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81) The brush voltage drop in a DC motors is usually of the order of

1. 2 V
2. 20 V
3. 50 V
4. 100 V
EXPLANATION
The voltage drop for metal graphite brush is least around 0.5V-1V whereas the voltage drop for carbon or graphite brushes is around 1.5V-2.5V.

82) The speed of a DC shunt motor can be controlled by

1. Changing the field resistance
2. Changing the terminal voltage applied to the armature
3. Inserting a resistor in series with the armature circuit
4. Any of the above
EXPLANATION
The different methods of speed control in DC shunt motors are:

1. Flux Control Method
2. Armature Control Method
3. Voltage Control Method

83) When the direction of power flow reverses, a diferentially compounded motor becomes

1. A diferentially compounded generator
2. A cumulatively compounded generator
3. A shunt motor
4. A series motor
EXPLANATION
Under Construction.

84) In a permanent magnet DC motor which method of speed control cannot be used?

1. Armature voltage variation
2. Armature resistance control
3. Varying the field current
4. All of the above
EXPLANATION
The speed control of the PMDC motor cannot be controlled by using flux control method as the flux remains constant in this type of motor. Both speed and torque can be controlled by armature voltage control, armature rheostat control, and chopper control methods. These motors are used where the motor speed below the base speed is required as they cannot be operated above the base speed.

85) When the direction of power flow reverses, a cumulatively compounded motor become

1. A diferentially compounded generator
2. A cumulatively compounded generator
3. A shunt generator
4. A series generator
EXPLANATION
Under Construction.

86) Frog leg winding is

1. Same as simplex winding
2. Same as duplex winding
3. Combined lap and wave winding on a single rotor
4. Duplex wave winding on a single rotor
EXPLANATION
Frog leg winding is a combination of a multiplex wave and a simplex lap winding in the same slots. It retains the advantages of both lap and wave windings without their inherit disadvantages.

87) The commutating winding is basically used in a DC machine

1. To reduce armature reaction
2. To improve commutation
3. To control the speed of machine
4. None of the above
EXPLANATION
No answer description required for this question.

88) The commutating pole winding is connected in

1. Series with the armature
2. Parallel with the armature
3. Series parallel with the armature
4. None of the above
EXPLANATION
The commutating pole winding is connected in series with the armature winding so that the flux it produces will vary with armature current. This series connection is necessary since the commutating e.m.f required for satisfactory commutation varies with armature current.

89) Commutating poles are also known as

1. Main poles
2. Primary poles
3. Interpoles
4. None of the above
EXPLANATION
No answer description required for this question.

90) In a DC compound machine,

1. Shunt field winding plays dominant role
2. Series field winding plays dominant role
3. Either can play dominant role
4. Both play equally dominant role
EXPLANATION
No answer description required for this question.

91) The compensating winding is connected in

1. Parallel with armature
2. Series with armature
3. Series parallel with armature
4. None of the above
EXPLANATION
No answer description required for this question.

92) A compensating winding is an auxiliary winding embedded in slots in

1. The pole faces of main poles
2. The armature
3. Partly in pole faces, partly in armature
4. None of the above
EXPLANATION
No answer description required for this question.

93) A 10 pole DC machine has a lap wound armature. It has 600 conductors, each of resistance 0.05 Ω. What is the armature resistance ?

1. 1.5 Ω
2. 1 Ω
3. 3 Ω
4. 0.3 Ω
EXPLANATION
There are 10 parallel paths, each of resistance = 0.05 ∗ (600/2) = 3 Ω.

Armature resistance, Ra = Resistance of each parallel path/10 = 3/10 = 0.3 Ω.

94) Motor starters are essential for

1. Accelerating the motor
2. Starting the motor
3. Avoiding excessive starting current
4. Preventing fuse blowing
EXPLANATION
No answer description required for this question.

95) The rheostat speed control method is very

1. Economical
2. Efficient
3. Unsuitable for rapidly changing loads
4. Suitable for getting speeds above the normal
EXPLANATION
No answer description required for this question.

96) With the increase in speed of a DC motor

1. Both back e.m.f and line current fall
2. Both back e.m.f and line current increases
3. Back e.m.f falls and line current increases
4. Back e.m.f increases and line current decreases
EXPLANATION
For a DC motor, line current = (V – Eb)/Ra.

If Eb =0, Ia = V/Ra. Since armature resistance Ra is very small, a very large armature current will burn the winding.

97) Speed control by variation of field flux results in

1. Variable power drive
2. Constant torque drive
3. Variable torque drive
4. Constant power drive
EXPLANATION
No answer description required for this question.

98) The efficiency of a DC shunt generator is maximum when the

1. Magnetic losses are equal to the mechanical losses
2. Field copper losses are equal to the constant losses
3. Stray losses are equal to the copper losses
4. Armature copper losses are equal to constant losses
EXPLANATION
No answer description required for this question.

99) In a DC generator, the brushes on the commutator are shifted from geometrical neutral positions to

1. Obtain highest generator voltage
2. Avoid sparking
3. Obtain highest generator efficiency
4. All the above
EXPLANATION
No answer description required for this question.

100) A simplex lap-wound 10-pole DC generator has 800 armature conductors. If each pole face covers 75% of pitch, compensating conductors required per pole face are

1. 80
2. 60
3. 6
4. 8