The cause of sparking at brushes may be

 Q701. The armature winding of a DC machine is placed on the

A) Stator
B) Rotor
C) Field poles
D) Yoke
Ans: B

Q702. The field winding of a DC machine is generally placed on the
A) Armature
B) Rotor
C) Stator
D) Commutator
Ans: C

Q703. The insulation between the commutator segments is made of
A) Mica
B) Paper
C) Plastic
D) Rubber
Ans: A

Q704. The function of a brush holder is to
A) Insulate the brushes
B) Hold the brushes against the commutator
C) Supply field current
D) Reduce eddy currents
Ans: B

Q705. The armature reaction in a DC machine reduces
A) Terminal voltage
B) Armature current
C) Field current
D) Power output
Ans: A

Q706. The number of commutator segments in a DC machine is equal to
A) Number of armature conductors
B) Number of armature coils
C) Half the number of armature conductors
D) Twice the number of armature coils
Ans: B

Q707. In DC generators, the brushes are placed along the
A) Geometrical neutral axis
B) Magnetic neutral axis
C) Axis of poles
D) Armature slot axis
Ans: B

Q708. The magnetic field in a DC machine is produced by
A) Armature winding
B) Field winding
C) Commutator
D) Brushes
Ans: B

Q709. In DC shunt motors, speed decreases slightly with load due to
A) Armature reaction
B) Field weakening
C) Increased back emf
D) High resistance
Ans: A

Q710. A DC series motor should never be started
A) With load
B) Without load
C) With low voltage
D) With weak field
Ans: B

Q711. The neutral plane in a DC generator is shifted because of
A) Armature reaction
B) Commutation
C) Magnetic leakage
D) Eddy currents
Ans: A

Q712. The purpose of interpoles is to produce
A) Cross-magnetizing flux
B) Demagnetizing flux
C) Neutralizing flux
D) Strengthening flux
Ans: C

Q713. The brushes in DC machines are made of
A) Copper
B) Carbon or graphite
C) Steel
D) Aluminum
Ans: B

Q714. The voltage regulation of a DC generator is the measure of
A) Efficiency
B) Change in voltage with load
C) Power factor
D) Speed regulation
Ans: B

Q715. The load characteristic of a DC shunt generator is
A) Rising
B) Drooping
C) Flat
D) Non-linear
Ans: B

Q716. The efficiency of a DC generator is maximum when
A) Iron losses = copper losses
B) Variable losses = constant losses
C) Copper losses = friction losses
D) Iron losses = friction losses
Ans: B

Q717. The speed-torque characteristic of a DC series motor is
A) Linear
B) Hyperbolic
C) Parabolic
D) Exponential
Ans: B

Q718. The speed control of DC shunt motor by armature voltage variation gives
A) Speed below normal
B) Speed above normal
C) Constant torque
D) None
Ans: A

Q719. The field control method in DC motor gives
A) Speed above normal
B) Speed below normal
C) Constant speed
D) Constant torque
Ans: A

Q720. The braking method used in DC motors includes
A) Regenerative braking
B) Dynamic braking
C) Plugging
D) All of the above
Ans: D

Q721. The direction of induced emf in DC generator is given by
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

Q722. The EMF equation of a DC generator is
A) E = (PΦZN)/(60A)
B) E = (ΦZN)/(P60A)
C) E = (60AΦZ)/N
D) E = (AΦZ)/(60PN)
Ans: A

Q723. In lap winding, the number of parallel paths is equal to
A) 2
B) Number of poles (P)
C) P/2
D) 2P
Ans: B

Q724. In wave winding, the number of parallel paths is always
A) 2
B) 4
C) P
D) P/2
Ans: A

Q725. The induced EMF in a DC generator depends on
A) Flux per pole
B) Speed
C) Number of conductors
D) All of the above
Ans: D

Q726. A DC shunt motor is used where
A) Constant speed is required
B) Variable speed is required
C) High torque is required
D) Speed reversal is needed
Ans: A

Q727. The speed of a DC series motor varies
A) Directly with load
B) Inversely with load
C) Linearly with current
D) Constantly
Ans: B

Q728. A differential compound generator has
A) Series field opposes shunt field
B) Series field aids shunt field
C) No series field
D) Weak shunt field
Ans: A

