Two simple tests with a wattmeter, ammeter and voltmeter reveal almost everything about a transformer. Here you will see how each test is connected, what every reading means and how to turn the numbers into efficiency and regulation.
The open circuit and short circuit tests let you find losses and the equivalent circuit without loading a transformer. Learn the connections, formulas, a fully solved example and the field mistakes that spoil results.
What Is the OC and SC Test of Transformer?
The OC and SC test of transformer is a pair of no load and short circuit measurements that find core loss, copper loss and the equivalent circuit parameters without loading the transformer fully. The open circuit test is done on the LV side at rated voltage, and the short circuit test is done on the HV side at rated current with the LV winding shorted.
- OC test: LV side energised at rated voltage, HV open
- OC test gives core loss and magnetising branch Rc and Xm
- SC test: HV side at rated current, LV shorted
- SC test gives full load copper loss, Req, Xeq and % impedance
- Together they give efficiency and voltage regulation at any load
Because each test supplies only the losses, the power drawn from the lab supply is tiny compared with the rating. That is the main reason these indirect tests are used for large units, where a real full load test would waste a lot of energy.
Open Circuit Test of Transformer
In the open circuit test, also called the no load test, a voltmeter, ammeter and wattmeter are connected on the LV winding and the HV winding is left open. Rated voltage at rated frequency is applied through a variac, and the three readings are noted.
Since the no load current is only a few percent of rated current, copper loss in the winding is negligible. Electrical Easy and Electrical4U both note that the wattmeter reading is therefore taken as the core loss, made of hysteresis and eddy current loss.
cos φ0 = W0 ÷ (V0 × I0)
Iw = I0 cos φ0, Iμ = I0 sin φ0
Rc = V0 ÷ Iw
Xm = V0 ÷ Iμ
The two currents represent the core loss component and the magnetising component of the no load current. The values of Rc and Xm found this way are referred to the LV side, because that is where the meters were connected.
The OC test is done on the LV side simply for safety and convenience. Rated LV voltage, such as 250 V, is easy to apply in a lab, while rated HV voltage would need a high voltage source and meters.
Short Circuit Test of Transformer
In the short circuit test, also called the impedance test, the LV winding is shorted with a thick link and the meters are placed on the HV side. The voltage is raised slowly from zero until the ammeter reads rated HV current.
Electrical4U notes that only about 5 to 10 % of rated voltage is needed to drive rated current. At this low voltage the core flux is very small, so core loss is neglected and the wattmeter reads full load copper loss.
Zeq = Vsc ÷ Isc
Req = Wsc ÷ Isc²
Xeq = √(Zeq² minus Req²)
% Z = Vsc ÷ Vrated × 100
These series values are referred to the HV side and form the approximate equivalent circuit. The percentage impedance from this test is the same figure used later when you calculate short circuit fault current on the LV bus.
Never apply full voltage with the LV side shorted. Always start the variac at zero and raise it slowly while watching the ammeter, stopping exactly at rated current.
OC and SC Test Connection Table
The table compares both tests side by side, which is the quickest way to remember the OC and SC test of transformer for exams and for site work.
| Point | Open Circuit Test | Short Circuit Test |
|---|---|---|
| Meters on | LV side | HV side |
| Other winding | Open | Shorted |
| Applied | Rated voltage | 5 to 10 % voltage, rated current |
| Current | 2 to 5 % of rated (no load) | Rated current |
| Wattmeter reads | Core (iron) loss | Full load copper loss |
| Gives | Rc, Xm, no load PF | Req, Xeq, Zeq, % impedance |
| Loss neglected | Copper loss | Core loss |
| Wattmeter type | Low power factor wattmeter | Normal wattmeter |
A low power factor wattmeter is used in the OC test because the no load power factor is often only 0.1 to 0.3. A normal wattmeter would read near the bottom of its scale and give poor accuracy.
Solved Example of OC and SC Test
Take a 25 kVA, 2500 V / 250 V single phase transformer, the same values used as defaults in the calculator below. Rated HV current is 25000 ÷ 2500 = 10 A.
cos φ0 = 105 ÷ (250 × 1.4) = 0.3
Iw = 0.42 A, Iμ = 1.336 A
Rc = 250 ÷ 0.42 = 595.2 Ω
Xm = 250 ÷ 1.336 = 187.2 Ω
Zeq = 125 ÷ 10 = 12.5 Ω
Req = 320 ÷ 100 = 3.20 Ω
Xeq = √(156.25 minus 10.24) = 12.08 Ω
% Z = 125 ÷ 2500 × 100 = 5 %
To refer Rc and Xm to the HV side, multiply them by the square of the turns ratio, here 10² = 100. Our page on transformer turns ratio explains why impedances scale with the square of that ratio.
Efficiency and Regulation From OC and SC Test
Once core loss and full load copper loss are known, efficiency at any load and power factor follows directly. Copper loss changes with the square of the load fraction, while core loss stays constant.
