OPEN CIRCUIT TEST ON A SINGLE PHASE TRANSFORMER
OBEJECT :
To find out iron loss of transformer by performing open circuit test and to observe its dependence on the primary voltage.
APPARATUS :
Sr.
no Name Range No.
Required Remarks
1. Single phase transformer
2. Variac
3. Voltmeter
4. Wattmeter
5. Ammeter
THEORY :
A transformer is a static apparatus used to transform A.C electrical power from one voltage to another voltage. On as iron core transformer windings are provided. The winding which is connected to the supply is called primary winding and the winding which is connected to the load is called secondary winding. A transformer works on the principle of mutual induction.
In a transformer there are two types of the losses.
1. Copper loss Wc and
2. Iron or core loss Wi
Wc =I21 R1 + I22 R2
=In2 Rn2
Where R is the equivalent resistance of the transformer from equation (1)
Wc ??I
Iron loss : Iron loss is the power wasted in the iron core. The iron loss can be divided into two parts,
1. Eddy current loss
2. Hysteresis loss
1. Eddy current loss : Since the alternating flux links with the core small e.m.f is induced within the iron core, which is a conductor. As the iron core forms the closed circuit the e.m.f sets up current within it. This current is called eddy current and it cause a power loss. The magnitude of this current depend upon the impedance of the current, frequency and flux density. Since the resistance of the path is constant.
i.e. : 1. We = k B2max f2 t2 V2 watts
2. We ? B2max f2
Where, B max = flux density
f = frequency of magnetic reversals
t = thickness of all laminations
V = volume of armature core
The frequency and the magnitude of the flux density depend upon those primary voltage. Therefore eddy current loss depends upon the voltage.
2 Hysteresis loss (Wc) : This power loss is due to the rapid increase and decrease and reversal of the fiux therefore such loss is very common where the a.c. current are related with the magnetic materials. The hysteresis loss depends upon the frequency and magnitude of the fiux density, which in turn depend on this of the primary voltage.
i.e. 1.Wh = ? B2m f2 V watts
2. Wh ? B2m f2
where, V = volume of core in m3
?? = steinment hysteresis co-efficient
Since the eddy current loss and hysteresis loss depends on primary voltage it means that iron loss depends upon the primary voltage.
The power equation of a tranfer can be wrriten as,
Input = Output = Cu loss + iron loss
= Va I2 cos? + I21 R21 + I22 R2
During the open circuit test secondary winding is open.
Therefore I2 = 0
Input = 0+ I21 R1 + I22 R2
During O.C. test current in the primary winding is also very small and hence copper loss will be very small and can be neglect.
Therefore Input = W1
Thus the input power to the t/r during O.C. test equal to the iron loss. The reading wattmeter cnnected in the circuit to measure input power during O.C. test gives directly the iron loss of the transformer.
PROCEDURE :
1. Set up the connections as per the diagram.
2. With the help of the variac the applied voltage is grdually increases from zero volts to the rated voltage.
3. Note down the wattmeter and ammeter readings.
4. Draw the graph iron loss v/s primary voltage.
OBSERVATION TABLE :
Sr No. Primary voltage in volts
Vp Primary current in amp
Ip Iron loss in watts
Wi
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
CALCULATIONS :
CONCLUSON :