11. Reducing the field current will give us ______
Constant torque and variable power speed control
Constant torque speed control
Variable power speed control
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.
Field
Armature resistance
Armature voltage
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 ______
Will start from origin
Will start from some positive value on power axis
Will start from some positive value on speed axis
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:
Torque will remain constant
Torque will change but power will remain constant
Torque and power both will change
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 ________
Higher than rheostatic control method but lower than shunted field control method
Lower than rheostatic control method
Higher than rheostatic control method and shunted field control method
Depends on load
Answer: 2. Lower than rheostatic control method
Explanation:
Drawbacks of Ward Leonard system:
Higher initial cost due to use of two additional machines (M-G set) of the same rating as the main dc motor.
Larger size and weight.
Require more floor area and costly foundation.
Lower efficiency due to higher losses.
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
Constant torque drive
Variable torque drive
Constant power drive
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.
Armature Control Method
Field Control Method
Rheostat control Method
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?
Changing the supply frequency
Injecting resistance in the rotor circuit
Field control method
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 _______
Is of small size
Is of large size
Size depends on the application
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
Back EMF
Method of starting
Field current
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.