Mainly two tests are carried out on the electrical transformer which are the open circuit test and short circuit test of the transformer also known as the SC and OC tests.
Table of Contents
Key takeaways:
- OC test: Measures no-load losses (core losses) and magnetizing characteristics, providing insights into core material quality, transformer efficiency, and no-load current.
- SC test: Analyzes the transformer’s behavior under short-circuit conditions, determining leakage reactance, copper losses, and voltage regulation under load.
The purpose of these tests is to determine the parameters of the equivalent circuit, voltage regulation, and efficiency of the single / three-phase transformer. this is the main reason for doing SC and OC tests on the transformer.
The power required during the test is equal to the power losses occurring in the three-phase transformer.
Open Circuit Test of Transformer
The circuit diagram for performing the OC test also called a no-load test on the transformer is shown in the figure
In the connection diagram voltmeter, ammeter, and wattmeter are connected on the low-voltage side of the transformer and the HV side is left open-circuited.
Rated frequency voltage is applied to the primary side of the transformer which is the low voltage side and this applied voltage is varied with the help of an auto transformer. when the voltmeter reading is equal to the rated voltage of the LV winding. all meter readings are recorded.
Ammeter records no-load current or exciting current Ie since the no-load current is small, the primary leakage impedance drop is negligible. the applied voltage V1 is equal to the induced EMF V1′ .
The input power recorded by the wattmeter consists of core loss and ohmic loss. the no-load current is about 2 to 6% of full load current and ohmic losses in the primary of the transformer vary from 0.04% to 0.36% of full load primary ohmic loss.
In fact, ohmic loss during the open-circuit test is negligible as compared to the normal core losses. hence the wattmeter reading can be considered equal to the core losses.
The phasor diagram is shown in the below figure
Parameters are obtained from O C test and equivalent circuit diagram
Calculations of open circuit tests :
Calculation no-load power factor cos θ =
Calculation magnetizing current Im
Calculation of core loss component of current Ic
Magnetising reactance Xm (LV)
Resistance representing the core loss
Short Circuit (SC)Test of Transformer
Why S.C. test is done on the transformer?
The purpose of the test is to find out losses in the transformer and equivalent circuit parameters. In the short circuit test of the transformer low voltage side of the transformer is short-circuited and the ammeter, voltmeter, and wattmeter are connected on the HV side. applied voltage to the HV side is adjusted by the autotransformer.
In the transformer primary side, mmf is almost equal to the secondary mmf , therefore rated current in the primary that is HV winding causes to flow rated current in the secondary that is LV winding.
2 to 12% of the rated voltage is sufficient to drive rated current in both primary and secondary winding.
Parameters are obtained from S.C test & equivalent circuit diagram
Calculations of SC Tests
Equivalent resistance referred to HV side re (HV)
Equivalent impedance ref. to HV side ze(HV)
Equivalent leakage reactance ref. to HV side xe(HV)
Differences Between Open Circuit Test and Short Circuit Test
| Open Circuit Test | Short Circuit Test |
| also called as NO load test on the transformer | called an impedance test |
| OC test is performed to calculate iron loss and exciting current | this test is performed to calculate copper loss or winding loss |
| voltmeter V0 connected across the primary winding to measure the applied voltage as shown in OC test wiring diagram | voltmeter V0 connected across the primary winding to measure the applied voltage as shown in OC test wiring diagarm |
| secondary winding kept open | ammeter connected in a series with primary to monitor no-load current |
| voltmeter V0 connected across the primary winding to measure the applied voltage as shown in OC test wiring diagram | ammeter connected in a series with primary to monitor short circuit current |
Conclusion:
Open Circuit Test (OC Test):
Open Circuit Test Primarily measures no-load losses (also called core losses) and no-load current. These losses represent energy dissipated in the core due to hysteresis and eddy currents, even without load. The open circuit test helps determine the iron loss component of overall transformer efficiency.
Short Circuit Test (SC Test):
Primarily measures copper losses (also called winding losses) and leakage reactance (Xʟ). Copper losses occur due to the resistance of the windings, and leakage reactance arises from the imperfect magnetic coupling between the primary and secondary sides. Knowing these values helps predict the transformer’s behavior under load
