Electrical Instruments and Classification of instruments

Measuring Instruments

Classification of instruments

(i). Depending on the quality measured

 (ii). Depending on the different principles used for their working (iii). Depending on how the quantity is measured

Depending on the quality measured

Voltmeter

Ammeter

Energy meter

Ohm meter

Depending on the different principles used for their working

Moving Iron type

Moving coil type

Dynamometer type

Induction type

Depending on how the quantity is measured

Deflecting type

Integrating type

Recording type

Deflecting Torque

The deflecting torque moves the moving system and the pointer from the zero position. The deflecting torque can be obtained through magnetic, thermal, electromagnetic or electro dynamic effects

Controlling torque

The controlling torque acts in a direction opposite to that of deflecting torque. When the controlling torque (TC) and the deflecting torque (TD) are numerically equal the pointer takes a definite position. In the absence of TC the pointer would deflect to maximum position irrespective of the quantity to be measured. Moreover TC also helps in bringing the moving system to zero position when the instrument is disconnected from the circuit. The controlling torque is obtained through spring control and gravity control

Spring Control:

The arrangement for spring control consists of two phosphor bronze spiral hair springs attached to a moving system. The springs are made of materials which (i). are not affected by fatigue. (ii). Have low temp-coefficient of resistance (iii). Have low specific resistance (iv). Are non-magnetic

As the pointer deflects the springs get twisted in the opposite direction. The combined twist produces the necessary controlling torque which is proportional to angle of deflection of moving system θ. If we consider a permanent magnet moving coil meter with spring control system the deflecting torque will be proportional to the current passing through it and the controlling torque will be proportional to the angle of deflection

Thus T_D α I

T_C α θ

Since T_D = T_C

We have θ α I

Thus the spring controlled instruments having uniform scale

Gravity control

In gravity controlled instruments, as shown in Fig. 12.2 (a) a small adjustable weight is attached to the spindle of the moving system such that the deflecting torque produced by the instrument has to act against the action of gravity. Thus a controlling torque is obtained. This weight is called the control weight. Another adjustable weight is also attached is the moving system for zero adjustment and balancing purpose. This weight is called Balance weight.

When the control weight is in vertical position as shown in Fig. 12.2 (a), the controlling torque is zero and hence the pointer must read zero. However, if the deflecting torque lifts the controlling weight from position A to B as shown in Fig.12.2 (b) such that the spindle rotates by an angle θ, then due to gravity a restoring (or controlling) torque is exterted on the moving system.

The controlling (or restoring) torque, Tc , is given by

Tc = Wl sin θ = k g sin θ where W is the control weight;

l is the distance of the control weight from the axis of rotation of the moving system; and k g is the gravity constant.

Equation shows the controlling torque can be varied quite simply by adjustment of the position of the control weight upon the arm which carries it. Again, if the deflecting torque is directly proportional to the current,

i.e., Td = kI

We have at the equilibrium position Td = Tc

kI = k g sin θ

I = g k sin θ / k

This relation shows that current I is proportional to sin θ and not θ. Hence in gravity controlled instruments the scale is not uniform. It is cramped for the lower readings, instead of being uniformly divided, for the deflecting torque assumed to be directly proportional to the quantity being measured.

Advantanges of Gravity Control

1. It is cheap and not affected by temperature variations.

2. It does not deteriorate with time.

3. It is not subject to fatigue.

Disadvantages of Gravity Control

1. Since the controlling torque is proportional to the sine of the angle of deflection, the scale is not uniformly divided but cramped at its lower end.

2. It is not suitable for use in portable instruments (in which spring control is always preferred).

3. Gravity control instruments must be used in vertical position so that the control weight may operate and also must be leveled otherwise they will give zero error. In view of these reasons, gravity control is not used for indicating instruments in general and portable instruments in particular.

Damping Torque

We have already seen that the moving system of the instrument will tend to move under the action of the deflecting torque. But on account of the control torque, it will try to occupy a position of rest when the two torques are equal and opposite. However, due to inertia of the moving system, the pointer will not come to rest immediately but oscillate about its final deflected position as shown in Fig and takes appreciable time to come to steady state. To overcome this difficulty a damping torque is to be developed by using a damping device attached to the moving system.

