Open-circuit Test(or No -Load Test)

Open-circuit test is used to determine the :

1. core(or iron excitation) loss,Pi
2. No-load current ,Io
3. shunt branch parameters Ro and Xo of equivalent circuit.

Open circuit Test on transformer

In open circuit test usually high-voltage winding is kept open circuited and the rated voltage at rated frequency is applied to the other winding.The core loss will be the same whether the measurements are made on lv winding or hv winding so long as the rated voltage of that winding is applied to it but in case the measurements are made on hv winding,the voltage required to be applied would be inconveniently large while the current Io would be inconveniently small.

An auto transformer or a voltage divider(VD) is used to vary the voltage to the low-voltage winding.Ammeter A and Watt meter W are connected to measure no-load current Io and input power Wo. voltmeter V is connected to measure the applied voltage.

Since no current flows in the open-circuited secondary,the current in the primary will be necessary to magnetize the core at normal voltage.This magnetizing current is a very small fraction of the full-load current(usually 3 to 10% of full-load current) and can be neglected.

when the normal voltage is applied to the primary normal flux will be set up in the core and therefore normal iron(or core) loss will occur which is recorded by a wattmeter W.We can compute the equivalent circuit constants Ro,Xo,no-load power factor cosΦo,no-load current Io and no load power loss(iron loss) of a Transformer .

Iron loss,Pi=Input power on no load

=Wo watts (say)

No-load current=Io amperes

Applied voltage to primary=V1 volts

Some important points to be noted:

• Since no-load current I0 is very small,pressure coils of wattmeter and the voltmeter should be connected such that the currents drawn by them do no flow through the current coils of the wattmeter and ammeter.
• LowPower factor wattmeter should be used to ensure accurate measurements ,since power factor at no load is quite low(i.e in range of 0.1-0.2 lag).
• Error due to power loss in ammeter can be eliminated by short-circuiting the ammeter while reading wattmeter.
• Proper insulation of the terminal is necessary.
• To indicate the induced emf in the secondary(hv winding),high resistance voltmeter is connected across secondary which helps to determine the transformation ratio K.

Short-circuit Test(or Impedance Test)

short circuit test is used to determine:

1. Full-load copper loss
2. Equivalent resistance and equivalent reactance

Short circuit test on transformer

In this test,the terminals of secondary winding are short circuited by a thick wire or strip or through an ammeter and variable low voltage is applied to the primary through an auto-transformer or potential divider.

The applied voltage Vs to the primary is gradually increased till the ammeter A indicates the full-load(rated) current of the metering side.Since applied voltage is very low(5-8% of the tared voltage),flux linking with the core is very small and therefore iron losses are so small that can be neglected.Thus the power input (reading of wattmeter W) gives total copper loss at rated load,output being nil.

Let the readings of voltmeter ammeter and wattmeter be Vs,Is and Ws.

This value is referred to the metering side(high voltage side in above case.

Questions:

What would happen if the measurements were made on hv and lv side?

Ans:Voltage required for the short-circuit test is about 5 percentage of the rated value.For a 200kVA,2200/220V transformer ,test on high voltage side would need the voltage of 2200*5/100 i.e 110 v and the current of 200*1000/2200 i.e 91 A.

If the test is conducted in the low voltage side of the above transformer ,the voltage needed would be 220*5/100=11V(very small) and the current would be 200*1000/220=910A(very high).At this low voltage ,high precision would not be readily obtainable with ordinary instruments.

Advantages of predetermining open and short circuit test

The advantage of predetermining the performance of electrical machines through open-circuit and short-circuit test data is that we can know the behavior and characteristics of the machine that will show during actual operating conditions and on this basis we can select the machine meeting our requirements.

For more notes on electrical engineering

Transformer Design