A DC shunt motor develops maximum torque when

 451. The speed–torque characteristic of a DC shunt motor is

A) Nearly constant speed
B) Hyperbolic
C) Drooping
D) Linear
Ans: A

452. The speed–torque characteristic of a DC series motor is
A) Hyperbolic
B) Linear
C) Constant
D) Flat
Ans: A

453. A cumulatively compounded motor combines the features of
A) Shunt and series motors
B) Shunt and induction motors
C) Series and synchronous motors
D) None
Ans: A

454. The armature resistance of DC motor is generally
A) Very low
B) High
C) Medium
D) Variable
Ans: A

455. The main function of brushes is to
A) Collect current from commutator
B) Generate emf
C) Control flux
D) Reduce losses
Ans: A

456. The function of commutator is
A) Convert AC to DC
B) Convert DC to AC
C) Reduce losses
D) Maintain torque
Ans: A

457. A generator running at constant speed produces constant voltage if
A) Field current is constant
B) Load current changes
C) Armature reaction varies
D) None
Ans: A

458. If the field of a shunt generator opens while running, the output voltage
A) Falls to zero
B) Rises to maximum
C) Becomes unstable
D) Remains constant
Ans: A

459. When DC motor is over-speeding, it may be due to
A) Weak field
B) Overload
C) Shorted turns
D) Commutation problem
Ans: A

460. Armature reaction in DC machine causes
A) Distortion and weakening of main flux
B) Increase of main flux
C) Increase of emf
D) None
Ans: A

461. The maximum efficiency of DC generator occurs when
A) Variable losses = constant losses
B) Copper losses = iron losses
C) Output power is max
D) Speed is max
Ans: A

462. In DC generator, armature reaction effect is more pronounced at
A) High load
B) No load
C) Constant speed
D) Low temperature
Ans: A

463. In DC motor, if polarity of supply is reversed
A) Direction of rotation reverses
B) Speed increases
C) Torque doubles
D) Nothing changes
Ans: A

464. To reverse the direction of rotation of DC motor
A) Reverse armature or field connections
B) Reverse supply
C) Change brushes
D) Increase field current
Ans: A

465. If both armature and field polarities are reversed
A) Direction remains same
B) Direction reverses
C) Motor stops
D) Speed increases
Ans: A

466. The speed control of DC shunt motor by field control is
A) Above normal speed
B) Below normal speed
C) Both above and below
D) Not possible
Ans: A

467. Armature voltage control of DC motor gives
A) Speeds below normal
B) Speeds above normal
C) Constant torque
D) None
Ans: A

468. The torque developed in DC motor is proportional to
A) ΦIa
B) Ia²
C) V × I
D) R × I
Ans: A

469. In DC motor, mechanical power developed =
A) Eb × Ia
B) V × Ia
C) Eb × If
D) V × If
Ans: A

470. The torque in DC motor is independent of
A) Armature resistance
B) Armature current
C) Flux
D) Supply voltage
Ans: A

471. The power developed in armature is equal to
A) Eb × Ia
B) V × Ia
C) (V - Eb) × Ia
D) Eb / Ia
Ans: A

472. The speed equation of DC motor is
A) N ∝ (V - IaRa)/Φ
B) N ∝ Φ/(V - IaRa)
C) N ∝ IaRa/Φ
D) N ∝ V × Φ
Ans: A

473. If the flux of DC motor is halved, speed becomes
A) Double
B) Half
C) Same
D) Zero
Ans: A

474. The armature torque of DC motor is given by
A) 0.159 × ZΦIa/P × A
B) 9.55 × Pout/N
C) EbIa/ω
D) All of these
Ans: D

475. In DC generator, armature reaction causes
A) Sparking at brushes
B) Decrease in voltage
C) Demagnetizing effect
D) All of these
Ans: D

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

477. The losses in DC machines that vary with load are
A) Copper losses
B) Iron losses
C) Friction losses
D) Windage losses
Ans: A

478. Constant losses in DC machine are
A) Iron and mechanical losses
B) Copper losses
C) Armature losses
D) None
Ans: A

479. In DC machine, efficiency is maximum when
A) Variable losses = constant losses
B) Load current is max
C) Armature losses are zero
D) Flux is max
Ans: A

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

481. The no-load speed of DC shunt motor is
A) Slightly above normal
B) Very high
C) Below normal
D) Zero
Ans: A

482. The mechanical losses in DC machines depend on
A) Speed
B) Load current
C) Field current
D) Supply voltage
Ans: A

483. The power losses due to brush contact is
A) Vb × Ia
B) I²R
C) Eb × Ia
D) None
Ans: A

484. In DC generator, residual magnetism is essential for
A) Voltage build-up
B) Commutation
C) Speed regulation
D) Field control
Ans: A

485. A DC shunt motor develops maximum torque when
A) Back emf = 0.5 supply voltage
B) Back emf = supply voltage
C) Flux = 0
D) Armature current = 0
Ans: A

486. The torque of DC series motor varies as
A) Ia²
B) Ia
C) 1/Ia
D) Constant
Ans: A

487. The torque of DC shunt motor varies as
A) Ia
B) Ia²
C) ΦIa
D) Constant
Ans: C

488. A differential compound motor is not used because
A) It is unstable
B) It is costly
C) It has high speed
D) It has low torque
Ans: A

489. The field current of DC shunt motor is
A) Small
B) Equal to armature current
C) High
D) Same as supply
Ans: A

490. The voltage across armature of DC generator is
A) V = Eg – IaRa
B) V = Eg + IaRa
C) Eg = V + IaRa
D) V = Eg × Ia
Ans: A

491. The armature reaction under load results in
A) Demagnetizing and cross magnetizing effect
B) Strengthening main field
C) Neutral effect
D) None
Ans: A

492. Compensating windings are provided in DC machine to
A) Neutralize armature reaction
B) Reduce eddy losses
C) Increase flux
D) Reduce heating
Ans: A

493. The function of yoke in DC machine is
A) Provide mechanical support
B) Carry flux
C) Complete magnetic circuit
D) All of these
Ans: D

494. Armature core is laminated to
A) Reduce eddy current losses
B) Increase strength
C) Reduce hysteresis
D) Increase inductance
Ans: A

495. Commutator segments are made of
A) Hard drawn copper
B) Brass
C) Aluminum
D) Steel
Ans: A

496. Brushes are usually made of
A) Carbon or graphite
B) Copper
C) Iron
D) Brass
Ans: A

497. The field poles of DC machine are made of
A) Cast steel or wrought iron
B) Aluminum
C) Copper
D) Brass
Ans: A

498. The armature windings are placed in
A) Slots of core
B) Pole shoes
C) Yoke
D) Shaft
Ans: A

499. In lap winding, number of parallel paths is equal to
A) Number of poles
B) 2
C) 1
D) Z/2
Ans: A

500. In wave winding, number of parallel paths is always
A) 2
B) 4
C) Equal to poles
D) None
Ans: A