Page 843 - Basic Electrical Engineering

where θ is the angle of deflection and K is the spring constant
s

Under steady-state deflection, T d av = T c

By proper design of the instrument, the rate of change of mutual inductance,

M of the moving coil and stationary coil with respect to deflection, θ, i.e.,

is kept constant over the range of deflection of the moving coil.
The deflection θ is written as

θ α VI cos φ

i.e., deflection of the moving coil is proportional to the power of the ac

circuit.
In ac circuits, the deflection of the moving system will always be positive.
This is because in ac as the polarities of supply change, current through both

the current coil and pressure coil will change, and hence the deflection
always remains positive. The wattmeter is suitable for both ac and dc power

measurements.

11.10.3 Sources of Error in Measurement Using Dynamometer-type
Wattmeters

A wattmeter has two coils, namely, the current coil which carries the load
current and the voltage coil or the pressure coil which is connected in series

with a non-inductive high-value resistance across the load terminals. Torque
is produced due to the interaction between the fields created by the current
flowing through these two coils. Since the coils are air cored, the fields