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