Dynamometer Wattmeter Principle

A dynamometer-type wattmeter operates based on the electrodynamic principle. It utilizes the interaction between the magnetic fields produced by fixed current coils (current coils) and a moving coil.

Power Measurement Mechanism

In a dynamometer wattmeter:

• The current coil is connected in series with the load, carrying the load current ($I$).
• The potential coil is connected across the load, carrying a current ($I_p$) proportional to the load voltage ($V$).

The torque produced is proportional to the product of the currents in the two coils and the sine of the angle between them. For a resistive load, this torque is proportional to the instantaneous power consumed by the load.

Response Value

For AC circuits, the instantaneous power ($p(t)$) fluctuates. The deflection of the wattmeter's pointer indicates the average value of this instantaneous power over a complete cycle. This average value corresponds to the true power or active power consumed by the load.

Mathematically, the average power ($P_{avg}$) measured is given by:

$P_{avg} = V_{rms} \times I_{rms} \times \cos(\phi)$

where $V_{rms}$ and $I_{rms}$ are the RMS voltage and current, and $\cos(\phi)$ is the power factor.

Conclusion

Therefore, the dynamometer-type wattmeter responds to the average value of the active power.