Output = 25 × 0.8 = 20 kW
Losses = 105 + 320 = 425 W
η = 20000 ÷ 20425 = 97.92 %
% R = I (Req cos φ + Xeq sin φ) ÷ V × 100
= 10 (3.2 × 0.8 + 12.08 × 0.6) ÷ 2500 × 100
= 3.92 %
Maximum efficiency occurs when copper loss equals core loss, at a load fraction of √(105 ÷ 320), or about 57 % of rating. Our article on transformer efficiency calculation shows how to plot the full efficiency curve from these two numbers.
OC and SC Test Calculator
Enter the rating, the three OC readings, the three SC readings and the load power factor. The SC readings must be taken at rated HV current for the efficiency and regulation to be correct.
The defaults reproduce the solved example above exactly. Change the PF to 1 to see regulation drop and efficiency rise, which is typical for resistive loads.
Step by Step Procedure for Both Tests
The VSSUT Electrical Machine Lab I manual follows exactly this order on a 3 kVA, 230/230 V single phase transformer, using an autotransformer to set voltage and current. Many Indian university labs use a similar setup for the OC and SC test of transformer in the third year.
Before any test, it is good practice to do an insulation resistance test on each winding. A weak winding can fail when energised, which damages instruments and the variac.
OC and SC Test in Factory and Field Practice
In factories the same measurements appear as routine tests, called no load loss and current measurement and load loss and short circuit impedance measurement in IEC 60076 1. Our guide on transformer routine and type tests lists where they sit among the other tests.
For 3 phase units, a 3 phase supply and three wattmeters or a power analyser are used, and load loss is corrected to a reference temperature. The vector group does not change the method, but you should confirm it first using transformer vector group markings.
Units that will run in parallel must have close % impedance values, which our article on the per unit system helps you compare on a common base. Tap position matters too, so note it if the unit has an on load tap changer.
- Needs only a small fraction of rated power
- Separates core loss and copper loss clearly
- Gives the full equivalent circuit
- Predicts efficiency and regulation at any load
- Does not show temperature rise under real load
- Ignores stray load loss variation with real load
- Core loss in SC test and copper loss in OC test are assumed zero
- Needs accurate low power factor wattmeters
Common Errors and Field Tips
- Use a low power factor wattmeter for the OC test
- Make the LV short with a link rated for full LV current
- Raise SC voltage slowly from zero, never switch on at full
- Record winding temperature with the SC readings
- Check the variac rating against rated HV current
- Keep HV terminals guarded during the OC test, since they rise to rated HV voltage
During the OC test the open HV terminals sit at full rated HV voltage, even though nothing is connected to them. Students often forget this, so the HV side must always be guarded.
Inrush current can also trip the supply when the OC test is switched on directly at rated voltage, a topic covered in transformer inrush current. Starting the variac from zero avoids this problem completely.
If OC power factor comes out above 0.5, recheck the wattmeter connections and scale. A healthy transformer core normally gives a low no load power factor.
Before starting any lab or site test, go through the electrical safety audit checklist for the test area. For regular upkeep after testing, our transformer maintenance checklist is a useful next step.
OC and SC Test of Transformer FAQ
The open circuit test is done on the low voltage side because its rated voltage is easier and safer to apply. Meters of normal range can then be used for the voltage and current readings.
The high voltage winding stays open, so no load current flows in it. The core flux is the same from either side, so core loss does not change.
Rated current on the high voltage side is much smaller than on the low voltage side. This lets the lab use a normal ammeter and a smaller variac for the test.
Only a few percent of rated voltage is needed, which is easy to supply on that side. The low voltage winding is shorted with a heavy copper link.
The open circuit test measures the core loss, made of hysteresis and eddy current loss in the steel. It also measures the no load current and the no load power factor of the transformer.
From these readings you can find the core loss resistance and the magnetising reactance. Together they form the shunt branch of the approximate equivalent circuit used in calculations.
The short circuit test measures the full load copper loss in both windings together. It also gives the equivalent resistance, reactance and percentage impedance.
These are series values referred to the side where the meters are connected during the test. Engineers use them to calculate voltage regulation, fault level and load sharing between transformers in parallel.
The applied voltage is only about 5 to 10 % of rated, so the core flux is very low. Core loss varies roughly with the square of flux, so it becomes negligible.
The wattmeter reading is therefore taken as copper loss alone in the windings. This assumption makes the OC and SC test of transformer simple and accurate enough.
Add the core loss from the open circuit test to the copper loss at the chosen load. Copper loss at any load equals full load copper loss times the square of load fraction.
Efficiency is output divided by output plus these total losses, expressed in percent. Maximum efficiency occurs when copper loss equals the constant core loss value.
Related Articles
- Transformer Routine and Type Tests
- Transformer Efficiency Calculation
- Eddy Current Loss and Hysteresis Loss
- Transformer Turns Ratio and Secondary Voltage
- Per Unit System in Power Systems
External References
- VSSUT Burla: Electrical Machine Lab I Manual (PDF)
- Electrical Easy: Open Circuit and Short Circuit Test on Transformer
- Wikipedia: Open circuit test
What We Learn Today
- The OC and SC test of transformer finds core loss on the LV side at rated voltage and copper loss on the HV side at rated current.
- Open circuit readings give the magnetising branch Rc and Xm, while short circuit readings give Req, Xeq and the percentage impedance.
- Using both losses together, you can calculate efficiency and voltage regulation at any load and power factor without a real full load test.