Use of Wattmeter

Author: E.E. Kimberly

A wattmeter should be connected in the circuit to be metered in the manner shown in Fig. 16-7. The connecting terminals of the potential coil at P are always insulated to the hand, and the connecting terminals of the series or current coil are almost never so insulated. The coil terminals may usually be identified by that difference.

Fig. 16-7. Single-Phase Wattmeter Connections

It is important that the potential-coil terminal marked zero or ± be the one connected to the line passing through the current coil and on the load side of that coil, as shown in Fig. 16-7. If the meter reading is negative, it should be corrected by reversing the current-coil connections. If the potential-coil connections were reversed instead, the moving-coil end of the potential-coil circuit would be at full line potential with respect to the closely adjacent current coil, and electrostatic forces would tend to influence the reading and introduce errors.

Inasmuch as the reading of a wattmeter is a function of the cosine of the load phase angle, as well as of the current in the circuit, a moderately high reading is no assurance that the current coil is not seriously overloaded unless the power factor is known to be comparatively high. To protect the current coil against such accidental abuse, an ammeter of the same current rating as the current coil of the wattmeter should always be used in the circuit, although readings of power only may be important as data.

With few exceptions, wattmeters may be used in either direct-current or alternating-current circuits. The rare induction type of wattmeter (Westinghouse) may be used on alternating-current circuits only.