50 Most Important MCQ Question of DC Motor | Objective type question of DC Motor With Explanation:

Que 1. No load speed of which of the following motor is highest?

  1. Differentially compound motor
  2. Cumulative compound motor
  3. Series Motor
  4. Shunt Motor

Ans 3. Series Motor

Explanation:

  • At the time of starting, flux is very less in DC series motor.
  • So as we know that inverse relationship between speed and flux i.e flux decrease then speed increase.
  • Therefore we never start DC series motor at no load.

 

Que 2. Which of the following load normally need starting torque more than the rated torque?

  1. Conveyors
  2. Blowers
  3. Centrifugal pump
  4. Air compressor

Ans 1. Conveyors

Explanation:

  • Starting torque is the amount required to overcome the inertia of a standstill.
  • It is also known as Pull up Torque.
  • Conveyors require high starting torque because the material is placed before the starting of Conveyors belt.
  • So obviously we require more torque to overcome the load at starting period.
  • Therefore conveyor required high starting torque and constant speed.
  • Series motor is suitable for conveyor belts operating in a particular region of its torque speed characteristics.
  • In industries  BLDC is used, as BLDC provides PWM speed controlling method which is easier to vary speed as well as with low maintains.

 

Que 3. Which of the following rule is used to determine the direction of rotation of D.C motor?

  1. Coloumb’s Law
  2. Lenz’s Law
  3. Fleming’s Right-hand Rule
  4. Fleming’s Left-hand Rule

Answer 4. Fleming’s left-hand rule.

Explanation:

 

  • When a current carrying conductor such as a wire attached to a circuit moves placed in a magnetic field, an electric current is induced in the wire due to Faraday’s law of induction.
  • The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other than
  • The thumb is pointed in the direction of the motion of the conductor relative to the magnetic field i.e direction of the force.
  • The fore finger is pointed in the direction of the magnetic field.
  • The middle finger represents the direction of the induced or generated current within the conductor.

 

Ques 4. Which part of DC motor can sustain maximum temperature rise?

  1. Armature Winding
  2. Field winding
  3. Slip Ring
  4. Commutator

Answer 4. Commutator

Explanation:

  • The commutator is made up of copper segments.
  • This copper segment is insulated from each other with the help of mica.
  • Mica has a very high dielectric stress of about 1180 kV/cm and the next best is Diamond.
  • It can withstand high temperatures, up to 600 deg C.

 

Ques 5. The ratio of starting torque to full load torque is least in

  1. Differential Compound Motor
  2. Shunt motor
  3. Series Motor
  4. Cumulative compound motor

Answer 1. Differential Compound motor

Explanation:

Differential Compound motor

  • In the differential compound, motor two field windings i.e shunt and series windings oppose each other.
  • This cause a reduction in flux and consequences decrease in a torque.

 

Ques 6. The number of pole in Small Dc Motor Up to 5 H.P are

  1. 2 poles
  2. 4 poles
  3. 8 poles
  4. 10 poles

Answer 1. 2 poles

Explanation:

  • Small H.P motors require only 2 poles because as the number of poles is inversely proportional to the speed therefore 2 pole motor run at high speed than 4 pole motor.
  • Two pole motor is better in efficiency.
  • Two pole motor has better rpm and noise performance.

 

Ques 7. The efficiency of DC motor at maximum power is

  1. 90%
  2. 100%
  3. Around 80%
  4. Less than 50%

Answer 4. Less than 50%

Explanation:

Why efficiency of D.C motor is below 50%

  • The DC motor develops maximum power when the Back EMF is half the applied voltage.
    Eb = V/2.
  • Practically it is not possible to develop exact 50% of maximum power because in that case, the current would be much beyond the rated current of the motor.
  • Some of the energy is wasted in the form of heat and other losses.
  • Therefore motor efficiency is below 50%.

 

Ques 8. In which of the following application DC series motor is used?

  1. Centrifugal Pump
  2. Motor Operation in DC and AC
  3. Water pump drive
  4. Starter for car

Answer 4. Starter for car

Explanation:

  • In DC series motor Torque (Ta) increase as the Square of armature current (Ia).
    Ta ∝ Ia2.
  • So DC motor provides high starting torque which is required to start a car.

 

Ques 9. In DC machine the fractional pitch winding is used

  1. To reduce the Harmonic in generated EMF
  2. Improve Cooling
  3. Increase EMF
  4. To reduce the copper losses

Answer 1. To reduce the Harmonic in generated EMF

Explanation:

  • In full pitched coils, as one conductor of a turn in a coil cuts N pole, the other conductor of the same turn cuts S pole resulting in the production of induced emf(E), i.e phase angle is 180 degree
  • In short pitched coils, both conductors of the same turn in a coil don’t cut the respective poles simultaneously. so phase angle is slightly less than 180 degree As a result, the magnitude of induced emf gets reduced to E × Cos(nα/2)
    Where E × Cos(nα/2) is called as pitch factor
  • For eliminating 3rd harmonic from Generated EMF,
    Cos(3α/2) = 0
    3α/2 = π/2
    α = π/3 = 60°

 

Ques 10. A three point starter is suitable for

  1. Shunt Motor
  2. Series Motor
  3. Shunt & Compound Motor
  4. Shunt, Series, and compound motor

Answer 3. Shunt & Compound motor

Explanation:

3 point starters in DC Shunt and Compound machines serve for following purposes.

  • It limits the high starting current into the armature by having the resistance high at the time of starting and reducing it during running condition.
  • It also protects the motor from overload and under voltage conditions.