1. The ability of a material to remain magnetized after removal of the magnetizing force is known as
2. When a solenoid is activated, the force that moves the plunger is
an electromagnetic field
a permanent magnetic field
a steady current
3. Which of the following capacitors is polarized
4. In a series resonant band-pass filter, a lower value of Q results in
a higher resonant frequency
a smaller bandwidth
a higher impedance
a larger bandwidth
5. A steady-state condition is reached when
the output voltage reaches the average value of the input voltage
the output voltage reaches the input voltage
the output voltage reaches approximately 63% of the input voltage
the output voltage reaches the effective value of the input voltage
6. An RC differentiator acts as a
7. A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
Explanation: NS = 120f /P = 120 x 50 /4 =1500rpm
N = NS ( 1-s) = 1500 (1-0.05) = 1425
\relative speed = 1500 – 1425 = 75 rpm
8. A ceiling fan uses
capacitor start and capacitor run motor
capacitor start motor.
9. The drive motor used in a mixer-grinder is a
10. A 1:5 step-up transformer has 120V across the primary and 600 ohms resistance across the secondary. Assuming 100% efficiency, the primary current equals
Explanation: I1= V1 /R1 = 120/600 = 0.2 (h = 100%, losses are zero \V1 = VR = I1R1)
11. A 50 Hz, 3-phase induction motor has a full load speed of 1440 r.p.m. The number of
poles of the motor are
Explanation: N= Ns (1-S) = NS –NS x S
1440 = Ns (1-S)
Ns = 1440 / (1-S)
Ns = (120 f/ p) = 120 x 50/p = 6000 p
Ns will be closer to N i.e 1440
When P=2 ; Ns = 3000 rpm , not close to N
When P=4 ; Ns = 1500 rpm , it is closer to N
Therefore P =4 for N=1440
12. In a 3-phase synchronous motor
the speed of stator MMF is always more than that of rotor MMF.
the speed of stator MMF is always less than that of rotor MMF.
the speed of stator MMF is synchronous speed while that of rotor MMF is zero
rotor and stator MMF are stationary with respect to each other.
Explanation: Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore,
rotor & stator mmf are stationary w.r.t each other.
13. In a three phase transformer, if the primary side is connected in star and secondary
side is connected in delta, what is the angle difference between phase voltage in the
delta side lags by -30°.
star side lags by -30°.
delta side leads by 30°.
star side leads by -30°.
Explanation: This is vector group and has +30° displacement. Therefore, delta side leads by +30°.
14. Slip of the induction machine is 0.02 and the stator supply frequency is 50 Hz.
What will be the frequency of the rotor induced emf?
Explanation: Given : s = 0.02 ; f = 50 Hz
Therefore, frequency of rotor induced emf = s f
= 0.02 x 50 = 1.0 Hz
15. A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4 mWb and the total number of conductors are 1000. What is the value of emf?
Explanation: Given N = 1500 rpm, F = 0.4 mWb, Z = 1000, P = 4, & A= 4
Therefore, Eb = NFPZ / 60 A
= 1500 x 0.4 x 4 x 1000 x 10-3 / 60 x 4
= 60/6 = 10 volts