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In this article, we will study about the controlling torque in the Electrical Instrument. In the previous article we had studied about DEFLECTING TORQUE. You can read that article as link is provided below;
Producing deflecting torque in Electrical instruments
WHAT IS CONTROLLING TORQUE?
The deflection of the pointer will be indefinite and pointer will go on moving, if there is no controlling or restoring torque. This controlling torque opposes the deflecting torque. Pointer will be at rest when controlling torque becomes equal to deflecting torque.
Controlling torque acts in the opposite direction to the deflecting torque.
Moreover, in the absence of controlling torque, the pointer once deflected, would not return to zero position on the removal of the current.
There are two methods of providing controlling torque in the instrument.
1. Spring Controlled.
2. Gravity Controlled.
We will discuss each method one by one.
1. SPRING CONTROL
In this method, two springs made up of phosphor bronze are used. One spring is placed above while other spring is placed below the spindle of the instrument. One end of each spring is attached to the spindle and its other end is attached to the fixed part of the instrument.
The arrangement in the spring control method is shown in this figure.
Both springs are wounded are in reverse direction to each other to compensate against temperature changes.
Materials used for making springs should be non magnetic, of low specific resistance and should have low temperature coefficient of resistance.
The controlling torque in this method depends upon the twisting of springs. The twisting force of spring is proportional to the angle through which the pointer moves.
When the instrument is not in use, the two springs are in their natural conditions and the controlling torque is zero. When the instrument is used to measure any electrical quantity then a deflecting torque is produced, and then pointer moves and one of the spring gets twisted while other spring gets unwounded. The resultant twist produces the controlling torque. The twist goes on increasing with the increase in the deflection of pointer and so does the controlling torque. Position comes when the deflecting torque becomes equal to the controlling torque and pointer stops.
Design of Springs: - Usually flat springs are used because they require less space.
The controlling torque developed by the spring is equal to
Tc = {Y*b*t3*�}/ 12* L ;
Where, Y = Young�s Modulus of spring material
b = width of the spring.
t = thickness of the spring.
� = deflection of the pointer.
L = length of the spring.
ADVANTAGES OF SPRING CONTROL METHOD
1. The instrument can be placed in any position i.e., horizontal as well as vertical.
2. This method does not increase the weight of the instrument as springs are light.
3. Scale is uniform in this method.
DISADVANTAGES OF SPRING CONTROL METHOD
a. Temperature variations can affect the length and design of the spring and hence also changes the controlling torque.
b. Springs get deterioted with the time and hence its accuracy is also lost.
2. GRAVITY CONTROL METHOD
In this method, natural downward pull due to gravity is employed. A small weight �W� is placed on the arm attached to the moving system.
At the zero position of the pointer, the control weight W is in the vertical position and therefore no controlling torque is produced. However, under the action of the deflecting force, the pointer deflects by an angle �ؔ from its zero position.
As shown in the figure below, as pointer moves weight will also move.
But due to gravity, the control weight would try to come back to its original vertical position and hence produces the necessary controlling torque. There are two components of the control weight Wsin� and Wcos�.
Only the component Wsin� will provide the necessary controlling torque and helps to retain the original position.
Thus, Tc = Wsin�
It is seen from above equation that controlling torque is directly proportional to the sine of the deflected angle.
ADVANTAGES OF GRAVITY CONTROL METHOD
1. It is simple and cheap method.
2. Not affected by temperature variations.
DISADVANTAGES OF GRAVITY CONTROL METHOD
1. Have non-uniform scale and thus reading become difficult.
2. It increases the weight of the system.
3. The instrument should be placed in the always in the vertical position and thus their use is limited to the indoors, otherwise it may produce error in the reading.
Now, we will study damping torque in next article as link is provided below.
Damping torque in Electrical Instruments
Thanks for reading.
Keep sharing and loving.
