DC Motor Principle MCQ || Objective Question with Answer for DC Motor Principle

11. Which part will surely tell that given motor is DC motor and not an AC type?

  1. Winding
  2. Shaft
  3. Commutator
  4. Stator

Answer:3. Commutator

Explanation:

  • Based on construction, the basic difference between DC & AC motor is the commutator.
  • The commutator is only present in DC motor, which performs the commutation phenomenon.
  • Commutation can be defined as the reversal of current (AC to DC or DC to A.
  • In DC motor commutator converts AC to DC ( Rectifier function )
  • In DC generator commutator converts DC to AC (Inverter function).

 

12. What will happen to torque if the back emf and speed of the DC motor is doubled?

  1. Remain unchanged
  2. Reduce to one-fourth value
  3. Increase four folds
  4. Be doubled

Answer:1. Remain unchanged

Explanation:

Torque on the DC Machine depends on armature current, so as long as armature current remains constant torque will not change.  For a constant load, the armature current remains the same so the input to the motor remains the same but the output decreases in proportion to the speed.

The speed of the DC motor is directly proportional to the back emf generated inside it and is inversely proportional to the flux produced in the machine.

 

13. Which of the following is used to convert electrical energy to mechanical energy?

  1. Generator
  2. Motor
  3. Transformer
  4. None of the above

Answer:2. Motor

Explanation:

  • An electric motor is an electrical machine that converts electrical energy into mechanical energy.
  • Most electric motors operate through the interaction between the motor’s magnetic field and electric current in a wire winding to generate force in the form of rotation of a shaft.
  • An electric generator is mechanically identical to an electric motor, but operates in the reverse direction, converting mechanical energy into electrical energy.
  • The transformer is a device, which is used to transform power at the same frequency.

 

14. In DC motor, which of the following part can sustain the maximum temperature rise?

  1. Commutator
  2. Field winding
  3. Slip rings
  4. Armature winding

Answer:2. Field winding

Explanation:

  • Field winding in a DC motor is not a rotating part.
  • It is present away from the rotating part of the machine.
  • So, temperature rise in the machine will not produce any effect on the machine rotation.
  • Thus, field winding can sustain the maximum temperature rise.

 

15. At the instant of starting, when a DC motor is put on supply, it will behave like _________

  1. Highly resistive circuit
  2. Low resistance circuit
  3. Capacitive circuit
  4. Inductive circuit

Answer:2. Low resistance circuit

Explanation:

  • At the starting of the DC machine when the back emf is very low, the machine will draw a large amount of armature current from the supply acting as a low resistive circuit.
  • Motor Starting Resistors are used to reduce the voltage at the motor terminals and also decrease the starting current.
  • Motor Starting Resistors are necessary because the self-resistance of a motor armature is very low. When the voltage is first applied, excessive current will flow. Because there will be no back electromotive force (emf) to limit this current, some series resistance can be added to the armature windings.
  • As the motor accelerates, the back emf will build, and the current through the resistor will thus decrease.
  • Additionally, the voltage drop on the resistor will reduce, causing the voltage across the terminals to increase. With this gradually increasing torque and voltage, the acceleration becomes smooth. The resistance is disconnected when the motor reaches a certain speed.

 

16. Two series motors are mechanically coupled. One machine is run as a motor and the other as a generator. The iron and friction losses of the machines will be identical when

  1. Their speeds are identical
  2. Their speeds and excitations are identical
  3. Their speeds are equal and back EMFs are half the supply voltage
  4. Their ratings and armature sizes are identical

Answer:2. Their speeds and excitations are identical

Explanation:

  • Field test is applicable to two similar DC series motors of large capacity
  • Series motors which are mainly used for traction work are easily available in pairs. The two machines are coupled mechanically
  • One machine runs normally as a motor and drives generator whose output is wasted in a variable load R
  • Iron and friction losses of two machines are made equal by joining the series field winding of the generator in the motor armature circuit so that both machines are equally exciting and by running them at equal speed.
  • Load resistance R is varied till the motor current reaches its full-load value indicated by the ammeter.
  • After this adjustment for full-load current, different ammeter and voltmeter readings are noted.

 

17. The current drawn by the armature of DC motor is directly proportional to _______

  1. Torque
  2. Speed
  3. The voltage across the terminals
  4. Cannot be determined

Answer:1. Torque

Explanation:

Torque on the DC Machine depends on armature current, so as long as armature current remains constant torque will not change.  For a constant load, the armature current remains the same so the input to the motor remains the same but the output decreases in proportion to the speed.

The speed of the DC motor is directly proportional to the back emf generated inside it and is inversely proportional to the flux produced in the machine.

 

18. Which of the following statements is/are true regarding the electric braking of motors

(I) In regeneration braking, the back emf Eb is greater than the supply voltage V.

(II) In reverse current breaking, connection to the armature terminals is reversed.

(III) In Dynamic braking, connections to the armature terminals are reversed and a variable resistance R is connected in series across it.

(IV) Plugging gives greater braking torque than rheostatic breaking.

  1. I, II, III, IV
  2. I, II, III
  3. I, II, IV
  4. II, III, IV

Answer:3. I, II, IV

Explanation:

Regenerative braking: In this type of braking back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Dynamic braking: In this type of braking, the DC motor is disconnected from the supply and a braking resistor (R) is immediately connected across the armature. The motor will now work as a generator and produces the braking torque.

Plugging: In this method, the terminals of supply are reversed, as a result of the generator torque also reverses which resists the normal rotation of the motor, and as a result the speed decreases. Plugging has the highest braking torque.

 

19. Which power is mentioned on a nameplate of a motor?

  1. Gross power
  2. Power drawn in kVA
  3. Power drawn in kW
  4. Output power available at the shaft

Answer:4. Output power available at the shaft

Explanation:

The motor nameplate provides information about the motor’s construction and performance characteristics. A nameplate contains a large amount of useful information relating to the motor including the type, RPM. frame size and power of the motor rated speed, rated current, rated voltage.

It also shows output power available at the shaft when all other quantities are set to rated values.

kW is an expression of the motor’s mechanical output rating – its ability to deliver the torque needed for the load at rated speed.

 

20. All others are advantages of DC motor over AC motor except ________

  1. Low cost
  2. Wide speed range
  3. Stability
  4. High starting torque

Answer:1. Low cost

Explanation:

DC motors have the advantage of higher starting torque, quick starting and stopping, reversing, variable speeds with voltage input and they are easier and cheaper to control than AC.

DC machines have many disadvantages, such as high maintenance costs for the commutator and brushes, high sensitivity to vibrations (due to the commutation), a high acquisition cost, the limitation of the product of power and speed to 1000MW -rpm (e.g. 750kW at 1500 rpm).

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