Page 837 - Basic Electrical Engineering

and since this magnetism is produced by the current to be measured, we can
write

T α I 2
d

and T α θ, as more the deflection, θ more is the control torque, T .
c
c
Thus, deflection of the pointer, θ is directly proportional to the square of
the current to be measured. Control torque is provided by spring attached to
the spindle and damping (reduction of oscillation of the pointer in the

deflected position) is provided by the air friction damping mechanism. As θ α
2
I , this instrument can be used for both ac and dc measurement.

11.9.4 Dynamometer-type Moving Coil Instruments

In dynamometer-type instruments, there is one fixed coil and one moving
coil. The fixed coil is however, made in two sections, and placed apart as

shown in Fig. 11.22. The moving coil is free to move on a spindle. The
pointer is attached with it and will move over a graduated scale when the
moving coil gets deflected. This instrument can be used as ammeter,

voltmeter, and wattmeter for the measurement of current, voltage, and power,
respectively. For use as ammeter and voltmeter, both the fixed coils and the

moving coil are connected in series as shown in Fig. 11.22. For use as
wattmeter, the fixed coils will carry the line current while the moving coil

will carry a current proportional to the voltage. That is, in a wattmeter, the
fixed coils, which are also called the current coils, are connected in series

with the load whose power is being measured and the moving coil, which is
also called the voltage coil or pressure coil, is connected across the supply
voltage. As wattmeter the deflecting torque will be proportional to power in

an ac circuit, i.e.,

P = VI cos ϕ

where cos ϕ is the power factor of the circuit.