In this article, we will study about the controlling torque in the Electrical Instrument. In the previous article we had studied about DEFLECTING TORQUE. You can read that article as link is provided below;
Producing deflecting torque in Electrical instruments
WHAT IS CONTROLLING TORQUE?
The deflection of the pointer will be indefinite and pointer will go on moving, if there is no controlling or restoring torque. This controlling torque opposes the deflecting torque. Pointer will be at rest when controlling torque becomes equal to deflecting torque.
Controlling torque acts in the opposite direction to the deflecting torque.
Moreover, in the absence of controlling torque, the pointer once deflected, would not return to zero position on the removal of the current.
There are two methods of providing controlling torque in the instrument.
1. Spring Controlled.
2. Gravity Controlled.
We will discuss each method one by one.
1. SPRING CONTROL
In this method, two springs made up of phosphor bronze are used. One spring is placed above while other spring is placed below the spindle of the instrument. One end of each spring is attached to the spindle and its other end is attached to the fixed part of the instrument.
The arrangement in the spring control method is shown in this figure.
Arrangement for Spring Control Method.
Both springs are wounded are in reverse direction to each other to compensate against temperature changes.
Materials used for making springs should be non magnetic, of low specific resistance and should have low temperature coefficient of resistance.
The controlling torque in this method depends upon the twisting of springs. The twisting force of spring is proportional to the angle through which the pointer moves.
When the instrument is not in use, the two springs are in their natural conditions and the controlling torque is zero. When the instrument is used to measure any electrical quantity then a deflecting torque is produced, and then pointer moves and one of the spring gets twisted while other spring gets unwounded. The resultant twist produces the controlling torque. The twist goes on increasing with the increase in the deflection of pointer and so does the controlling torque. Position comes when the deflecting torque becomes equal to the controlling torque and pointer stops.
Design of Springs: - Usually flat springs are used because they require less space.
The controlling torque developed by the spring is equal to
Tc = {Y*b*t3*�}/ 12* L ;
Where, Y = Young�s Modulus of spring material
b = width of the spring.
t = thickness of the spring.
� = deflection of the pointer.
L = length of the spring.
ADVANTAGES OF SPRING CONTROL METHOD
1. The instrument can be placed in any position i.e., horizontal as well as vertical.
2. This method does not increase the weight of the instrument as springs are light.
3. Scale is uniform in this method.
DISADVANTAGES OF SPRING CONTROL METHOD
a. Temperature variations can affect the length and design of the spring and hence also changes the controlling torque.
b. Springs get deterioted with the time and hence its accuracy is also lost.
2. GRAVITY CONTROL METHOD
In this method, natural downward pull due to gravity is employed. A small weight �W� is placed on the arm attached to the moving system.
At the zero position of the pointer, the control weight W is in the vertical position and therefore no controlling torque is produced. However, under the action of the deflecting force, the pointer deflects by an angle �ؔ from its zero position.
As shown in the figure below, as pointer moves weight will also move.
Arrangement of gravity control method.
But due to gravity, the control weight would try to come back to its original vertical position and hence produces the necessary controlling torque. There are two components of the control weight Wsin� and Wcos�.
Only the component Wsin� will provide the necessary controlling torque and helps to retain the original position.
Thus, Tc = Wsin�
It is seen from above equation that controlling torque is directly proportional to the sine of the deflected angle.
ADVANTAGES OF GRAVITY CONTROL METHOD
1. It is simple and cheap method.
2. Not affected by temperature variations.
DISADVANTAGES OF GRAVITY CONTROL METHOD
1. Have non-uniform scale and thus reading become difficult.
2. It increases the weight of the system.
3. The instrument should be placed in the always in the vertical position and thus their use is limited to the indoors, otherwise it may produce error in the reading.
Now, we will study damping torque in next article as link is provided below.
Damping torque in Electrical Instruments
Thanks for reading.
Keep sharing and loving.
Mr. Muhammad Saqib, M.Sc. (PAK), Research Fellow (PAK), a member of IEEE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from University Of Engineering And Technology Taxila, PAKISTAN.
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