Open Circuit Test Or No-Load Test

Figure 1.35 Open Circuit Test

From this test, we can determine core loss and no-load current (I₀) of the transformer. Figure 1.35 shows the schematic diagram of the transformer. The high-voltage side is generally kept open because the current in high-voltage winding is less compared to that on low-voltage winding. In low-voltage side, a voltmeter, an ammeter and a wattmeter are connected to measure the input voltage, no-load current and the core loss of the transformer. Since no-load current is generally small, the copper loss at no-load condition is negligible. The wattmeter reading practically gives the iron loss of the transformer.

To measure the induced emf in secondary winding, a high-resistance voltmeter is connected across the secondary to calculate the turns ratio (a).

Let I₀ be the reading of the ammeter, V₁ be the reading of the voltmeter and W be the reading of the wattmeter.

We have W=V₁I₀cosθ₀

i.e., 

Therefore, I_W=I₀cosθ₀ (1.27)
and I_μ=I₀sinθ₀ (1.28)

During no-load condition, the voltage drop across the primary impedance is small. Therefore, we have

and 

The total iron loss depends on the frequency as well as the maximum flux density. Hysteresis loss and eddy current loss are the two parts of the total iron loss, which are described below.

1. Hysteresis loss: P_h = k₁B_max^1,6f (1.31)
2. Eddy current loss: P_e = k₂B_max²f² (1.32)

where k₁ and k₂ are the constants in the above two equations, which can be obtained from the experi-ment. The hysteresis and eddy current losses can be calculated by knowing k₁ and k₂. Figure 1.36 shows the variation of iron loss with the applied voltage.

Figure 1.36 Variation of Iron Loss with Applied Voltage