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The measurement of power

Understanding the operating costs of electric machinery and systems requires knowledge of power and energy usage. Power measurements are particularly important in evaluating performance of a-c motors both before and after repairs, as in determining the reusability of a possibly damaged stator core, evaluating no-load friction and windage loss, and estimating motor efficiency. Yet many service centers are not well equipped for power measurement.

Power, measured in watts, is represented by the product of voltage and current. Techniques first developed for d-c circuits were not suited to alternating current circuits, in which wattage depends upon the phase displacement between current and voltage, as well as their magnitudes. When that phase difference amounts to a 90 degree angle--that is, a zero "power factor"--the power in watts becomes zero. The frequent literature references to "real" and "reactive" power do not help clarify the relationships involved.

The most common a-c power measuring instrument is the electrodynamometer wattmeter in which a fixed coil carries current, while a moving coil is energized by the voltage, rotating against a restraining spring according to the torque created by magnetic interaction between the two coils. A two-element wattmeter contains two sets of current coils, and two voltage coils coupled to a common pointer shaft.

In three-phase circuits, it was proven in the 19th Century that only two wattmeters are needed to correctly assess total power. This is equally true for both wye- and delta-connected load circuits. Total power is the algebraic sum of the two meter readings, which will vary according to the power factor. When that factor is low (below 50%), inaccuracies arise from reduced moving coil torque as well as from the ratio of the two readings, such that wattmeters intended for such circuits be compensated for such errors. The newest instruments use complex electronic circuitry to avoid variations inherent in electromechanical metering.

To convert power to energy requires coupling the wattmeter function to the time during which the power is supplied. In the typical watthour meter, current and voltage combine to rotate an induction disc at a speed proportional to power. A gear train converts that rate of rotation to a series of calibrated dial readings that indicate the energy used.

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