DFCCIL'21 EE: Daily Practices Quiz 09-Aug-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.

Q1. A 3-phase induction motor is running at 4 % slip. If the input to the rotor is 1000 W, then the mechanical power developed by the motor is……
(a) 0.96 W
(b) 96 W
(c) 960 W
(d) 9,600 W

Q2. A washing machine generally employs a …………motor.
(a) Shaded pole
(b) Resistance split phase
(c) Single-phase series
(d) Hysteresis

Q3. A transformer has 400W as iron loss at full load. The iron loss at half full load will be:
(a) 400 W
(b) 800 W
(c) 100 W
(d) 200 W

Q4. A 220 V DC machine has an armature resistance 1 Ω. If the full load current is 20 A, the difference of induced voltage between generator and motor is
(a) 0 V
(b) 20 V
(c) 40 V
(d) 60 V

Q5. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency will
(a) Increase
(b) Decrease
(c) Remains unchanged
(d) Can’t say

Q6. The voltage gain of an ideal voltage amplifier is
(a) 0
(b) 1
(c) <1
(d) Infinity

SOLUTIONS

S1. Ans.(c)
Sol. P_m=(1-s) P_g=(1-0.04)×1000=960 W.

S2. Ans.(b)
Sol. A washing machine generally employs a resistance split-phase motor or a split-phase induction motor.

S3. Ans.(a)
Sol. Iron losses are constant for any transformer irrespective of load.

S4. Ans.(c)
Sol. in case of generator: E_g=V_t+I_a R_a
In case of motor: E_b=V_t-I_a R_a
∴E_g-E_b=2I_a R_a=2×20×1=40 V

S5. Ans.(c)
Sol. The string efficiency is independent of system frequency.

S6. Ans.(b)
Sol. For ideal voltage follower, V_out=V_in
Voltage gain unity
Both input and output are in same phase.
Used as buffer amplifier