Q729. In a long shunt compound generator, the shunt field is connected
A) Across armature
B) Across both armature and series field
C) In series with armature
D) Across series field only
Ans: B

Q730. The function of yoke in DC machine is
A) To provide mechanical support
B) To carry magnetic flux
C) To provide protection
D) All of the above
Ans: D

Q731. The efficiency of a DC motor is maximum when
A) Copper losses = constant losses
B) Copper losses = iron losses
C) Input = output
D) Output = losses
Ans: A

Q732. The stray losses in a DC machine include
A) Windage loss
B) Friction loss
C) Brush contact loss
D) All of the above
Ans: D

Q733. The generated emf in DC generator is proportional to
A) Armature current
B) Field current
C) Speed × flux
D) Speed only
Ans: C

Q734. The flux per pole in DC machine depends on
A) Field current
B) Armature current
C) Speed
D) Load
Ans: A

Q735. The magnetic neutral axis (MNA) shifts due to
A) Armature reaction
B) Commutation
C) Speed change
D) Load increase
Ans: A

Q736. A DC series motor is used in
A) Traction
B) Lathes
C) Fans
D) Elevators
Ans: A

Q737. The main advantage of wave winding over lap winding is
A) High current capacity
B) High voltage for same size
C) Simple design
D) Low cost
Ans: B

Q738. The speed of DC motor is directly proportional to
A) Back emf
B) Supply voltage
C) Flux per pole
D) Both A and B
Ans: D

Q739. For same load, the armature current of series motor is
A) Greater than shunt motor
B) Less than shunt motor
C) Same
D) Variable
Ans: A

Q740. The speed control of DC shunt motor by field control is
A) Below normal speed
B) Above normal speed
C) Constant speed
D) None
Ans: B

Q741. The armature reaction in DC machine produces
A) Cross magnetizing effect
B) Demagnetizing effect
C) Both A and B
D) No effect
Ans: C

Q742. The brushes are shifted to new positions due to
A) Armature reaction
B) Poor commutation
C) Voltage drop
D) Temperature rise
Ans: A

Q743. The load on DC generator increases, terminal voltage
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Ans: B

Q744. The field coils in DC machine are wound on
A) Yoke
B) Pole core
C) Armature
D) Shaft
Ans: B

Q745. The commutator in DC machine is made of
A) Copper segments insulated by mica
B) Aluminum plates
C) Iron discs
D) Steel plates
Ans: A

Q746. The cause of sparking at brushes may be
A) Poor commutation
B) Weak brush spring
C) High load current
D) All of the above
Ans: D

Q747. The magnetic losses in a DC machine are
A) Eddy current and hysteresis losses
B) Copper losses
C) Mechanical losses
D) Brush losses
Ans: A

Q748. The iron losses in DC machines occur in
A) Armature core
B) Yoke
C) Commutator
D) Brushes
Ans: A

Q749. The brushes should be placed at
A) Geometrical neutral axis
B) Magnetic neutral axis
C) Armature axis
D) Axis of poles
Ans: B

Q750. The armature of DC machine rotates because
A) Force acts on current-carrying conductor in magnetic field
B) Magnetic field is stationary
C) Supply voltage is DC
D) Commutator reverses current
Ans: A

Eddy current loss in armature core can be reduced by

 Q651. In a DC machine, armature reaction causes

A) Increased main flux
B) Distortion of main field flux
C) Uniform air gap flux
D) No effect
Ans: B

Q652. Interpoles in DC machines are connected in
A) Series with armature
B) Parallel with armature
C) Series with shunt field
D) Parallel with shunt field
Ans: A

Q653. The main purpose of a commutator in a DC generator is
A) To convert DC into AC
B) To convert AC into DC
C) To increase voltage
D) To reduce eddy current
Ans: B

Q654. The armature of a DC machine is laminated to
A) Reduce hysteresis loss
B) Reduce eddy current loss
C) Increase flux
D) Strengthen armature
Ans: B

Q655. If the speed of a DC generator increases, its generated EMF
A) Decreases
B) Increases
C) Remains same
D) Becomes zero
Ans: B

Q656. In a DC shunt generator, the voltage drops to zero when
A) Load increases
B) Field circuit opens
C) Armature speed decreases
D) Brush contact improves
Ans: B

