open circuit test of transformer
The set up for open circuit test of transformer is shown in the figure. This test is carried on the transformer to determine its iron losses.
In this test generally, AC voltage is applied on low voltage side and the secondary is kept open.
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.

This test is performed to determine the iron losses, Ro and Xo of transformer.

Procedure for Open Circuit Test of Transformer


  • 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 Io and Wo.

  • 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 Wo = Pi = 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

The two parameters which can be calculated from the open circuit test of transformer are Ro and Xo. They are calculated as follows.
Step 1: Calculate no load power factor (cos φo)
The wattmeter reads the real power input.
Therefore, Wo = VoIocos φo
or cosφo = Wo/VoIo
We can calculate φo from this.
Step 2: Calculate Im and Iw :
Im = Iosin φo
Iw = Iocos φo
Step 3: Calculate Ro and Xo
Ro = Vo/Iw Ω
Xo = Vo/Im 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.

Transformer — 8 | Objective Type Question Answers

#1 In a power transformer the efficiency should be maximum at

full load

#2 For minimum weight of a transformer, the weight of iron should be

equal to the weight of copper

#3 The iron loss of a transformer can be calculated by knowing the weights of

cores and yokes

#4 H.R.C. fuses on a transformer provide protection against

external  faults

#5 Which of the following insulating materials is used in power transformers ?

All of the above

#6 Simple porcelain bushing, an used for transformers up to

11 kV

#7 Oil impregnated paper condenser bushing for generally used on transformer operating

132 kV

#8 The color of fresh dielectric oil for a transformer is

pale yellow

#9 Which of the following test on a transformer provides information about regulation, efficiency and heating under load conditions?

Back to back test

#10 Buchholz relay is used on

oil cooled transformers

#11 The kVA rating ratio of transformers operating in parallel, as a general rule, should be within


#12 Which value of flux is involved in the e.m.f equation of the transformer?

Maximum value

#13 In Scott connections the main transformer has centre tap on

both primary and secondary winding

#14 In a transformer over currents affect

all of the above

#15 For transformers up to a capacity of ……… kVA air blast cooling is provided.


#16 As the supply frequency of a transformer increases its rating


#17 Which of the following methods is used for cooling of transformers up to a capacity of 10 MVA?

oil natural cooling

#18 The nominal short-circuit voltage of the transformer is defined as percentage of ….

the rated primary voltage

#19 A distribution transformer has low flux density because

it is constantly connected to the supply

#20 Spiral core transformer which has core made up of steel strip has the following advantage.

All above

#21 A transformer designed for installation in a tropical country if installed in a cold country

is to be loaded in excess of the name plate kVA rating



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