Open Circuit Test: One winding is connected to a normal voltage supply, while the other is left open-circuited. The input power (P₀) is measured using a wattmeter, and the no-load current (I₀) is measured with an ammeter. Voltmeters measure the primary and secondary voltages. The wattmeter reading directly indicates the core losses (iron losses), which are primarily due to hysteresis and eddy currents in the transformer core. These losses are relatively constant regardless of the load.

Short Circuit Test: One winding is short-circuited through an ammeter, and a reduced voltage is applied to the other winding. The applied voltage is adjusted to circulate the full-load current through the short-circuited winding. Because the core flux is proportional to the applied voltage, and the voltage is kept low, the core losses are negligible. The wattmeter reading primarily represents the copper losses (I²R losses) in the windings. These losses are dependent on the load current.

• Equivalent resistance (Req) is the total resistance of the transformer windings referred to either the primary or secondary side.
• It represents the combined resistance of the primary and secondary windings, taking into account the turns ratio of the transformer.
• Equivalent resistance is used in calculations related to voltage drop, power loss, and efficiency of the transformer.
• It is determined from the short circuit test.

In simpler terms: Imagine the transformer windings as a single resistor. The equivalent resistance is the value of that single resistor that would have the same effect on the circuit as the actual windings.