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