Open Circuit Test of a Transformer: Circuit Diagram, Test Procedure, Formula, Calculations & Example

The open circuit test is one of the first tests every electrical engineering student learns about transformers. It answers a simple but important question: how much power does a transformer waste just by being connected to a supply, even when no load is attached?

Imagine a distribution transformer installed in your neighborhood. It stays energized 24 hours a day, 7 days a week, whether people are using electricity or not. The losses it produces during those idle hours add up to a massive energy bill over a year. The open circuit test measures exactly these losses and gives us the information we need to design more efficient transformers.

In this technical guide, we will discuss everything about the open circuit test — what it is, how to perform it step by step, how to calculate the results, and what those results actually tell you about the transformer.

1. What is the Open Circuit Test (No Load Test)?

The open circuit test also called the no-load test is a method of measuring core losses and magnetizing parameters of a transformer. During this test, you apply rated voltage to one winding while keeping the other winding completely disconnected. No external circuit is attached to the open side, which means no load current flows through it.

Here is how the setup works in practice:

• The low-voltage (LV) winding receives rated voltage from an AC supply
• The high-voltage (HV) winding remains open — nothing is connected to its terminals
• Three instruments measure voltage, current, and power on the LV side

When you energize the LV winding with rated voltage and leave the HV winding open, the transformer draws only a small current from the supply. This current is called the no-load current (I₀) and is usually about 2% to 5% of the full-load current. Because this current is so small, the I²R losses (copper losses) in the winding become negligible. As a result, almost all the power recorded by the wattmeter comes from iron losses in the core.

These iron losses have two components:

1. Hysteresis loss — caused by the repeated magnetization and demagnetization of the core material during each AC cycle
2. Eddy current loss — caused by small circulating currents induced within the core laminations

Both of these losses depend on the applied voltage and frequency, not on the load. So whether the transformer supplies zero load or full load, the core losses remain the same as long as the voltage stays constant.

2. Why is the Test Performed on the Low Voltage Side?

You might wonder if the test just needs rated voltage on one winding, why not apply it to either side? There are two strong reasons for choosing the LV side.

2.1 Safety and Convenience

Think about a transformer rated 240V/11,000V. If you wanted to energize the HV side, you would need an 11,000V supply in your laboratory. Working with such high voltages demands expensive equipment, specialized safety barriers, and extreme caution. On the other hand, energizing the 240V LV side requires only a standard lab supply or variac. The test becomes far simpler and safer.

2.2 Better Measurement Accuracy

The no-load current is already very small just a few percent of rated current. When this tiny current is referred to the HV side, it becomes even smaller because of the turns ratio. For example, if the turns ratio is 1:10, the no-load current on the HV side would be one-tenth of what it is on the LV side. Measuring such a tiny current accurately with standard ammeters becomes difficult, and even small instrument errors can produce misleading results.

By testing from the LV side, the no-load current is at its largest value, making it much easier for standard instruments to measure accurately.

3. Circuit Diagram and Test Setup

As shown in the diagram, the test setup is organized into three sections — the Variac, the Measuring Instruments, and the Transformer. Let us walk through each section from left to right.

The AC source on the far left feeds into a variac (variable autotransformer). The variac gives you control over the output voltage so you can increase it gradually from zero to rated value. Never switch on the full voltage at once, always begin from zero and raise it step by step.

From the variac output, the current path enters the measuring instruments section. Here, two instruments are connected in series with the line. The wattmeter’s current coil comes first. It sits in the series path so that the full no-load current passes through it. Next in the series path is the ammeter (marked “A”), which reads the no-load current I₀ flowing into the transformer.

Two more instruments connect in parallel across the LV winding terminals. The wattmeter’s voltage coil (pressure coil) taps across the supply line to sense the voltage. The voltmeter (marked “V”) also connects across the same terminals and gives you a direct reading of the applied voltage V₁. Both of these instruments appear between the two lines in the diagram because they measure voltage rather than carrying the line current.

On the far right of the diagram, you can see the transformer with its two windings clearly labeled LV on the left and HV on the right. The LV winding receives the controlled supply through the instruments described above. The HV winding, however, is left completely disconnected.

4. Equipment You Need

Here is the list of instruments and equipment required:

4.1 Measuring Instruments

4.2 Why use a Low Power Factor Wattmeter?

This is a point many students overlook. The power factor during the no-load test is very low somewhere between 0.1 and 0.3 lagging. A standard wattmeter designed for power factors near unity will give inaccurate readings at such low power factors because the current and voltage are nearly 90° apart. A low-power-factor wattmeter is built specifically to handle this condition and produce reliable readings.

