12.1. Potential Transformers (P.T.)
A potential transformer is a step down transformer used along with a low range voltmeter for measuring a high voltage. The primary is connected across the high voltage supply and the secondary to the voltmeter or potential coil of the wattmeter. Since the voltmeter (or potential coil) impedance is very high, the secondary current is very small and the potential transformer behaves as an ordinary two winding transformer operating on no-load. Fig. 36 shows a potential transformer used to measure the voltage of a circuit. It may be noted that the secondary is grounded. This is done so that if the insulation breaks down, the high voltage does not endanger personnel who may be reading the meters.
Fig. 36. Potential transformer connections.
- These transformers are made with high quality iron core operating at very low flux densities so that the magnetising current may be very small. Careful design ensures minimum variation of voltage ratio with load and minimum phase shift between input .and output voltages. Potential transformers secondary are commonly designed for an output of 110 V.