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

Brandon
 
This test gives the noload iron loss or core loss of the transformer. It also gives the noload current I which is used to calculate the parameters R_{0} 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 noload current. When the rated voltage is applied to the transformer using an autotransformer, the armature gives the noload current 10, the wattmeter gives the total power loss and the ratio of voltmeter readings, V_{1} /V_{2}, gives the ratio of the number of turns. The primary noload current 10, is usually less than 5% of the fullload current and therefore the copper loss in the winding is less than 1/400 of the copper loss on fullload, 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 Opencircuit test on a transformer From the noload 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, k_{1} and k_{2} 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 k_{1} and k_{2}, the hysteresis and eddy losses can be calculated  
 
Maninder
 
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 I_{0} 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, V_{1} is the reading of the voltmeter, and I_{0} is the reading of the ammeter. If cos Φ_{0} is the angle between V_{1 }and I_{0}, we get where, cos Φ_{0} is called the open circuit or noload power factor.  
 
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