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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.
How Does a Wattmeter Work
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.

Wattmeter Circuit

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
Have you understood how does wattmeter work? Don't forget to share and subscribe! Happy learning!
Reference:  Fundamentals of electric circuits by Charles K. Alexander and Matthew N. O. Sadiku.
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