The Compensated Wattmeter

Author: E.E. Kimberly

Inasmuch as in Fig. 16-7 the potential-coil current flows through the current coil, the wattmeter so connected will read too high by the amount of the power lost in the potential coil. When the power factor of the load is specially low, the wattmeter reading will be relatively low, and the error introduced by the potential-coil power may be too great to be ignored. In this case a compensated wattmeter is convenient. In the compensated wattmeter, a compensating coil having the same number of turns as the current coil is wound on the current coil and is connected in series with the potential coil. The current in the compensating coil flows in a direction opposite to that of the current in the current coil, and therefore its effect on the wattmeter field is nullified. In some cases it is convenient also to have the effect of the voltmeter current nullified in the wattmeter, so that the voltmeter need not be disconnected when the wattmeter is read.

Fig. 16-8. Schematic Diagram of Compensated Wattmeter

Such a connection is shown in Fig. 5-25. Fig. 16-8 shows schematically a wattmeter compensated for its own potential-coil error, and also one compensated for both its own potential-coil error and that which would be introduced by an accompanying voltmeter.