Open Circuit Test and Short Circuit Tests

To determine the circuit constant, efficiency and regulation of a transformer, without actually loading the transformer, we perform Open-Circuit and Short-Circuit tests. These tests give more accurate result than compared with the fully loaded transformer. And the power consumption in these tests is minimal as compared with the transformer's output on full load.

Open Circuit Test

The circuit diagram of the open circuit test for the transformer is shown below:

Fig: Open Circuit Test on a Transformer

A voltmeter V, an ammeter A, and a wattmeter W are connected in the low voltage (lv) side of transformer. The voltmeter v gives the rated voltage V₁ of the primary. A very small current I₀, called the no-load current, flows in the primary side because the secondary side is open circuited. The ammeter A, therefore, reads the no-load current I0. The power loss in the transformer occurs due to core loss and a very small I₂R loss in the primary. There is no I2R loss in the secondary since it is open and I₂ = 0. Since the no-load current I₀ is very small, the I₂R loss in the primary winding can be neglected. The instrument readings obtained in open circuit test are as follows:

• Ammeter reading = no-load current I₀
• Voltmeter reading = primary rated voltage V₁
• Wattmeter reading = iron or core loss P₁

From the above readings we can determine the no-load equivalent circuit:

Where X₀ is the inductive reactance, I_W is the core loss current, I_μ is the magnetizing current.

Short Circuit Test

Fig: Short Circuit Test on a Transformer

In the short circuit test, usually, the low voltage side is short-circuited by a thick conductor. As we can see that in the figure an ammeter, a voltmeter, and a wattmeter are connected on the high-voltage side. The reason for short-circuiting the low voltage side is as follows:

1. The rated current on the high voltage (hv) side of a transformer is less than the low voltage (lv) side of the transformer. And we can measure this current with the help of available laboratory ammeters.
2. Greater accuracy in the reading of the voltmeter is possible when we use the hv side as the primary because the applied voltage is less than 5 percent of the rated voltage of the winding.

The high voltage winding is supplied at the reduced voltage from a variable voltage supply. This supply voltage is gradually increased until full-load primary current flows. When the rated full load current flows in the primary winding, then it will also flow in the secondary winding by the transformer action.

The readings of the instruments on the short circuit test are as follows:

• Ammeter reading = Full load primary current, I_1SC
• Voltmeter reading = Short circuit voltage, V_1SC
• Wattmeter reading = full-load copper loss of the transformer P_cfl

The equivalent resistance of the transformer referred to primary

Equivalent impedance referred to primary

Equivalent reactance referred to primary

When short circuit test is performed only on one side the equivalent circuit constants referred to another side can also be calculated as follows: