# Electric Drives Questions and Answers – Electric Motors – Dynamic Braking

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Electric Motors – Dynamic Braking”.

1. Which braking method is the best method for obtaining high braking torque?
a) Regenerative braking
b) Plugging
c) Dynamic braking
d) Rheostatic braking

Explanation: Plugging is the best braking method among all braking techniques. In plugging the value of the armature current reverses and the mechanical energy is extracted. A very high braking torque is produced in case of plugging.

2. The polarity of back e.m.f changes in which of the method?
a) Plugging
b) Regenerative braking
c) Dynamic braking
d) Rheostatic braking

Explanation: In case of Plugging braking technique polarity of the back e.m.f voltage changes due to which current direction changes and mechanical, electrical energy are wasted in the form of heat in resistors.

3. Full form of WRIM.
a) Wound round induction motor
b) World cage induction motor
c) Wolf cage induction motor
d) Squirrel cage inverter motor

Explanation: WRIM stands for wound round induction motor. WRIM rotor is made up of armature conductors. There are no copper bars, rods involved in it.

4. Calculate the energy stored in the capacitor if the voltage across the capacitor is 20 V and capacitance value is 2 F.
a) 400 J
b) 200 J
c) 100 J
d) 50 J

Explanation: The energy stored in the capacitor is .5×C×V2. It is the total amount of energy stored in the capacitor in the steady state condition. E=.5×C×V2=.5×2×20×20=400 J.

5. Calculate the energy stored in the inductor if the current value is 4 A and inductance value is 1 H.
a) 4 J
b) 2 J
c) 8 J
d) 5 J

Explanation: The energy stored in the inductor is .5×L×I2. It is the total amount of energy stored in the inductor in the steady state condition. E=.5×L×I2=.5×1×4×4=8 J.

6. All networks are always circuits.
a) True
b) False

Explanation: The network is defined as the interconnection of electrical elements that may or may not has a closed path. The circuit is defined as the interconnection of an element that must have at least one closed path. All networks are not always circuits but vice-versa is true.

7. The Thevenin voltage of a dead circuit is zero volts.
a) True
b) False

Explanation: In the case of the dead network there are no independent sources available in the electrical circuit. The Thevenin voltage of the circuit is zero because no force is available to drive the current in the circuit.

8. During the open circuit condition, the current is equal to ______
a) 1 A
b) 0 A
c) 4 A
d) 7 A

Explanation: During the open circuit condition, the current is equal to zero amperes. According to Ohm’s law, I=V/R=0 A. The value of the voltage can be positive, negative or zero.

9. The minimum value of the impedance in case of the series RLC network is ___________
a) R
b) 0
c) Xl
d) Xc

Explanation: The minimum value of the series impedance in case of the series RLC network is R. During resonance condition Xl=Xc so Z=(R2+(Xl-XC)2).5=R.

10. Calculate the quality factor for the purely inductive coil.
a) inf
b) 10
c) 3
d) 6

Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The purely inductive coil always absorb reactive power. The value of active power consumed is 0. Quality factor=Q/0=inf.

11. Calculate the quality factor for the pure capacitor.
a) inf
b) 0
c) 1
d) 2

Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The pure capacitor always absorbs reactive power. The value of active power consumed is 0. Quality factor=Q/0=inf.

12. Calculate the equivalent inductance when n inductors are connected in series of values L.
a) n2L H
b) nL H
c) .5L H
d) 2 H

Explanation: When two inductors are connected in series their equivalent inductance is equal to the sum of the individual inductances. Leq=L+L+…..+n times L=nL H.

13. Calculate the equivalent resistance when two resistances are connected in series of values 12 Ω, 10 Ω.
a) 32 Ω
b) 22 Ω
c) 47 Ω
d) 17 Ω

Explanation: When two resistances are connected in series their equivalent resistance is equal to the sum of the individual resistances. Req=R1+R2=22 Ω.

14. Reactive power is positive in the case of the inductor.
a) True
b) False

Explanation: The reactive power is defined as the product of the voltage, current, and sine of the difference between the phase angle of voltage and current. In the case of an inductor, the voltage leads the current. The value of sine is positive and reactive power is positive.

15. Reactive power is positive in the case of the capacitor.
a) True
b) False 