DFCCIL’21 EE: Daily Practices Quiz 22-July-2021

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

Q1. In an induction motor, air gap power is 12 kw and mechanically developed power is 8 kw. What would be the ohmic losses?
(a) 4 kW
(b) 1.5 kW
(c) 2 kW
(d) 0.5 kW

Q2. The power input in blocked rotor test performed on a three-phase induction motor is approximately equal to:
(a) Eddy current loss in the core
(b) Iron loss in the core
(c) Hysteresis loss in the core
(d) I^2 R loss in the winding

Q3. In star connected system the phase angle difference between line and phase voltage is:
(a) 30°
(b) 120°
(c) 60°
(d) 90°

Q4. The phase-lead compensation is used to
(a) Increase rise time and decrease overshoot
(b) Decrease both rise time and overshoot
(c) Increase both rise time and overshoot
(d) Decrease rise time and increase overshoot.

Q5. The type of cascade compensator to be used for improving damping of underdamped control system is
(b) Phase-lag
(d) Notch filter

Q6. A voltage follower is used as
1. Isolation amplifier
2. A buffer amplifier
Which of the above statements is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) None of the above

SOLUTIONS
S1. Ans.(a)
Sol. Ohmic losses=air gap power or rotor input power-mechanical power(P_m )
=12-8=4 kW

S2. Ans.(d)
Sol. Blocked rotor test of a 3-phase induction motor:
Same as short-circuit test of a transformer
Low voltage is applied on the stator terminal
Full load current passes through stator by which full load copper loss(I^2 R)
Can be calculated.

S3. Ans.(a)
Sol. for star-connection

Note: – for balanced star-connected system V_L=√3 V_ph and line voltage leads the phase voltage by 30°.

S4. Ans.(b)
Sol. Phase-lead compensator is used to decrease both rise time and overshoot.

S5. Ans.(a)
Sol. Phase-lead compensators are used to reduce steady state error as well as to improve damping. It is also used to decrease both rise time and overshoot.

S6. Ans.(c)
Sol. For Voltage follower:
both the input and output voltages are the same.
voltage gain is equivalent to 1.
It is also known as unity gain, buffer & isolation amplifier.
A voltage follower can be used as a buffer because it draws very little current due to the high input impedance of the amplifier, thus eliminating loading effects while still maintaining the same voltage at the output.

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