Q657. A cumulatively compounded DC motor is preferred for
A) Constant speed operation
B) Variable load with heavy starting torque
C) Light load
D) High-speed operation
Ans: B

Q658. The commutation in DC machine is improved by
A) Increasing armature resistance
B) Decreasing brush contact area
C) Using interpoles
D) Decreasing field current
Ans: C

Q659. The mechanical power developed in a DC motor is maximum when
A) Back emf is half of supply voltage
B) Back emf equals supply voltage
C) Armature current is maximum
D) Field current is zero
Ans: A

Q660. The direction of rotation of a DC motor can be reversed by
A) Changing armature connections
B) Changing field connections
C) Changing either field or armature connections
D) Changing both simultaneously
Ans: C

Q661. In a DC machine, the function of brushes is to
A) Conduct current to commutator
B) Conduct current to field winding
C) Increase magnetic flux
D) Cool the armature
Ans: A

Q662. The armature reaction in a DC generator causes
A) Neutral plane shift
B) Increase in flux
C) Voltage rise
D) Field strengthening
Ans: A

Q663. The losses in a DC generator are
A) Constant losses only
B) Variable losses only
C) Constant + variable losses
D) Core losses only
Ans: C

Q664. For maximum efficiency of a DC generator,
A) Variable losses = constant losses
B) Variable losses > constant losses
C) Variable losses < constant losses
D) Constant losses = 0
Ans: A

Q665. The back emf in a DC motor is proportional to
A) Armature current
B) Field flux
C) Speed
D) Product of field flux and speed
Ans: D

Q666. The speed of a DC shunt motor can be controlled by
A) Armature resistance control
B) Field flux control
C) Voltage control
D) All of the above
Ans: D

Q667. The torque of a DC motor is directly proportional to
A) Armature current only
B) Field flux only
C) Product of flux and armature current
D) Speed
Ans: C

Q668. The armature current in a DC motor is maximum when
A) Motor starts
B) Motor runs at full speed
C) Motor runs at no load
D) Field is open
Ans: A

Q669. Sparking at brushes in DC machine can be reduced by
A) Decreasing brush pressure
B) Using interpoles
C) Using carbon brushes only
D) Reducing speed
Ans: B

Q670. In a DC generator, residual magnetism helps in
A) Commutation
B) Self-excitation
C) Cooling
D) Armature reaction
Ans: B

Q671. The type of DC generator used for arc welding is
A) Shunt generator
B) Series generator
C) Compound generator
D) Differential generator
Ans: B

Q672. Armature reaction in DC machine results in
A) Flux strengthening
B) Cross magnetization
C) Constant flux
D) Improved commutation
Ans: B

Q673. The lap winding in DC machine is used for
A) High voltage, low current
B) Low voltage, high current
C) High speed
D) High efficiency
Ans: B

Q674. A wave wound DC machine is used for
A) Low voltage
B) High current
C) High voltage, low current
D) Low speed
Ans: C

Q675. In a DC shunt generator, voltage builds up only if
A) Residual magnetism is present
B) Field resistance < critical resistance
C) Speed > critical speed
D) All of the above
Ans: D

Q676. The purpose of compensating winding is
A) To reduce armature reaction
B) To increase flux
C) To increase efficiency
D) To improve voltage regulation
Ans: A

Q677. In a DC motor, if field current increases, speed
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Ans: B

Q678. When a DC motor is overloaded, it will
A) Run faster
B) Slow down
C) Stop immediately
D) Speed up then stop
Ans: B

Q679. A DC motor converts
A) Electrical energy into mechanical energy
B) Mechanical energy into electrical energy
C) AC to DC
D) DC to AC
Ans: A

Q680. In a DC generator, generated EMF is directly proportional to
A) Flux
B) Speed
C) Flux × Speed
D) Armature current
Ans: C

Q681. In a DC shunt motor, speed is nearly constant because
A) Flux is constant
B) Armature current is constant
C) Field current varies
D) Torque is constant
Ans: A

Q682. The field winding of a DC shunt motor is made of
A) Thick wire
B) Thin wire
C) Aluminum wire
D) Iron wire
Ans: B

