Electrical Engineering ⇒ Topic : Open Circuit Test (O.C. Test)

This test gives the no-load iron loss or core loss of the transformer. It also gives the no-load current I which is used to calculate the parameters R₀ and X_M of the magnetising circuit (Figure). The transformer is connected as shown in Figure (a). One of the windings, usually the low voltage winding is connected to the supply voltage source while the high voltage winding is kept open. This ensure magnification of the no-load current. When the rated voltage is applied to the transformer using an auto-transformer, the armature gives the no-load current 10, the wattmeter gives the total power loss and the ratio of voltmeter readings, V₁ /V₂, gives the ratio of the number of turns. The primary no-load current 10, is usually less than 5% of the full-load current and therefore the copper loss in the winding is less than 1/400 of the copper loss on full-load, and therefore it can be neglected in comparison with the iron loss of the transformer. Therefore, the reading of the wattmeter can be taken as only the iron loss of the transformer.

                                                FIGURE  Open-circuit test on a transformer

From the no-load phasor diagram shown in Figure (a), it follows that the wattmeter reading W_o can be expressed as

                                                                   .........1

                                  and          ..........2

                                                             ...........3

                                                      ..........4

The total iron loss depends on the frequency and the maximum flux density used. The total iron loss has two components, which are given as

(a) Hysteresis loss                                  ..........5

and (b) Eddy current loss                           ..........6

The total core loss, P_i = P_h + P_e = sum of hysteresis and eddy current losses, k₁ and k₂ in the above two equations are two constants which can be found separately by conducting two experiments at two different frequencies but at the same maximum flux density. Knowing k₁ and k₂, the hysteresis and eddy losses can be calculated

Open Circuit Test (0.C. Test)

Figure (a) shows the connection diagram for open circuit test. During this test, generally, the high voltage side is kept open

                          Figure (a) Open circuit test of a transformer

The rated voltage is applied to the low voltage side. The wattmeter W, voltmeter V, and an ammeter A are placed on the low voltage side. The pressure coil of the wattmeter and the voltmeter are connected in such a way that the current in the pressure coil and in the voltmeter does not pass through the wattmeter. Due to normal voltage applied in the primary, normal flux will be set up in the core and hence normal iron loss will occur. Practically, the wattmeter reading indicates the core loss under no load condition. In this test, very small current I₀ will flow through the ammeter because secondary side is an open circuit. So the copper loss in the primary side will be very small and it can be neglected.

A voltmeter is generally connected to the secondary. The reading of this voltmeter will be helpful to calculate the turn ratio a.

Let, W is the wattmeter reading, V₁ is the reading of the voltmeter, and I₀ is the reading of the ammeter. If cos Φ₀ is the angle between V₁ and I₀, we get 

where, cos Φ₀ is called the open circuit or no-load power factor.