The damping torque is proportional to the speed of rotation of the moving system, that is Tv = kv d dt θ

where kv = damping torque constant

d dt θ = speed of rotation of the moving system

Depending upon the degree of damping introduced in the moving system, the instrument may have any one of the following conditions as depicted in Fig.

1. Under damped condition: The response is oscillatory

2.   Over damped condition: The response is sluggish and it rises very slowly from its zero position to final position.

3.  Critically damped condition: When the response settles quickly without any oscillation, the system is said to be critically damped.

In practice, the best response is slightly obtained when the damping is below the critical value i.e., the instrument is slightly under damped.

The damping torque is produced by the following methods: Air Friction Damping & Fluid friction damping

Air Friction Damping

In this type of damping a light vane or vanes having considerable area is attached to the moving system to develop a frictional force opposing the motion by reason of the air they displace. Two methods of damping by air friction are depicted in Fig.

The arrangement shown in Fig consists of a light aluminum vane which moves in a quadrant (sector) shaped air chamber. The chamber also carries a cover plate at the top. The vane is mounted on the spindle of the moving system. The aluminum vane should not touch the air-chamber walls otherwise a serious error in the deflection of the instrument will be introduced. Now, with the motion, the vane displaces air and thereby a damping force is created on the vane that produces a torque (damping) on the spindle. When the movement is quicker the damping force is greater; when the spindle is at rest, the damping force is zero.

The arrangement of Fig. consists of a light aluminum piston which is attached to the moving system. This piston moves in a fixed chamber which is closed at one end. Either circular or rectangular chamber may be used. The clearance (or gap) between the piston and chamber walls should be uniform throughout and as small as possible. When the piston moves rapidly into the chamber the air in the closed space is compressed and the pressure of air thus developed opposes the motion of the piston and thereby the whole moving system. If the piston is moving out of the chamber, rapidly, the pressure in the closed space falls and the pressure on the open side of the piston is greater than that on the opposite side. Motion is thus again opposed. With this damping system care must be taken to ensure that the arm carrying the piston should not touch the sides of the chamber during its movement. The friction which otherwise would occur may introduce a serious error in the deflection.

The air friction damping is very simple and cheap. But care must be taken to ensure that the piston is not bent or twisted. This method is used in moving iron and hot wire instruments.

Fluid Friction Damping

This form is damping is similar to air friction damping. The action is the same as in the air friction damping. Mineral oil is used in place of air and as the viscosity of oil is greater, the damping force is also much greater. The vane attached to the spindle is arranged to move in the damping oil. It is rarely used in commercial type instruments. The oil used must fulfill the following requirements. It should not evaporate quickly . It should not have any corrosive effect on metals. Its viscosity should not change appreciably with temperature. It should be good insulator.

Advantages of Fluid Friction Damping

1. The oil used for damping can also be used for insulation purpose in some forms of instruments which are submerged in oil.

2.  The clearance between the vanes and oil chamber is not as critical as with the air friction clamping system.

3.   This method is suitable for use with instruments such as electrostatic type where the movement is suspended rather than pivoted.

4. Due to the up thrust of oil, the loads on bearings or suspension system is reduced thereby the reducing the frictional errors.

Disadvantages of Fluid Friction Damping

1. The instruments with this type of damping must be kept always in a vertical position.

2. It is difficult to keep the instrument clean due to leakage of oil.

It is not suitable for portable instruments. The fluid friction damping can be used for laboratory type electrostatic instruments.

Eddy current damping

Eddy Current Damping

Eddy current damping is the most efficient form of damping. The essential components in this type of damping are a permanent magnet; and a light conducting disc usually of alumninum. When a sheet of conducting material moves in a magnetic field so as to cut through lines of force, eddy currents are set up in it and a force exists between these currents and the magnetic field, which is always in the direction opposing the motion. \

This force is proportional to the magnitude of the current, and to the strength of field. The former is proportional to the velocity of movement of the conductor, and thus, if the magnetic field is constant, the damping force is proportional to the velocity of the moving system and is zero when there is no movement of the system.