Q20Basic Electrical Engineering

To accurately predict the voltage regulation and efficiency of a transformer at any load without physically subjecting it to massive loads, engineers perform two standard, low-power tests: the Open Circuit (OC) test and the Short Circuit (SC) test. From these, the exact parameters of the transformer's electrical equivalent circuit are derived.

1. Open Circuit (OC) Test: - Procedure: The High Voltage (HV) winding is left completely open-circuited. Meters (Voltmeter, Ammeter, Wattmeter) are connected to the Low Voltage (LV) winding. Rated voltage at rated frequency is applied to the LV side. - Purpose: Because the secondary is open, the current drawn by the primary is only the tiny no-load exciting current ($I_0$, approx 2-5% of full load). Therefore, primary copper losses ($I_0^{2}R$) are negligible. The Wattmeter reading directly represents the continuous Core (Iron) Losses (Hysteresis and Eddy current). - Parameters Found: This test determines the shunt branch parameters of the equivalent circuit: the core loss resistance ($R_0$) and the magnetizing reactance ($X_0$).

2. Short Circuit (SC) Test: - Procedure: The LV winding is solidly short-circuited using a thick copper wire. Meters are connected to the HV winding. A greatly reduced variable voltage (typically 5-10% of rated voltage) is applied to the HV side and slowly increased until the ammeter shows the full-load rated current flowing. - Purpose: Since the applied voltage is tiny, the magnetic flux in the core is minimal, making Core Losses negligible. However, full-load rated current is flowing through both windings. Therefore, the Wattmeter reading directly represents the Full-Load Copper Losses ($I^{2}R$). - Parameters Found: This test determines the series branch parameters: the equivalent resistance ($R_{eq}$) and equivalent leakage reactance ($X_{eq}$) referred to the measuring side.

Equivalent Circuit Development: Using the data from both tests, a complete equivalent circuit is constructed. It features the shunt branch ($R_0||X_0$) representing the core characteristics, placed in parallel with the source, and the series branch ($R_{eq}+j{X}_{eq}$) representing the winding characteristics, placed in series with the ideal transformer.