Open and short circuit tests are performed on a transformer to determine the: Equivalent circuit of the transformer, Voltage regulation of the transformer, and Efficiency of the transformer.
Open-circuit Test (or No -Load Test)
The open-circuit test is used to determine the :
- core( or iron excitation) loss, Pi
- No-load current, Io
- shunt branch parameters Ro and Xo of the equivalent circuit.
Open circuit Test on the transformer
In an open circuit test, usually high-voltage winding is kept open circuited and the rated voltage at the 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 autotransformer 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, the pressure coils of the wattmeter and the voltmeter should be connected such that the currents drawn by them do not flow through the current coils of the wattmeter and ammeter.
- A low Power factor wattmeter should be used to ensure accurate measurements since the power factor at no load is quite low(i.e in the range of 0.1-0.2 lag).
- Error due to power loss in the ammeter can be eliminated by short-circuiting the ammeter while reading the wattmeter.
- Proper insulation of the terminal is necessary.
- To indicate the induced emf in the secondary(hv winding), high resistance voltmeter is connected across the secondary which helps to determine the transformation ratio K.
Short-circuit Test(or Impedance Test)
short circuit test is used to determine:
- Full-load copper loss
- Equivalent resistance and equivalent reactance
Short circuit test on transformer
In this test, the terminals of the 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 the 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 the rated load, the output being nil.
Let the readings of voltmeter ammeter and wattmeter be Vs,Is and Ws.
This value is referred to as the metering side(high voltage side in the above case.
What would happen if the measurements were made on the hv and lv sides?
The voltage required for the short-circuit test is about 5 percent of the rated value. For a 200kVA,2200/220V transformer, a test on the 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 on 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.