Hello readers, in this article we will study about AIR FRICTION DAMPING.

AIR FRICTION DAMPING

Hello friends and welcome to ELECTRICAL ENCYCLOPEDIA, in this article we will study about the Hot Wire Instruments , it construction , working , advantages and disadvantages.

These type of instruments are mainly used as Ammeter but can be used as Voltmeter by connecting a high resistance in series.


CONSTRUCTION OF HOT WIRE INSTRUMENT

 As shown in the figure , this type of instrument consists of a wire AB which is made up of platinum-iridium because it can withstand oxidation at higher temperature. Wire AB is stretched between a fixed end B and adjusting screw at A. When current is to be measured , it is then passed through the wire AB.

There is a another wire CD which is usually made up of phosphor-bronze. One end of wire CD is is attached to the center of AB while other end of CD is taken over the pulley by silk thread. This silk thread is pulled by the spring S.

   ALSO CHECK
- Moving Iron Instruments
-Permanent magnet moving coil Instruments
WORKING OF HOT-WIRE INSTRUMENT


Working principle of hot wire instrument is based upon the heating effect of Electric Current. When the current is passed through the wire AB , it expands. This produces the sag in wire AB , since the wire CD is attached to AB. So it will produce a slack in CD, this slack in CD is taken up by the silk thread which after passing through the pulley is attached to the spring. As soon as the silk thread is pulled by the spring, the pulley moves , therefore produces a deflection in the pointer which in turn gives the reading on calibrated scale.

 
It can be seen that the deflection of the pointer is proportional to the extension of wire AB which is proportional to the  square of the current (I�).

Damping :  Generally eddy current damping is provided in hot wire instrument. A thin light aluminium disc is attached to the pulley such that it moves between the poles of permanent damping magnet ,M.

ADVANTAGES OF HOT WIRE INSTRUMENTS

1. Such instruments can be used for both a.c and d.c.

2. There readings are independent of waveform and frequency.

3. They are not affected by the stray magnetic field.

DISADVANTAGES OF HOT WIRE INSTRUMENTS

1. They have high power consumption.

2. They are fragile.

3. Hot wire instruments are sluggish (slow) in nature i.e. it takes time to heat up.

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Hello friends, welcome to �beingelectricalengineer�.
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
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