Question

How will you perform open circuit test?

Hint:

To easily remember:

• Open Circuit Test = Measuring Iron Losses (performed on LV winding, keeping HV open).
• Short Circuit Test = Measuring Copper Losses (performed on HV winding, shorting LV).

Answer 1

Step 1: Objective of the Test:
The Open Circuit (OC) or No-Load test is performed on a transformer to determine:

• Core losses (Iron losses), which consist of hysteresis and eddy current losses.
• No-load parameters ($R_0$ and $X_0$).

Step 2: Experimental Circuit Setup:

• Winding Configuration: The High Voltage (HV) winding of the transformer is kept completely open-circuited, while all measuring instruments are connected to the Low Voltage (LV) winding. This is done because it is safer and easier to apply rated voltage on the LV side.
• Instruments Connected:
  • A voltmeter (across the input to measure rated LV voltage, $V_0$).
  • An ammeter (in series to measure no-load current, $I_0$).
  • A Low Power Factor (LPF) wattmeter (to measure no-load input power, $W_0$).
• Applied Input: Rated voltage at rated frequency is applied to the LV winding.

Step 3: Test Analysis:
Since the secondary side is open, the current drawn ($I_0$) is very small (typically $2\%\text{ to }5\%$ of full-load current). Thus, copper losses ($I_0^2 R$) in the primary are negligible. The wattmeter reading ($W_0$) therefore directly measures the constant core losses ($W_c$): $$W_0 \approx \text{Core/Iron Loss } (W_c)$$