Q683. The series field winding in a DC motor carries
A) Full armature current
B) Half armature current
C) No current
D) Shunt current
Ans: A

Q684. For regenerative braking,
A) Motor acts as generator
B) Motor runs faster
C) Motor absorbs energy
D) Motor reverses polarity
Ans: A

Q685. The main advantage of DC series motor is
A) High starting torque
B) Constant speed
C) Low cost
D) No commutation problem
Ans: A

Q686. The direction of rotation of a DC motor is given by
A) Fleming’s left-hand rule
B) Fleming’s right-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

Q687. A differential compound motor is never used for
A) Variable load
B) Heavy torque
C) Industrial drives
D) Hoisting applications
Ans: D

Q688. The copper losses in a DC motor depend on
A) Field current
B) Armature current
C) Both A and B
D) Speed
Ans: C

Q689. Armature reaction is more in
A) Loaded condition
B) No-load condition
C) Open circuit
D) Short circuit
Ans: A

Q690. The commutator converts
A) DC to AC
B) AC to DC
C) DC to DC
D) AC to AC
Ans: B

Q691. The sparkless commutation is achieved by
A) Interpoles
B) Resistance in armature
C) Large brushes
D) Field weakening
Ans: A

Q692. The brush contact drop in DC machine is about
A) 0.2–0.4 V per brush
B) 1 V per brush
C) 2 V per brush
D) 5 V per brush
Ans: A

Q693. The efficiency of large DC machine is in the range of
A) 40–50%
B) 60–70%
C) 80–90%
D) 90–95%
Ans: D

Q694. In DC generator, open circuit characteristic shows relation between
A) Voltage and current
B) Voltage and field current
C) Current and speed
D) Speed and torque
Ans: B

Q695. The induced EMF in a DC generator is due to
A) Electrostatic induction
B) Electromagnetic induction
C) Mechanical action
D) Chemical reaction
Ans: B

Q696. In a DC shunt motor, if supply voltage decreases, speed
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Ans: B

Q697. The function of equalizer rings in DC machine is to
A) Prevent circulating current
B) Improve commutation
C) Increase voltage
D) Increase torque
Ans: A

Q698. Eddy current loss in armature core can be reduced by
A) Laminating core
B) Increasing speed
C) Using copper core
D) Using solid iron core
Ans: A

Q699. The critical speed in DC generator is that speed at which
A) Voltage just builds up
B) Maximum voltage occurs
C) Minimum voltage occurs
D) No load current is zero
Ans: A

Q700. The commutation in DC machine is the process of
A) Current reversal in armature coil
B) Voltage reversal in field
C) Flux reversal in core
D) Torque reversal
Ans: A

The load torque of a hoist motor is

 

651. The yoke of a DC machine is used to
A) Provide mechanical support and carry flux
B) Increase resistance
C) Reduce weight
D) Hold brushes
Ans: A

652. The material used for yoke is usually
A) Cast steel
B) Copper
C) Aluminum
D) Brass
Ans: A

653. The main function of armature core is to
A) Carry armature winding and provide path for flux
B) Reduce weight
C) Store magnetic energy
D) None
Ans: A

654. The armature core is laminated to
A) Reduce eddy current losses
B) Reduce hysteresis loss
C) Increase strength
D) Decrease resistance
Ans: A

655. The armature winding is placed in
A) Armature slots
B) Field poles
C) Yoke
D) Commutator
Ans: A

656. The winding used for high current and low voltage is
A) Lap winding
B) Wave winding
C) Frog-leg winding
D) None
Ans: A

657. The winding used for high voltage and low current is
A) Wave winding
B) Lap winding
C) Series winding
D) Shunt winding
Ans: A

658. The number of parallel paths in lap winding is
A) Equal to number of poles
B) Two
C) Four
D) One
Ans: A

659. The number of parallel paths in wave winding is
A) Two
B) Equal to poles
C) Four
D) Six
Ans: A

660. The commutator converts
A) AC to DC
B) DC to AC
C) AC to AC
D) DC to DC
Ans: A

661. The function of brushes is to
A) Collect current from commutator
B) Support armature
C) Carry magnetic flux
D) Increase speed
Ans: A

