DC Motor Speed Control MCQ || DC Motor Speed Control Questions and Answers

11. Reducing the field current will give us ______

  1. Constant torque and variable power speed control
  2. Constant torque speed control
  3. Variable power speed control
  4. Constant power speed control

Answer: 4. Constant power speed control

Explanation: 

In DC Motor Speed control is a form of torque control.

In a d.c. motor, if the armature voltage is reduced, but the load torque kept constant, speed will fall so that the back e.m.f. reduces sufficiently to increase the current to almost its original value (flux being kept constant) and give an unchanged machine torque.

Similarly, a reduction in field flux would cause a reduction in speed so that the increased current can give the same torque, the flux-current product tending to be maintained constant. If the load torque was allowed to fall as flux decreased, motor speed would increase to compensate for lost torque and so keep the power constant.

 

12. Ward Leonard control is basically a _______ control method for below-rated speed.

  1. Field
  2. Armature resistance
  3. Armature voltage
  4. Field diverter

Answer: 3. Armature voltage

Explanation:

Ward-Leonard method of speed control is achieved by varying the applied voltage to the armature. Hence it is basically an armature voltage control method.

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. The speed control of the D.C. motor can be obtained by varying the applied voltage to the armature.

At the base speed, the motor armature is fed at rated voltage and its field current is adjusted to the maximum value, i.e. the field is excited at rated voltage. Reducing the armature voltage provides a constant- torque speed control where the speed can be reduced below the base value.

 

13. Speed-power characteristic for Ward Leonard speed control method ______

  1. Will start from origin
  2. Will start from some positive value on power axis
  3. Will start from some positive value on speed axis
  4. Depends on other parameters

Answer: 1. Will start from origin

Explanation:

The ward-Leonard method is the basic adjustable armature voltage control method of controlling the speed of a DC shunt motor over the whole range from zero (origin) to normal speed in both directions, accomplished by means of an adjustable voltage generator.

 

14. For a DC shunt motor if the excitation is changed then:

  1. Torque will remain constant
  2. Torque will change but power will remain constant
  3. Torque and power both will change
  4. All torque, speed and power will change

Answer: 2. Torque will change but power will remain constant

Explanation:

Field current or excitation is directly proportional to the torque, therefore torque will be changed. If the excitation or field current is changed, the armature current changes and back emf also will be changed accordingly and hence power will remain constant.

 

15. Efficiency of Ward Leonard method is ________

  1. Higher than rheostatic control method but lower than shunted field control method
  2. Lower than rheostatic control method
  3. Higher than rheostatic control method and shunted field control method
  4. Depends on load

Answer: 2. Lower than rheostatic control method

Explanation:

Drawbacks of Ward Leonard system:

  1. Higher initial cost due to use of two additional machines (M-G set) of the same rating as the main dc motor.
  2. Larger size and weight.
  3. Require more floor area and costly foundation.
  4. Lower efficiency due to higher losses.
  5. Frequent maintenance is needed.

Since the efficiency, is essentially the product of the individual efficiencies of the two larger machines, the efficiency of this method is not as high as the rheostat speed control or field speed control method.

 

16. Speed control by the variation of flux in a DC shunt motor will give

  1. Constant torque drive
  2. Variable torque drive
  3. Constant power drive
  4. Variable power drive

Answer: 3. Constant power drive

Explanation:

​​A constant speed drive maintains constant speed irrespective of load on the drive.

In DC shunt motor the power is given as

P = (2πNT)/60

As speed is constant(N = k),

⇒ P ∝ T

So, the drive consumes more power if we increase the load on the machine.

Hence, this drive is also called a constant speed variable power drive.

DC shunt motor has these types of characteristics

Applications:

  • Constant speed applications
  • Fans, Forced air circulators, Centrifugal pumps, Hydraulic pumps
  • Lathe machines in the iron industry

 

17. ________ is an ideal choice for the motor which undergoes frequent starting, stopping, speed reversal.

  1. Armature Control Method
  2. Field Control Method
  3. Rheostat control Method
  4. Ward Leonard Method

Answer: 4. Ward Leonard Method

Explanation: 

  • Ward Leonard Method is an ideal choice for the motor which undergoes frequent starting, stopping, speed reversal.
  • Ward-Leonard method of speed control is achieved by varying the applied voltage to the armature. Hence it is basically an armature voltage control method.
  • Ward Leonard’s method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. The speed control of the D.C. motor can be obtained by varying the applied voltage to the armature.
  • At the base speed, the motor armature is fed at rated voltage and its field current is adjusted to the maximum value, i.e. the field is excited at rated voltage. Reducing the armature voltage provides a constant- torque speed control where the speed can be reduced below the base value.

 

18. Which of the following control methods is not preferred for AC drives?

  1. Changing the supply frequency
  2. Injecting resistance in the rotor circuit
  3. Field control method
  4. Changing the supply voltage

Answer: 3. Field control method

Explanation: 

The speed control of three-phase induction (AC drive) are classified as:

  • V/f control or frequency control
  • Changing the number of stator poles
  • Controlling supply voltage
  • Adding rheostat in the stator circuit
  • Adding external resistance on the rotor side
  • Cascade control method
  • Injecting slip frequency emf into rotor side

From the above options, the Field control method is not preferred for AC drive.

 

19. Starting gear used in Ward Leonard method _______

  1. Is of small size
  2. Is of large size
  3. Size depends on the application
  4. Is absent

Answer: 4. Is absent

Explanation:

Advantages of the Ward-Leonard system:

  • Smooth speed control of dc motor over a wide range in both directions is possible.
  • This method is used for the speed control of large motors.
  • The speed can be controlled in both the direction of rotation of the motor easily.
  • No special starting gear is required. As the generator-induced voltage is gradually raised from zero, the motor starts up smoothly.
  • The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of the generator
  • It has inherent regenerative braking capacity.

 

20. For a DC shunt motor, speed cannot be varied by varying

  1. Back EMF
  2. Method of starting
  3. Field current
  4. Armature circuit

Answer: 2. Method of starting

Explanation:

In armature resistance control the speed by increasing armature resistance. This method gives only below base speeds. This method is a constant torque and variable power drive.

In Field Control Method, By varying flux, we can increase the speed more than its base speed. This method is constant power and variable torque drive.

In a DC motor, the back emf is directly proportional to speed and flux.

Eb ∝ Nϕ

Where N is speed and ϕ is flux.

In a DC shunt motor, flux is constant.

So, the back emf is directly proportional to speed.

Note:- For a DC shunt motor, speed cannot be varied by the method of starting.

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