Page 841 - Basic Electrical Engineering

where V and I are the rms values of voltage and current and cos ϕ is the
power factor of the circuit.

11.10.2 Measurement of Power in Single-phase Ac Circuit

For measurement of power we generally use an electrodynamometer-type
wattmeter. The wattmeter has two coils; one is a stationary or fixed coil and

the other is a moving a coil. The moving coil terminals and stationary coil
terminals are brought out in the instrument for connection to the circuit for
power measurement. The fixed coil is connected in series with the load

whereas the moving coil is connected in series with a high resistance and is
connected in parallel, i.e., across the load as shown in Fig. 11.23.

Figure 11.23 Measurement of power in an ac circuit using a wattmeter

It can be seen from Fig. 11.23 that the coil cc, called the current coil carries
the current, i which is the same as the load current because this coil is
c
connected in series with the load. The coil pc, called the pressure coil or
voltage coil is connected in parallel with the load. This coil carries a current,

i . Current, i flows through the pressure coil and through the high resistance
p
p
R . Neglecting the impedance of the pressure coil, i can be taken as equal to
s
p
.

Instantaneous deflecting torque