Dynamometer type wattmeter - Engineering notes

Dynamometer type wattmeter:

A dynamometer type wattmeter is most commonly employed for measurement of power in a.c as well as d.c circuits.

Principle of Dynamometer type wattmeter:

It is based on the principle that mechanical force exists between two current carrying conductors.

Construction of Dynamometer type wattmeter:

It essentially consists of two coils, namely fixed coil and 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 between the two fixed coils and is placed on the spindle to which the pointer is attached.

the fixed coils are connected in series with the load and carry the circuit current. It is, therefore called current coil. The moving coil is connected across the load and carries current proportional to the voltage. It is therefore called potential coil. Generally, a high resistance is connected in series with potential coil to limit the current through it.

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

Dynamometer type wattmeter 

Working of Dynamometer type wattmeter:

When power is to be measured in a circuit, the instrument is suitably connected in the circuit. The current coil is connected in series with load so that it carries the circuit current. The potential coil is connected across the load so that it carries current proportional to the voltage.

Due to the current in the coils, mechanical force exists between them. 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.

Reversing the current, reverses the field due to fixed coil as well as the current in the moving coil so that the direction of the deflection torque remains unchanged. Therefore, such instruments can be used for the measurement of a.c as well as d.c power.

Dynamometer type wattmeter circuit diagram

Deflecting torque:

It can be easily proved that deflecting torque is proportional to the power in the circuit.

Operation on d.c:

suppose that in a d.c circuit

V = voltages across load

I = current through load

Current through fixed coil If is proportional to I

Current through moving coil Im is proportional to V

Deflecting torque Td is due to the current If and Im

Td  proportional   Im.If  proportional  V.I  proportional    power

Operation on a.c:

suppose that in an a.c circuit

e = instantaneous voltage across load

i = instantaneous current through load

If the load has a lagging power factor of cos a, then equation become

e = Em. sinwt

i = Im sin (wt - a)

Current through fixed coil If is proportional to i

Current through moving coil Im is proportional to e

Due to large inertia of the moving system, the deflection will be proportional to the average torque.

Mean deflecting torque   proportional   Average of im.if

proportional    E I cos a

hence, dynamometer type wattmeter can be used for the measurement of a.c as well as d.c power.

We have seen that, 

Td   proportional    power 

Tc  proportional   deflection 

Deflection   proportional   power

Dynamometer type wattmeter instruments have uniform scale.