The DC machines are replaced by AC machines because

 551. The back emf of a DC motor is zero when

A) Motor is stationary
B) Motor is running at rated speed
C) Armature current is zero
D) Flux is constant
Ans: A

552. The back emf of a DC motor is proportional to
A) Flux × Speed
B) Flux / Speed
C) Current × Speed
D) Resistance × Speed
Ans: A

553. The back emf of a DC motor opposes
A) Supply voltage
B) Armature current
C) Field current
D) Torque
Ans: A

554. The purpose of back emf is to
A) Regulate armature current
B) Increase torque
C) Reduce speed
D) Increase flux
Ans: A

555. The armature of DC motor is made of
A) Soft iron laminated core
B) Cast iron solid core
C) Aluminum alloy
D) Copper bar
Ans: A

556. The pole shoes in DC machine are laminated to
A) Reduce eddy current losses
B) Increase weight
C) Increase field flux
D) Reduce cost
Ans: A

557. The iron loss in DC machine depends on
A) Speed and flux density
B) Armature current
C) Field resistance
D) Load torque
Ans: A

558. The mechanical losses depend on
A) Speed
B) Load current
C) Field current
D) Flux
Ans: A

559. The copper losses depend on
A) Square of current
B) Speed
C) Flux
D) Voltage
Ans: A

560. Stray losses in DC machine are usually
A) 1% of full load
B) 10% of full load
C) 0.1% of full load
D) 5% of full load
Ans: A

561. If flux is reduced by 50% in a DC motor, speed becomes
A) Double
B) Half
C) Same
D) Zero
Ans: A

562. The terminal voltage of a generator decreases due to
A) Armature reaction and IR drop
B) Increased field current
C) Increased speed
D) None
Ans: A

563. The commutation time depends on
A) Brush width and commutator segment size
B) Speed
C) Flux
D) Load
Ans: A

564. The brushes are made of carbon because
A) They provide lubrication and low friction
B) They have high conductivity
C) They are strong
D) They reduce flux
Ans: A

565. The purpose of brush spring is
A) Maintain constant contact pressure
B) Reduce torque
C) Increase voltage
D) Cool the brush
Ans: A

566. In a wave wound armature, the number of parallel paths is
A) 2
B) Equal to poles
C) 4
D) 6
Ans: A

567. In a lap wound armature, the number of parallel paths is
A) Equal to poles
B) 2
C) 4
D) 6
Ans: A

568. The armature reaction in DC machine can be reduced by
A) Compensating windings
B) Increasing speed
C) Decreasing load
D) Increasing armature resistance
Ans: A

569. In a DC motor, torque is proportional to
A) Flux × Armature current
B) Flux / Armature current
C) Speed × Flux
D) Armature resistance
Ans: A

570. The induced emf in DC generator is
A) E = (PΦNZ) / (60A)
B) E = (60A) / (PΦNZ)
C) E = (ΦN) / Z
D) E = (ZΦ) / A
Ans: A

571. For DC shunt motor, torque is proportional to
A) Armature current
B) Square of armature current
C) Speed
D) Voltage
Ans: A

572. The starting torque of DC series motor is proportional to
A) Ia²
B) Ia
C) 1/Ia
D) Constant
Ans: A

573. The condition for maximum power in DC motor is
A) Eb = ½ V
B) Eb = V
C) Eb = 2V
D) Eb = 0
Ans: A

574. The efficiency of a DC generator is maximum when
A) Variable losses = Constant losses
B) Speed = Maximum
C) Torque = Constant
D) Armature current = Maximum
Ans: A

575. The stray losses include
A) Eddy current and stray load losses
B) Copper losses
C) Hysteresis losses only
D) None
Ans: A

576. The iron losses occur in
A) Armature core
B) Field winding
C) Commutator
D) Brushes
Ans: A

577. The brush contact drop is nearly
A) 1 to 2 V per brush
B) 10 V
C) 0.1 V
D) 5 V
Ans: A

578. The effect of armature reaction is to
A) Distort main field
B) Increase flux
C) Decrease emf
D) All of these
Ans: D

579. The speed of DC motor can be controlled by
A) Field control
B) Armature voltage control
C) Armature resistance control
D) All of these
Ans: D

580. The braking methods of DC motor are
A) Regenerative, dynamic, plugging
B) Static, dynamic, kinetic
C) Forward, reverse, dynamic
D) None
Ans: A

581. In regenerative braking, energy is
A) Returned to supply
B) Dissipated in resistance
C) Stored in field
D) None
Ans: A

582. Dynamic braking is suitable for
A) Quick stopping
B) Continuous speed control
C) Increasing torque
D) None
Ans: A

583. Plugging braking involves
A) Reversal of supply polarity
B) Open circuiting the armature
C) Reducing voltage
D) Shorting the field
Ans: A

584. The torque under plugging condition is
A) Opposite to direction of rotation
B) Same as rotation
C) Constant
D) Zero
Ans: A

585. The regenerative braking is not possible in
A) DC series motor
B) DC shunt motor
C) DC compound motor
D) Separately excited motor
Ans: A

586. When the field circuit of DC shunt motor opens,
A) Speed increases dangerously
B) Speed decreases
C) Torque increases
D) Motor stops
Ans: A

587. A DC shunt motor should never be started
A) With open field circuit
B) At full voltage
C) With load
D) Without starter
Ans: A

588. A DC series motor should never be started
A) Without load
B) Without starter
C) At full voltage
D) With high resistance
Ans: A

589. The speed control of DC shunt motor above base speed is done by
A) Field weakening
B) Armature voltage
C) Resistance control
D) None
Ans: A

590. For DC series motor, speed control below base speed is done by
A) Series resistance
B) Field weakening
C) Voltage increase
D) None
Ans: A

591. The advantage of Ward–Leonard system is
A) Wide speed control
B) Low efficiency
C) High losses
D) Slow response
Ans: A

592. In Ward–Leonard system, the motor used is
A) DC shunt motor
B) DC series motor
C) AC induction motor
D) DC compound motor
Ans: A

593. The main disadvantage of Ward–Leonard system is
A) High cost
B) Poor control
C) High efficiency
D) Small speed range
Ans: A

594. The main advantage of DC motor is
A) Easy speed control
B) Compact size
C) Low maintenance
D) Low cost
Ans: A

595. The main disadvantage of DC machine is
A) High maintenance due to commutator
B) Poor torque
C) Poor regulation
D) High noise
Ans: A

596. The DC machines are replaced by AC machines because
A) AC supply is available easily
B) Commutator maintenance is avoided
C) Efficiency is higher
D) All of these
Ans: D

597. The DC generator used for welding purpose is
A) Series generator
B) Shunt generator
C) Compound generator
D) Differential generator
Ans: A

598. The generator used for charging batteries is
A) Shunt generator
B) Series generator
C) Differential generator
D) None
Ans: A

599. The voltage regulation of DC generator is defined as
A) (No-load V – Full-load V) / Full-load V × 100
B) (Full-load V – No-load V) / No-load V × 100
C) (No-load V – Full-load V) / No-load V × 100
D) (Full-load V / No-load V) × 100
Ans: C

600. For sparkless commutation, brush position should be on
A) Magnetic neutral axis
B) Geometric neutral axis
C) Pole axis
D) At 90°
Ans: A