DFCCIL’21 EE: Daily Practices Quiz 16-July-2021

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

Q1. In repulsion motor, zero torque is developed when
(a) Brush axis is 45⁰ electrical to field axis
(b) Brush axis is 90⁰ electrical to field axis
(c) Brush axis coincides with the field axis
(d) Both b & c

Q2. A synchronous motor working at leading power factor can be used as
(a) Current booster
(b) Voltage booster
(c) Power factor correction
(d) All of the above

Q3. Reactance relay is used for protection in:
(a) Long transmission lines
(b) Short transmission lines
(c) Medium transmission lines
(d) Both long transmission lines and short transmission lines

Q6. A dynamometer type wattmeter responds to the
(a) average value of active power
(b) average value of reactive power
(c) peak value of active power
(d) peak value of reactive power

SOLUTIONS
S1. Ans.(d)
Sol. In repulsion motor, no electromagnetic torque is developed when the angle between the stator and rotor magnetic flux axis is either 0 or 90°. But actually, the brush axis occupies a position somewhere in between α = 0° and α = 90° as shown in figure below.

NOTE: In repulsion motor, maximum torque is developed when Brush axis is 45⁰ electrical to field axis.

S2. Ans.(c)
Sol. An over-excited synchronous motor has a leading power factor. This makes it useful for power factor correction of industrial loads. If DC field excitation of a synchronous motor is such that back EMF Eb is greater than applied voltage V, then the motor is said to be over excited. An over excited synchronous motor draws leading current.

S3. Ans.(b)
Sol. Relay used for different configuration of transmission lines……
• Short transmission line: reactance relay
• Medium transmission line: impedance relay
• Long transmission line: mho relay

S6. Ans.(a)
Sol. A dynamometer gives average value of active power over a cycle.
P=V_rms I_rms cosϕ

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