A. Magnetic field
B. Electrostatic field
C. Nuclear field
D. Gravitational field

Answer: A

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.

32. In DC series machine ∅ is directly proportional to Ia.

A. True
B. False

Answer: A

In a 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∝∅.

33. For faster braking, the series field is separated from the armature circuit.

A. True
B. False

Answer: A

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

34. 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

Answer: A

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

35. 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

Answer: B

The fundamental time period of the cot wave is π. The time period of x(t) is π÷π=1 sec. The time period is independent of phase shifting and time-shifting.

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36. In dynamic braking, DC series motor behaves as a ____________

A. Separately excited motor
B. Transformer
C. Induction motor
D. Thyristor

Answer: A

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

37. 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

Answer: A

In a DC series motor, field windings are connected in series with the armature circuit.

F=NIa=R∅. Ia∝∅

∅=F÷R=5 wb.

38. The shape of the speed-armature current characteristics in DC series motor is __________

A. Hyperbolic
B. Parabola
C. Circle
D. Ellipse

Answer: A

The shape of the speed-armature current characteristics in the DC series motor is hyperbolic. The motor equation is

Eb=Vt-IaRa=Km∅ωm.

N=(Vt/KmIa)-Ra/Km.

N∝1÷Ia.

39. The shape of the speed-torque current characteristics in DC series motor is __________

A. Rectangular Hyperbolic
B. Parabola
C. Circle
D. Ellipse

Answer: A

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.

40. DC series motors are used where the high ________ torque is required.

A. Maximum
B. Minimum
C. Starting
D. Stalling

Answer: C

DC series motors are used where a 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.