This time, we will try to understand the principle behind how does a wattmeter work.
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Wattmeter Power Measurement
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.
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| 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.
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| 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
and the wattmeter measures the average power given by
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)
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| 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
The voltage across the (8 – j6) Ω impedance is
The complex power is
The wattmeter reads
