To Perform Open circuit test on Transformer

Significance

• The No test on the single phase transformer is performed in order to find out no load parameters of the transformer. 
• This test is performed to find out

• No load current
• Magnetizing current
• Loss component of no load current
• No load equivalent circuit parameters : Exciting susceptance B₀ and Exciting conductance G₀
• Iron loss and small amount of copper loss
• Voltage transformation ratio

Instruments required

AC voltmeter : __________

AC ammeter  : __________

Wattmeter     : ___________

Multi meter   : __________

Theory

• The single phase transformer consists of mainly high voltage winding and low voltage winding. 
• The high voltage winding is kept open circuited whereas the single phase supply is given to the low voltage winding. 
• The supply is given as per rating of the transformer. As the normal voltage is given to the primary winding, normal flux sets up in the core of the winding. 
• The no load current is only 2 to 5% of the full load current therefore the wattmeter indicates iron loss plus small amount of copper loss. 
• The no load current of the transformer consists of two components : magnetizing current and loss component of the no load current. 
• The function of the magnetizing current is to create flux in the core where the function of the core loss component of the no load current is to supply small amount of copper loss plus iron loss.

Circuit diagram

Procedure

• Make connection as per circuit diagram.
• Switch on the supply and note down the reading of voltmeter, ammeter and wattmeter.

Observation table

Supply voltage	No load current	Wattmeter reading	Secondary voltage
V0	I0	W0	V2

 Calculation

No load input Power	W0 = V0 I0 Cos Φ0
No load input power factor	Cos Φ0 = W0 / V0 I0    Φ0 = Cos – 1( W0 / V0 I0 )
Magnetizing current	Iµ = I0 Cos Φ0
Loss component of no load current	Iw = I0 Sin Φ0
Exciting admittance	Y0 = I0 / V0
Exciting conductance	G0 = W0 / V02
Exciting susceptance	B0 = √ ( Y0 )2  – ( G0 )2
Voltage transformation ratio	k  = V2 / V0

 Conclusion

The no load current is only 2 to 5% of full load current.

Questions

Why the no load current is very small as compared to full load current in the transformer?

• The core of the transformer has very low reluctance path therefore the no load current is 2 to 5% of the full load current.

Why the copper losses are neglected in the open circuit test of the transformer?

• The no load current is only 2 to 5% of the full load current and no flow of current through secondary winding of the transformer therefore the copper losses are neglected in the transformer.

Which winding is kept open circuited of the transformer winding? High voltage or Low voltage

The high voltage winding is kept open circuited in the single phase transformer due to following reasons.

• Low voltage supply is easily available from practical point of view.
• The no load current of low voltage winding is high as compared to high voltage winding which is measurable.

How the iron losses are calculated from the open circuit test of the transformer?

• The wattmeter reading indicates combined iron losses and copper loss of the transformer. 
• If we neglect copper loss, the wattmeter indicates total iron losses of the transformer. 
• However the copper loss of the primary winding is calculated as below.
• Measure primary winding reading by voltage – current method or multi meter. 
• The copper loss of the primary winding is I₁²R₁. 
• Iron loss = Wattmeter reading – Copper loss of the primary winding =  W₀ – I₁²R₁

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