The open circuit test of transformer is conducted to determine no-load loss or core loss and no-load I₀ which is helpful in finding X₀ and R₀.
 
One winding of the transformer whichever is convenient but usually high voltage winding is left open and the other is connected to its supply of normal voltage and frequency.
 
The primary reason of performing open circuit test on the low voltage side is that it draws significantly large no load current for convenient reading. Also, it requires only low voltage supply which is easily available and safe to perform the test.
 
A wattmeter W, voltmeter V and an ammeter A are connected in the low voltage winding i.e. primary winding. With normal voltage applied to the primary, normal flux will be set up in the core, hence normal iron losses will occur which are shown by the wattmeter.
 
As the primary no-load current I₀ is small copper loss at this current is negligibly small in primary and nil in secondary (as it is open). Hence, the wattmeter reading shows practically the core loss under no-load condition.
 
It should be noted that since I₀ is itself very small, the pressure coils of the wattmeter and the voltmeter are connected such that the current in them does not pass through the current coil of the wattmeter.
 
Sometimes, a high-resistance voltmeter is connected across the secondary. In that case, the reading of the voltmeter gives the induced EMF in the secondary winding. This helps to find transformation ratio K.

Open Circuit Test Procedure

• Connect the circuit as shown in the figure.
• Keep the autotransformer at its minimum output voltage position.
• Switch on the power supply and adjust the autotransformer to get the rated supply voltage.
• Now note down the current and power shown by the ammeter and wattmeter respectively. Let these are I_o and W_o.

• The wattmeter reads the no load input power to the transformer. The no load current of the transformer is very small as compared to full load current (about 3 to 5% of the full load value) and hence the copper loss in the winding connected to the supply is small.
• As the high voltage winding is kept open therefore the copper loss in that winding is zero. Therefore the total copper loss is very small and can be neglected. Hence the watt meter reading represents the iron losses.

i.e W_o = P_i = Iron losses

While performing the open circuit test of a single phase transformer, high voltage winding should not be touched because it may cause a serious electric shock.

Calculation of Parameters From Open Circuit Test

The two parameters which can be calculated from the open circuit test of transformer are R_o and X_o. They are calculated as follows.
 
Step 1: Calculate no load power factor (cos φ_o)
 
The wattmeter reads the real power input.
Therefore, W_o = V_oI_ocos φ_o
or cosφ_o = W_o/V_oI_o
We can calculate φ_o from this.
 
Step 2: Calculate I_m and I_w :
 
I_m = I_osin φ_o
I_w = I_ocos φ_o
 
Step 3: Calculate R_o and X_o
 
R_o = V_o/I_w Ω
X_o = V_o/I_m Ω
 
The value of power factor of a transformer at no load is very small. Therefore the watt meter used while performing the open circuit test of a single phase transformer should be able to show accurate readings on small power factors.

Separation of Core Losses

The core loss of a transformer depends upon the frequency and the maximum flux density when the volume and the thickness of the core laminations are given. The core loss is made up of two parts

• hysteresis loss W_h = PB_max⁶f as given by Steinmetz’s empirical relation and
• eddy current loss W_e = QB_max²f² where Q is a constant.

So, the total core loss = W_h + W_e =  PB_max^1.6f + QB_max²f²
 
If we carry out two experiments using two different frequencies but the same maximum flux density, then we will be able to find the constants P and Q and hence we can calculate hysteresis and eddy current losses separately.

Thanks for reading about “open circuit test of single phase transformer”.
 

• Ideal Transformer
• Construction of Three Phase Transformer
• Types of Transformers
• Equivalent Resistance and Reactance of Transformer
• Equivalent Circuit of Single Phase Transformer
• Power Loss in a Transformer
• Open Circuit Test of Single Phase Transformer
• Short Circuit Test on Single Phase Transformer
• Transformer Efficiency
• Regulation of Transformer
• Autotransformer
• Instrument Transformers
• Polarity of Transformer Windings
• Significance of Vector Group of Transformer
• Buchholz Relay Construction | Working
• Why current transformer secondary should not be opened
• Dielectric Strength Test of Transformer Oil
• Transformer Moisture Removal Process

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