# Electric Drives Questions and Answers – Dynamic Braking of DC Series Motors

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

1. Dynamic braking requires a strong _________
a) Magnetic field
b) Electrostatic field
c) Nuclear field
d) Gravitational field

Explanation: Dynamic braking requires a strong magnetic field to convert energy from the magnetic domain into the electrical domain. The kinetic energy is wasted at a higher rate.

2. In DC series machine ∅ is directly proportional to Ia.
a) True
b) False

Explanation: In series DC machine, the field winding is in series with the armature winding. The armature current flows through the field winding. F=NIa=R∅. Ia∝∅.

3. For faster braking, the series field is separated from the armature circuit.
a) True
b) False

Explanation: For faster braking, the series field is separated from the armature circuit. The series field is excited by a different voltage source.

4. At the starting, the field is very strong but it decreases with time because of the ________
a) Reduction in armature current
b) Increase in armature current
c) Reduction in speed
d) Increase in speed

Explanation: At the starting, the field is very strong but it decreases with time because of the reduction in armature current. ∅∝Ia.

5. Calculate the time period of the waveform x(t)=1cot(πt+7π).
a) 0.5 sec
b) 1 sec
c) 3 sec
d) 5 sec

Explanation: The fundamental time period of the cot wave is π. The time period of x(t) is a π÷π=1 sec. The time period is independent of phase shifting and time shifting.
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6. In dynamic braking, DC series motor behaves as a ____________
a) Separately excited motor
b) Transformer
c) Induction motor
d) Thyristor

Explanation: In dynamic braking, DC series motor behaves as a separately excited motor as the motor armature is disconnected from the source and connected across resistance RB.

7. Calculate the flux produced by the DC series field winding using the following data: Ia=5 A, N=23, R=23.
a) 5 Wb
b) 6 Wb
c) 9 Wb
d) 8 Wb

Explanation: In DC series motor, field windings are connected in series with the armature circuit. F=NIa=R∅. Ia∝∅. ∅=F÷R=5 wb.

8. The shape of the speed-armature current characteristics in DC series motor are __________
a) Hyperbolic
b) Parabola
c) Circle
d) Ellipse

Explanation: The shape of the speed-armature current characteristics in DC series motor is hyperbolic. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt/KmIa)-Ra/Km. N∝1÷Ia.

9. The shape of the speed-torque current characteristics in DC series motor are __________
a) Rectangular Hyperbolic
b) Parabola
c) Circle
d) Ellipse

Explanation: The shape of the speed-torque characteristics in the DC series motor is a rectangular hyperbola. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt/KmIa)-Ra/Km. Ia∝√T .N∝1÷√T.

10. DC series motors are used where the high ________ torque is required.
a) Maximum
b) Minimum
c) Starting
d) Stalling

Explanation: DC series motors are used where the high starting torque is required. At starting the torque produced by DC series motor is very high. To start a motor torque should be greater than load torque.

11. The shape of the current-torque current characteristics in DC series motor is a __________
a) Rectangular Hyperbolic
b) Parabola
c) Circle
d) Ellipse

Explanation: The shape of the current-torque characteristics in the DC series motor is a parabola. The motor equation is Eb=Vt-IaRa=Km∅ωm. T=Km∅Ia. ∅∝Ia. T∝Ia2.

12. Calculate the current in DC series field winding using the following data: Ia=3 A.
a) 4 A
b) 2 A
c) 3 A
d) 6 A

Explanation: In DC series motor, field windings are connected in series with the armature circuit. The armature current will also flow through the series winding. If=Ia=3 A.

13. For a given torque, increasing diverter resistance of a DC series motor causes the ________
a) decrease in speed and less armature current
b) decrease in speed but armature current remains the same
c) increase in speed and more armature current
d) increase in speed but armature current remains the same

Explanation: In DC series motor, increasing diverter resistance increases the field current and field flux and also decreases the armature current and therefore decreases the speed.

14. The field diverter resistance for a DC series motor is kept ________
a) High
b) Low
c) zero
d) one

Explanation: The field diverter resistance for a DC series motor is kept low in order to increase the speed of the motor. This will reduce the field flux and increase the armature current.

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