4.3 Other Equipment

• Circuit breaker or isolator: For connecting and disconnecting the supply safely
• Variac (autotransformer): Allows you to increase voltage gradually from zero to rated value
• Connecting wires: Properly rated for the test voltage and current
• Safety gear: Insulated gloves, safety glasses, and proper grounding connections

5. Step-by-Step Testing Procedure

5.1 Step 1: Pre-Test Checks

Before you touch any terminal, complete these preparatory checks:

• Inspect the transformer physically for any visible damage — cracked insulation, burnt marks, loose terminals, or oil leaks (for oil-filled types)
• Run an insulation resistance test (using a megger) to confirm the insulation between windings and between windings and core is healthy
• Measure winding resistance using a DC bridge or micro-ohmmeter to establish a baseline
• Confirm that all measuring instruments have valid calibration dates
• Ground the transformer frame and all test equipment properly

5.2 Step 2: Connect the Circuit

Follow this connection sequence with the supply completely switched off:

1. Connect the variac output terminals to the LV winding terminals of the transformer
2. Connect the variac input to the main AC supply through a circuit breaker
3. Place the ammeter in series between the variac output and one LV terminal
4. Connect the wattmeter current coil in series with the same line
5. Connect the wattmeter voltage coil (pressure coil) directly across the LV terminals
6. Connect the voltmeter across the LV terminals
7. Verify that the HV terminals are completely open — no wires, no instruments (except an optional voltmeter if you want to check turns ratio)
8. Double-check all connections for correct polarity, especially the wattmeter terminals (look for ± markings)

5.3 Step 3: Perform the Test

1. Set the variac output to zero volts
2. Close the circuit breaker to energize the variac from the mains
3. Slowly increase the variac output voltage — do not jump straight to rated voltage
4. Raise the voltage in small steps. For a 200V rated LV winding, increase in steps of about 20V-30V
5. At each step, wait a few seconds for readings to stabilize, then note down the voltmeter (V₁), ammeter (I₀), and wattmeter (W₀) readings
6. Continue until the voltmeter shows the rated voltage of the LV winding
7. At rated voltage, take your final readings carefully — these are the most important measurements
8. While the transformer is energized, listen for any unusual buzzing, humming, or rattling sounds. Watch for vibrations, overheating, or any smell of burning insulation. If anything seems abnormal, reduce voltage to zero immediately and investigate

5.4 Step 4: Shut Down

1. Gradually reduce the variac output back to zero — do not suddenly switch off at rated voltage, as this can cause voltage spikes
2. Once the variac output reads zero, open the circuit breaker
3. Disconnect all instruments and wires
4. Record all readings in a test report

5.5 Recording Table Example

6. How to Calculate Test Parameters

The three readings from the test \(V_1\), \(I_0\), and \(W_0\) are all you need to calculate several transformer parameters.

6.1 Core Loss (Iron Loss)

The wattmeter reading directly gives you the core loss:

\( P_i = W_0 \)

In our example: \(P_i = 100\,W\)

This means the transformer wastes 100 watts just from core magnetization, regardless of how much load it carries.

6.2 No-Load Power Factor

The no-load power factor tells you how much of the no-load current actually contributes to real power (losses) versus reactive power (magnetization):

\(cos\phi _0​=\frac{W_0}{V_0 \times I_0}\)

Example calculation:

\(cos\phi _0​=\frac{100}{200\times 1.5}=\frac{100}{300} = 0.33 \text{(lagging)}\)

A power factor of 0.33 means only about one-third of the apparent power drawn from the supply goes toward actual losses. The remaining two-thirds is reactive power used purely for magnetizing the core. This is why the no-load power factor is always very low and lagging.

6.3 Components of No-Load Current

The no-load current \(I_0\) has two parts that are 90° apart from each other, like two sides of a right triangle:

Working Component \((I_w)\): Also called the active or core-loss component. This part of the current produces the real power that gets dissipated as heat in the core:

\( I_w = I_0 \cos \phi_0 = \frac{W_0}{V_1} \)

Example:

\( I_w = \frac{100}{200}=0.5 A\)

Magnetizing Component \((I_m)\): This part of the current creates the magnetic flux in the core. It does not consume real power but is necessary to maintain the magnetic field:

\( I_m = I_0 \sin \phi_0 = \sqrt{I_0^2 – I_w^2} \)

Example:

\( I_m =\sqrt{1.5^2 – 0.5^2} = \sqrt{2.25 – 0.25} = \sqrt{2} = 1.414A\)

Notice that \(I_m\) (1.414 A) is much larger than Iw (0.5 A). This is because most of the no-load current goes toward magnetizing the core, not toward losses.

You can verify these components using the Pythagorean relationship:

\(I_0=\sqrt{I_w^2+I_m^2} = \sqrt{0.5^2+1.414^2}=\sqrt{0.25+2.0}=\sqrt{2.25} = 1.5 A\)

6.4 Equivalent Circuit Parameters (Shunt Branch)

The transformer’s equivalent circuit has a shunt branch (parallel branch) that models the core behavior. The open circuit test gives us the two components of this branch:

Core Loss Resistance \((R_0)\): This is a fictitious resistance that, if it existed, would dissipate the same amount of power as the actual core losses.

