Dynamometer type instruments - Engineering notes

Dynamometer type instruments:

These instruments are the modified form of permanent magnet moving coils type. Here operating field is produced by a permanent but by another fixed coil. The moving system and the control system are similar to those of permanent magnet type. Such instruments can be used for both a.c and d.c circuits. They can be used as ammeters and voltmeters but are generally used as wattmeters.

Principle of Dynamometer type instruments:

These instruments are based on that principle the mechanical force exists between the current carrying conductors.

Construction of Dynamometer type instruments:

A dynamometer type instrument as shown in fig essentially consists of a fixed coil and a moving coil. The fixed coil is split into two  equal parts which are placed close together and parallel to each other. The moving coil is pivoted in between the two fixed coils. The fixed and moving coils may be excited separately or they may be connected in series depending upon the use to which the measurement is put. The moving coil is attached to the moving system so that under the action of deflecting torque, the pointer moves over the scale.

Dynamometer type instrument

The controlling torque is provided by two springs which also serve the additional purpose of leading the current into and out of the moving coil. Air friction damping is provided in such instruments.

Working of Dynamometer type instruments:

When instrument is connected in the circuit, operating currents flow through the coils. Due to this, mechanical force exists between the coils. The result is that the moving coil moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque.

by reversing the current, the field due to fixed coils is reversed as well as the current in the moving coil, so that the direction of deflecting torque remains unchanged. Therefore, such instruments can be used for both d.c and a.c measurements.

Deflecting torque of Dynamometer type instruments:

Let 

If = current through fixed coil

Im = current through moving coil

Since If = Im because the fixed and coils are in series,

Td = I^2

Since the control is by springs, therefore, 

controlling torque is proportional to the angle of deflection

Tc  proportional  deflection 

The pointer will come to rest at a position when Td = Tc

we get deflection  proportional  I^2

It is clear that deflection of the pointer is directly proportional to the square of the operating current. Hence, the scales of these instruments is non - uniform being crowded in their lower parts and spread out at the top.

Advantages of Dynamometer type instruments:

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

2. Such instruments are free from hysteresis and eddy current errors.

Disadvantages of Dynamometer type instruments:

1. Since torque / weight ratio is small, therefore, such instruments have  frictional errors which reduce sensitivity.

2. Scale is not uniform.

3. A good amount of screening of the instruments are required to avoid the effect of stray fields.

4. These instruments are costlier than types and, therefore, they are rarely used as ammeters and voltmeters.