How Does a Wattmeter Work?

This time, we will try to understand the principle behind how does a wattmeter work.

Some wattmeters do not have coils; the wattmeter considered here is the electromagnetic type.
 
Make sure to read what is ac circuit first.

Wattmeter Power Measurement

The average power absorbed by a load is measured by an instrument called the wattmeter.

The wattmeter is the instrument used for measuring the average power.

Figure.(1) shows a wattmeter that consists essentially of two coils : the current coil and the voltage coil.

A current coil with a very low impedance (ideally infinite) is connected in parallel with the load as shown in Figure.(2) and responds to the load voltage.

How Does a Wattmeter Work
Figure 1. A wattmeter

The current coil acts like a short circuit because of its low impedance; the voltage coil behaves like an open circuit because of its high impedance.

As a result, the presence of the wattmeter does not disturb the circuit or have an effect on the power measurement.

How Does a Wattmeter Work
Figure 2. The wattmeter connected to the load

When the two coils are energized, the mechanical inertia of the moving system produces a deflection angle that is proportional to the average value of the product v(t)i(t).

If the current and voltage of the load are v(t) = Vm cos(ωt + θv) and i(t) = Im cos(ωt + θi), their corresponding rms phasors are

How Does a Wattmeter Work

and the wattmeter measures the average power given by

How Does a Wattmeter Work

As shown in Figure.(2), each wattmeter coil has two terminals with one marked ±.

To ensure upscale deflection, the ± terminal of the current coil is toward the source, while the ± terminal of the voltage coil is connected to the same line as the current coil.

Reversing both coil connections still results in upscale deflection.

However, reversing one coil and not the other results in downscale deflection and no wattmeter reading.

This wattmeter can also be used for three-phase power measurement.

How Does a Wattmeter Work Example

Find the wattmeter reading of the circuit in Figure.(3)

How Does a Wattmeter Work
Figure 3

Solution :
In Figure.(3), the wattmeter reads the average power absorbed by the (8 – j6) Ω impedance because the current coil is in series with the impedance while the voltage coil is in parallel with it. The current through the circuit is

How Does a Wattmeter Work

The voltage across the (8 – j6) Ω impedance is

How Does a Wattmeter Work

The complex power is

How Does a Wattmeter Work

The wattmeter reads

How Does a Wattmeter Work