\( R_0 = \frac{V_1}{I_w} = \frac{V_1^2}{W_0} \)

Example:

\( R_0 = \frac{200}{0.5} = 400\omega \)

Magnetizing Reactance \((X_0)\): Magnetizing Reactance is the inductive behavior of the core’s magnetizing circuit.

\( X_0 = \frac{V_1}{I_m} \)

Example:

\( X_0 = \frac{200}{1.414}=141.4\omega \)

These values of \(R_0\) and \(X_0\) are referred to the LV side (the side where the test was performed). If you need them referred to the HV side, multiply each by the square of the turns ratio:

\(R_{0(HV)}=R_0\times (\frac{N_2}{N_1})^2 = R_0 \times K^2\)

For our example transformer (200/400V, so K = 400/200 = 2):

\(R_{0(HV)}= 400 \times 2^2 = 400\times 4 = 1600 \omega\)

\(X_{0(HV)}= 141.4 \times 4 = 565.5 \omega\)

6.5 Summary Table of Results

7. How to Interpret Your Test Results

Getting numbers from the test is only half the job. You also need to know whether those numbers indicate a healthy transformer or one with problems.

7.1 What “Normal” Looks Like

For a healthy transformer:

• No-load current should be between 2% and 5% of the full-load rated current. In our example, the full-load current on the LV side is 5000/200 = 25 A. The no-load current of 1.5 A is 6%, which is slightly above the expected range and might warrant further investigation.
• No-load power factor should fall between 0.1 and 0.3 for most transformers. Our calculated value of 0.33 is marginally above this range, which could indicate slightly higher-than-normal core losses.
• For three-phase transformers, the no-load currents in all three phases should be roughly equal. A difference of more than 5% from the average value across phases may point to winding faults or core asymmetry.

7.2 Warning Signs

7.3 Comparing with Reference Values

Always compare your results against:

1. Manufacturer’s test report — the factory test data provided when the transformer was new serves as your baseline
2. Previous periodic test results — taken under the same conditions (same voltage, same temperature). A variation greater than 5% between successive tests taken under identical conditions should trigger further investigation
3. Industry standards — IEEE C57.12.90 (for North American practice) and IEC 60076-1 (for international practice) specify acceptable test methods and tolerances

8. Relationship with the Short Circuit Test

The open circuit test does not work alone. It gives you only the shunt branch parameters (R₀ and X₀) of the transformer equivalent circuit. To get the series branch parameters (equivalent resistance Rₑ and equivalent reactance Xₑ), you need the short circuit test.

Here is how the two tests complement each other:

Together, the two tests provide all the parameters needed to build the complete equivalent circuit of the transformer. From this equivalent circuit, you can predict transformer performance at any load including efficiency, voltage regulation, and maximum efficiency point without ever needing to actually load the transformer to its full rating.

9. Practical Applications and Importance

The open circuit test serves multiple purposes in transformer engineering and maintenance, extending well beyond simple parameter measurement.

9.1 Efficiency Calculation

One of the primary applications involves calculating transformer efficiency without actually loading the transformer. Since core losses remain constant at all load levels (when voltage is constant), the open circuit test provides the fixed loss component needed in efficiency calculations:

\( Efficiency = \frac{Output Power}{Output Power + Core Losses + Copper Losses} \times 100% \)

9.2 Voltage Regulation

The shunt branch parameters \((R₀\) and \(X₀)\) obtained from the open circuit test contribute to voltage regulation calculations. Voltage regulation measures how well a transformer maintains constant secondary voltage as load current varies:

\( Voltage Regulation = \frac{V_{no-load} – V_{full-load}}{V_{full-load}} \times 100% \)

9.3 Quality Control During Manufacturing

Transformer manufacturers run OC tests on every unit before shipping. The measured core losses must fall within the guaranteed values specified in the purchase contract. If a unit shows losses higher than guaranteed, the manufacturer may need to re-work the core or accept a financial penalty.

9.4 Condition Monitoring

Power utilities perform open circuit tests periodically on their transformers — during manufacturing, during commissioning, and at regular maintenance intervals. By tracking the no-load current and core loss values over time, engineers can spot gradual deterioration before it leads to failure. A steady upward trend in core losses over several years, for example, could indicate that the lamination insulation is breaking down due to aging or thermal stress.

10. Open Circuit Test Parameter Calculator

11. Quiz on Open Circuit Test

12. Conclusion

The open circuit test gives you the core loss, magnetizing current, no-load power factor, and shunt branch equivalent circuit parameters of a transformer – all from just three simple measurements. By performing the test on the LV side with the HV side disconnected, you keep the procedure safe and the measurements accurate.

The results from this test, combined with those from the short circuit test, allow you to construct the full equivalent circuit of the transformer. From that circuit, you can predict how the transformer will behave at any load – its efficiency, voltage regulation, and maximum efficiency operating point without ever needing to connect an actual full-rated load.

13. Frequently Asked Questions (FAQs)