=tan(39.1°)=.81 Ω.
The slope of the I-V curve is reciprocal to resistance.
9. If induction motor air gap power is 48 KW and gross developed power is 28 KW, then rotor ohmic loss will be _________ KW.
A. 10
B. 20
C. 30
D. 40
Answer: B
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
=48-28=20 KW.
10. The power factor of a squirrel cage induction motor is ___________
A. Low at light load only
B. Low at heavy loads only
C. Low at the light and heavy loads both
D. Low at rate load only
Answer: A
At light loads, the current drawn is largely a magnetizing current due to the air gap and hence the power factor is low.
