Transformer Operation and Testing Principles

Principle of Operation

The principle of operation of the transformer can be explained by the so-called single-phase ideal transformer, which is a machine powered by an AC source. Essentially, a transformer consists of a core of magnetic material that forms a closed magnetic circuit. The columns or legs of this core localize the windings: one called the "primary" receives power, and the other, the "secondary," closes on a circuit to use the delivered energy. The two windings are electrically isolated from each other.

Three-Phase Transformer

These transformers have three windings in the primary and three in the secondary. They can be connected in star (Y) (with or without a neutral wire) or delta ($\Delta$), including combinations such as: $\Delta$-$\Delta$, $\Delta$-Y, Y-$\Delta$, and Y-Y. Keep in mind that even with a 1:1 ratio, changing from $\Delta$ to Y or vice versa causes the phase voltages to vary.

Autotransformer

In an autotransformer, the primary and secondary windings are connected in series, forming a single winding. It weighs less and is cheaper than a standard transformer, making it commonly used for converting 220 V to 125 V and similar applications. Its disadvantage is that it does not provide galvanic isolation between the primary and secondary circuits.

Short Circuit Test (Three Phase)

The short circuit test involves closing the terminals of one winding with a connection of negligible resistance (short circuit) and feeding the other winding with a reduced voltage. This applied voltage is a small percentage of the winding's rated voltage, ensuring that the windings carry their rated currents. Under these conditions, the rated currents are measured, along with the power absorbed. Because the applied voltage is small compared to the rated voltage, the core losses can be considered negligible. Therefore, the entire power consumption is due to Joule losses in the primary and secondary windings.

Short Circuit Test (Single Phase)

This test is conducted at a reduced voltage until a nominal current flows through the circuit. In this case, the magnetization branch is ignored because only a small voltage is required to achieve the rated currents due to the impedance being limited by the leakage impedance of the coils. Consequently, the flux density in the core will be small, and the core losses and magnetizing current will be even smaller.

• The reduced voltage ($V_{cc}$), often called impedance voltage, is set so that the short circuit current ($I_{cc}$) does not damage the coils.
• $I_{cc}$ is usually chosen to be the full load (nominal) current.
• Usually, this test is performed on the high voltage side so that the current is smaller.