662. The brushes are usually made of
A) Carbon or graphite
B) Copper
C) Aluminum
D) Steel
Ans: A

663. The main disadvantage of carbon brush is
A) High contact resistance
B) High conductivity
C) High cost
D) Low lubricating property
Ans: A

664. The efficiency of a DC generator is maximum when
A) Variable losses = Constant losses
B) Speed = Maximum
C) Load = Minimum
D) Field current = Maximum
Ans: A

665. The speed of a DC motor is inversely proportional to
A) Flux
B) Torque
C) Current
D) Voltage
Ans: A

666. The torque developed by DC motor is proportional to
A) Flux × Armature current
B) Speed × Flux
C) Voltage × Flux
D) Resistance × Current
Ans: A

667. The back emf acts in
A) Opposite direction to supply voltage
B) Same direction as supply voltage
C) Random direction
D) None
Ans: A

668. The back emf increases with
A) Speed
B) Armature resistance
C) Field resistance
D) Load
Ans: A

669. The load torque of a hoist motor is
A) Inversely proportional to speed
B) Constant
C) Proportional to speed
D) None
Ans: B

670. The speed-torque characteristic of DC series motor is
A) Hyperbolic
B) Linear
C) Parabolic
D) Constant
Ans: A

671. The DC shunt motor is preferred for
A) Constant speed applications
B) Variable speed applications
C) High starting torque
D) Intermittent load
Ans: A

672. The DC series motor is preferred for
A) High starting torque applications
B) Constant speed drives
C) Light load
D) None
Ans: A

673. The DC compound motor combines advantages of
A) Shunt and series motors
B) Shunt and synchronous motors
C) Induction and DC motors
D) None
Ans: A

674. The differential compound motor is used when
A) Speed should fall with increase in load
B) Speed should rise with load
C) Constant speed required
D) None
Ans: B

675. The commutation is the process of
A) Reversal of current in armature coil
B) Reversal of supply
C) Magnetization of poles
D) None
Ans: A

676. Sparking at brushes occurs due to
A) Poor commutation
B) High resistance brushes
C) High flux density
D) All of these
Ans: A

677. The interpoles in DC machine are connected in
A) Series with armature
B) Parallel with armature
C) Series with field
D) Parallel with field
Ans: A

678. The flux per pole in a DC machine is measured in
A) Weber
B) Henry
C) Tesla
D) Joule
Ans: A

679. The emf generated in armature conductor is due to
A) Relative motion between conductor and flux
B) Resistance of conductor
C) Current through conductor
D) None
Ans: A

680. The generated emf in DC generator is directly proportional to
A) Speed and flux
B) Armature current
C) Field resistance
D) Load
Ans: A

681. The armature resistance drop in generator causes
A) Reduction in terminal voltage
B) Increase in emf
C) Constant voltage
D) Increase in flux
Ans: A

682. The function of field regulator is to
A) Control generator output voltage
B) Control speed
C) Control torque
D) Reduce losses
Ans: A

683. The field rheostat is connected in
A) Series with shunt field
B) Parallel with shunt field
C) Series with armature
D) Across armature
Ans: A

684. The speed of DC shunt motor increases if
A) Field current decreases
B) Armature current increases
C) Load increases
D) Supply voltage decreases
Ans: A

685. The speed of DC shunt motor decreases if
A) Flux increases
B) Field current decreases
C) Load decreases
D) Supply voltage increases
Ans: A

686. The speed of DC series motor is inversely proportional to
A) Armature current
B) Field current
C) Torque
D) All of these
Ans: D

687. The speed regulation of DC shunt motor is usually
A) 5–10%
B) 50%
C) 0%
D) 20%
Ans: A

688. The speed regulation of DC series motor is usually
A) Poor
B) Excellent
C) Constant
D) None
Ans: A

689. The losses in DC machine include
A) Copper, iron, mechanical, stray losses
B) Electrical only
C) Magnetic only
D) None
Ans: A

