Single Phase Transformers

Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.

Transformers are stationary electric machines that step up or down AC voltage. They are typically formed of primary and secondary coils or windings, where the voltage on the primary is stepped up or down at the secondary, or the other way around. When a voltage is applied to one of the windings and current flows in that winding, flux is induced in the magnetic core, coupling both windings. With an AC current, AC flux is induced, and its rate of change induces voltage on the secondary winding (Faraday's law). Flux linkage between both windings depends on the number of turns of each winding; therefore, if the primary windings have more turns than the secondary winding, voltage will be higher on the primary than on the secondary, and vice versa.

This experiment characterizes a single-phase transformer by finding its equivalent circuit parameters. Three tests are performed: open-circuit test, short-circuit test, and the DC test.

1. DC Test

Turn on the low-voltage DC power supply available on the bench.
Set its voltage output to 0 V, and set the current limit to 0.8 A.
Double-check the circuit connections, then connect the power supply output across the primary side windings (IN1 and IN2). Leave the secondary side windings (OUT1 and OUT2) open.
Turn on the supply and slightly increase the voltage until the current limit is reached. Note that the supply might already be current-limited when the supply is t

By performing the DC, open-circuit, short-circuit, and load tests, the transformer's equivalent circuit parameters were identified; therefore, simulating, operating, and analyzing realistic transformer behavior become possible.

The short circuit test is usually performed by applying an increasing voltage on the high voltage side, since only small voltages on that side may cause rated current to flow on the shorted low voltage side. This is useful in operating the transformer at rated curre

The described tests are critical in evaluating the impedance of a transformer and in determining its equivalent circuit parameters. Since transformer applications vary from simple chargers to high power AC transmission, appropriately characterizing different transformers for various applications is essential. Transformer impedance is used in power systems to determine possible fault impedances on either side of a transformer, approximate the efficiency of a transformer, calculate its line

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