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

DC Motor, MCQ / March 18, 2022

1. Ward-Leonard system of speed control is not recommended for _________

  1. Constant speed operation
  2. Wide speed
  3. Frequent-motor reversed
  4. Very slow speed

Answer:1. Constant speed operation

Explanation: 

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor.

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.
  • The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of the generator
  • It has inherent regenerative braking capacity.

Note:- Since Ward Leonard’s method is used for wide range of speed control therefore it is not practically used in constant speed drive.

 

2. In Ward Leonard speed control method for lowering the speed of the motor ______________

  1. Reduce armature voltage
  2. Increase armature voltage
  3. Increase field current
  4. Decrease field current

Answer: 1. Reduce armature voltage

Explanation: 

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.

 

3. Reducing the armature voltage will give us _________

  1. Variable torque speed control
  2. Constant torque speed control
  3. Variable and constant both can be achieved
  4. Cannot comment on torque

Answer: 2. Constant torque speed control

Explanation:

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.

 

4. In Ward Leonard speed control method for increasing the speed of the motor ______________

  1. Reduce armature voltage
  2. Increase armature voltage
  3. Increase field current
  4. Decrease field current

Answer:4. Decrease field current

Explanation: 

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. For obtaining speeds above base speed the field is gradually weakened maintaining armature voltage at rated value.

 

5. 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.

 

6. 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.

 

7. 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.

 

8. 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 rheostat speed control or field speed control method.

 

9. ________ 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.

 

10. 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.

 

11. To get the speed of DC motor below the normal speed without wastage of electrical energy we use ________

  1. Ward Leonard control
  2. Rheostatic control
  3. Any of the Ward Leonard or rheostatic method can be used
  4. Not possible

Answer: 1. Ward Leonard control

Explanation: 

  • The Ward Leonard system has the ability to “regenerate,” that is, to return stored energy in the machine to the supply lines.
  • For example, when a heavy load is first lifted and then lowered by the dc drive motor of a Ward Leonard system, the dc motor starts to act as a generator as the load is coming down, using its counter-torque as a brake.
  • Under these conditions, the “generator” itself starts to operate in the motor mode, driving the synchronous machine as a generator and supplying power back to the ac system. This process is known as regeneration.

 

12. A 220 V d. c. shunt motor running at 1000 rpm takes an armature current of 17.5 A. It is required to reduce the speed to 600 rpm. What must be the value of resistance to be inserted in the armature circuit if the original armature resistance is 0.4 Ω. Take armature current to be constant during this process.

  1. 4.8 Ω
  2. 4.4 Ω
  3. 0.4 Ω
  4. 5.4 Ω

Answer: 1. 4.8 Ω

Explanation:

Given

V = 220 V

N1 = 1000 rpm

Ia = 17.5 A

N2 = 600rpm

Ra = 0.4 ohm

Eb1 = V – Ia1Ra

= 220 – 17.5 × 0.4

Eb1 = 213 V

With constant flux,

Eb ∝ N

N2/N1 = EB2/EB1

EB2 = 600 × 213/100

EB2 = 127.8 V

To calculate External resistance value,

EB2 = V− Ia(R + R V

127.8 = 200 − 17.5(R + 0.4)

R = 4.8Ω

 

13. Speed control by Ward Leonard method, can give uniform speed variation ________

  1. In both directions
  2. In one direction
  3. Below normal speed only
  4. Above normal speed only

Answer: 1. In both directions

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.

 

14. In Ward Leonard’s control of DC motor, the lower limit of speed is imposed by ____________

  1. Residual magnetism of the generator
  2. Core losses of motor
  3. Mechanical losses of motor and generator together
  4. Cannot be determined

Answer:1. Residual magnetism of the generator

Explanation: 

In the ward Leonard method when the field excitation is controlled (i.e. capable of supplying current in one direction only), the minimum speed that can be obtained is 10% of the nominal speed. This is due to the presence of residual magnetism in the field circuit of the generator field. Thus, a lower limit of speed is imposed by the residual magnetism of the generator.

Due to the presence of residual magnetism, even with no field excitation enough voltage is generated to create crawling in the machine, especially at light load. To prevent crawling the following methods are adopted:

(i) The armature circuit is opened.

(ii) A differential field winding in the generator is connected across the armature terminals.

This field will oppose the residual flux and thereby reducing crawling.

 

15. The disadvantage of the Ward Leonard control method is ___________

  1. High initial cost
  2. High maintenance cost
  3. Low efficiency at high loads
  4. High cost, high maintenance and low efficiency

Answer: 4. High cost, high maintenance and low efficiency

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 Ward Leonard method gives uniform speed variation

  1. In one direction
  2. in both direction
  3. below normal speed only
  4. above normal speed only

Answer: 2. In both direction

Explanation: 

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

Advantages of the Ward Leonard drive-

  • Smooth speed control of DC motor over a wide range in both the direction is possible
  • It has an inherent braking capacity
  • The lagging reactive volt-amperes are compensated by using an overexcited synchronous motor as the drive and thus, the overall power factor improves
  • When the load is intermittent as in rolling mills, the drive motor is an induction motor with a flywheel mounted to smooth out the intermittent loading to a low value

 

17. Variation in speed by Ward-Leonard control method of D.C. motors occurs due to variation in

  1. Field excitation
  2. Armature current
  3. Armature voltage
  4. Supply voltage

Answer: 3. Armature voltage

Explanation: 

Speed control of dc motors can also be obtained by varying the applied voltage to the armature. Ward Leonard’s system of speed control is based on this principle. In this method, the adjustable voltage for the armature is obtained from an adjustable voltage generator while the field circuit is supplied from a separate source.

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

 

18. The application of Ward Leonard’s method of speed control is

  1. Elevator
  2. Rolling Mills
  3. Ship propulsion
  4. All of the above

Answer: 4. All of the above

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 to normal speed in both directions, accomplished by means of an adjustable voltage generator.

Ward-Leonard method of speed control is employed where fine speed adjustment and smooth acceleration are required, as in driving rolling mills, ship propulsion, colliery winders, and elevators in both directions.

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