690. The stray load losses in DC machine are about
A) 1% of output
B) 10% of output
C) 5% of output
D) Negligible
Ans: A

691. The hysteresis loss depends on
A) Frequency and maximum flux density
B) Current
C) Resistance
D) Voltage
Ans: A

692. Eddy current loss depends on
A) Square of flux density and frequency
B) Current
C) Load
D) Armature current
Ans: A

693. The copper losses vary with
A) Square of current
B) Current
C) Speed
D) Flux
Ans: A

694. The temperature rise of DC machine should not exceed
A) 40–50°C
B) 100°C
C) 10°C
D) 25°C
Ans: A

695. The cooling of DC machine is improved by
A) Ventilation
B) Increasing resistance
C) Increasing load
D) Reducing voltage
Ans: A

696. The air gap in DC machine is kept
A) As small as possible
B) Very large
C) Constant and large
D) Variable
Ans: A

697. Large air gap in DC machine causes
A) Increased mmf requirement
B) Reduced leakage
C) Increased flux
D) None
Ans: A

698. The commutation improves when
A) Brush shift is correct
B) Interpoles are used
C) Resistance is reduced
D) Both A and B
Ans: D

699. The flux weakening control in DC motor is used for
A) Speed above rated
B) Speed below rated
C) Constant speed
D) Torque control
Ans: A

700. The short-shunt compound generator has
A) Shunt field connected in parallel with armature
B) Shunt field connected in parallel with series field and armature
C) Series field connected across load
D) None
Ans: A

The critical speed is

 601. The torque of a DC motor is directly proportional to

A) Φ × Ia
B) Ia²
C) Φ / Ia
D) V × Ia
Ans: A

602. The emf equation of DC generator is
A) E = (PΦNZ)/(60A)
B) E = (AΦNZ)/(60P)
C) E = (ΦP)/(NZ)
D) E = (60A)/(PΦNZ)
Ans: A

603. The open circuit characteristic of a DC generator gives relation between
A) Generated emf and field current
B) Armature current and voltage
C) Load current and torque
D) Voltage and speed
Ans: A

604. In a DC motor, torque is zero when
A) Armature current is zero
B) Flux is zero
C) Both A and B
D) Supply voltage is zero
Ans: C

605. The commutation process is improved by
A) Using interpoles
B) Increasing brush pressure
C) Increasing load
D) Decreasing field flux
Ans: A

606. The function of interpoles in DC machine is
A) Neutralize armature reaction and improve commutation
B) Increase field strength
C) Decrease flux leakage
D) Reduce eddy current losses
Ans: A

607. The field winding of DC shunt motor has
A) Many turns of thin wire
B) Few turns of thick wire
C) Many turns of thick wire
D) Few turns of thin wire
Ans: A

608. The field winding of DC series motor has
A) Few turns of thick wire
B) Many turns of thin wire
C) Many turns of thick wire
D) Few turns of thin wire
Ans: A

609. The DC compound motor combines
A) Shunt and series windings
B) Armature and field windings
C) Parallel and series motors
D) None of these
Ans: A

610. The differential compound motor is rarely used because
A) It is unstable
B) It is costly
C) It has low torque
D) It has poor speed control
Ans: A

611. The mechanical characteristic of a DC shunt motor is
A) Nearly constant speed
B) Variable speed
C) Steep drooping
D) Inverse speed
Ans: A

612. The mechanical characteristic of a DC series motor is
A) Speed varies inversely with load
B) Speed increases with load
C) Constant speed
D) None
Ans: A

613. The armature voltage control of DC motor is used for
A) Speed below rated value
B) Speed above rated value
C) Constant torque
D) None
Ans: A

614. Field control of DC motor gives
A) Speed above base speed
B) Speed below base speed
C) Constant speed
D) None
Ans: A

615. In DC motors, torque is maximum when
A) Back emf is half of supply voltage
B) Back emf equals supply voltage
C) Current is zero
D) Speed is maximum
Ans: A

616. The electrical equivalent of mechanical load is
A) Back emf
B) Torque
C) Armature resistance
D) Speed
Ans: A

617. For a DC motor, mechanical output power =
A) Eb × Ia
B) V × Ia
C) Ia²Ra
D) Torque × Speed
Ans: D

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

619. The function of a starter in DC motor is to
A) Limit starting current
B) Increase torque
C) Reduce flux
D) Regulate voltage
Ans: A

