Potential Transformers

Author: E.E. Kimberly

It is common practice to use wattmeters and voltmeters with a nominal voltage range of 0 to 150 volts on all a-c circuits of 100 volts or more. When such a meter is to be used on a circuit whose voltage is greater than 150 volts, a potential transformer or voltage transformer is used between the meter potential coil and the circuit. The potential transformer is relatively small because its secondary burden is only a few volt-amperes taken by one, two, or three meters. Fig. 16-10 shows two potential transformers properly connected for the measurement of polyphase power by the two-wattmeter method. All instruments used in the secondary of a potential transformer must be connected in parallel, as are the voltmeter and the potential coil of W₁ in Fig. 16-10.

Fig. 16-10. Three-Phase Power Measurement by Two-Wattmeter Method Using Two Potential Transformers and Two Current Transformers