620. A 3-point starter cannot be used for
A) DC compound motor
B) DC series motor
C) DC shunt motor
D) None
Ans: B

621. The no-load speed of DC series motor is
A) Very high
B) Zero
C) Constant
D) Low
Ans: A

622. The armature reaction causes
A) Distortion of main field
B) Weakening of main field
C) Sparking at brushes
D) All of these
Ans: D

623. In DC machine, demagnetizing ampere-turns are proportional to
A) Ia × sin(θ/2)
B) Ia × cos(θ/2)
C) Ia²
D) Speed
Ans: A

624. The brush shift in DC generator is done to
A) Reduce sparking
B) Increase emf
C) Reduce losses
D) Increase torque
Ans: A

625. The neutral plane shifts due to
A) Armature reaction
B) Load current
C) Speed variation
D) Temperature
Ans: A

626. The residual magnetism in DC generator is essential for
A) Voltage build-up
B) Torque generation
C) Speed control
D) Commutation
Ans: A

627. If residual magnetism is lost, the generator fails to
A) Build up voltage
B) Deliver current
C) Rotate
D) Maintain speed
Ans: A

628. The residual magnetism in DC generator can be restored by
A) Field flashing
B) Reducing load
C) Increasing speed
D) Changing brushes
Ans: A

629. The magnetization curve of DC generator is also called
A) Open-circuit characteristic
B) Load characteristic
C) Efficiency curve
D) Torque-speed curve
Ans: A

630. The emf in DC generator depends on
A) Flux and speed
B) Current and resistance
C) Voltage and current
D) Field current only
Ans: A

631. If the speed of DC generator doubles, the generated emf
A) Doubles
B) Halves
C) Remains same
D) Becomes zero
Ans: A

632. The direction of induced emf in a conductor is given by
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

633. The direction of torque in DC motor is given by
A) Fleming’s left-hand rule
B) Fleming’s right-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

634. A generator fails to build up voltage due to
A) Wrong field connection
B) No residual magnetism
C) High field resistance
D) Any of these
Ans: D

635. The critical field resistance is
A) The maximum field resistance at which the generator just excites
B) Minimum field resistance for excitation
C) Average resistance
D) Armature resistance
Ans: A

636. The critical speed is
A) The minimum speed below which the generator fails to excite
B) Maximum speed
C) Rated speed
D) Constant speed
Ans: A

637. The load characteristic of DC generator shows relation between
A) Terminal voltage and load current
B) Field current and emf
C) Speed and torque
D) Flux and voltage
Ans: A

638. The terminal voltage of a DC shunt generator decreases with load because of
A) Armature reaction and IR drop
B) Increased field current
C) Increased flux
D) Reduced resistance
Ans: A

639. In a DC compound generator, the series field assists the shunt field in
A) Cumulative compounding
B) Differential compounding
C) Over compounding
D) None
Ans: A

640. The purpose of differential compounding is
A) To make voltage drop with load
B) To increase voltage
C) To maintain constant voltage
D) To decrease flux
Ans: A

641. In a cumulatively compounded generator, voltage
A) Increases with load
B) Decreases with load
C) Constant
D) Zero
Ans: A

642. The voltage regulation of DC generator can be improved by
A) Using interpoles
B) Increasing speed
C) Reducing load
D) Decreasing flux
Ans: A

643. The field winding of DC generator is excited by
A) DC supply
B) AC supply
C) Induction
D) None
Ans: A

644. A shunt generator has its field winding connected
A) In parallel with armature
B) In series with armature
C) Separately excited
D) Across load only
Ans: A

645. The purpose of equalizer connection in compound generator is
A) To avoid circulating current between generators
B) To increase efficiency
C) To reduce flux
D) To increase voltage
Ans: A

646. The equalizer bar connects
A) Armature of compound generators
B) Field windings
C) Load terminals
D) Neutral points
Ans: A

647. The brush contact drop remains almost
A) Constant
B) Proportional to current
C) Proportional to voltage
D) Variable with speed
Ans: A

648. The commutator is made of
A) Hard drawn copper segments
B) Steel laminations
C) Aluminum
D) Carbon
Ans: A

649. The insulation between commutator segments is
A) Mica
B) Paper
C) Cotton
D) Plastic
Ans: A

650. The field poles are fixed on
A) Yoke
B) Armature core
C) Shaft
D) Commutator
Ans: A

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