There are various types of tests that are performed to determine the performance of a transformer, one of which is the open circuit test. This test is usually performed to verify the performance of a power transformer in a no-load condition.
The open circuit test is usually performed on the low-voltage (LV) side of the transformer, and the high-voltage (HV) side is kept open (i.e., open). The main purpose of this test is to determine the core loss or iron loss and magnetizing current of the transformer.
In this test, the input voltage, current, and power of the transformer are measured using a voltmeter, ammeter, and wattmeter. Since the secondary winding is open, no load current flows. Therefore, the power obtained can be considered as core loss only.
Purpose of the open circuit test is:
- Determine the core loss
- Know the value of the magnetizing current.
- Evaluate the physical properties of the transformer.
This test provides important insights into the design and performance of the transformer, which later plays a helpful role in its use and maintenance.
What is Transformer open circuit test?
Transformer Open Circuit Test is a test used to determine the losses in the core and magnetizing circuit of a transformer. This test is also called no-load test.
Important Transformer Open Circuit Testing
- Determination of transformer loss without load:
Only iron loss or core loss can be determined through open circuit test, because at this time the secondary winding of the transformer is open and there is no load. In this, only the loss in the core can be measured. - Determination of Excitation Current and Power Factor:
In this test, a very small current flows in the primary winding, which is known as the excitation current. Through this, the magnetizing current and power factor are known. - Performance Analysis:
The information from the open circuit test is very useful for calculating the voltage regulation and efficiency of the transformer. - Simple and safe:
This is a simple and safe test, because it does not involve a heavy load and can be measured with a small power meter.
How to perform an open circuit test:
- Voltage is applied to the low-voltage side (LV side).
- The high voltage side (HV side) is open, i.e., no load is connected.
- A voltmeter, ammeter and wattmeter are connected to the LV side.
Purpose of Transformer Open Circuit Test
- Core Loss Determination: In this test, only voltage is applied, and the secondary circuit is open. As a result, the current is very low, and copper loss becomes negligible. Therefore, core loss can be easily determined.
- Magnetizing Current and Reactance Determination: The magnetizing current, reactive power and excitation admittance can be determined through the test.
- Performance Analysis: Using this information, the performance of the transformer such as efficiency, regulation, etc. can be analyzed.
Where is the performed transformer Open Circuit Test?
This test is usually performed on the low-voltage side.
Reason:
- Testing from the low-voltage side makes it easier and safer to use the metering instruments.
- In this test, the other side (high voltage side) is kept open (i.e., no load is connected).
Required Equipment
- Voltmeter: For high voltage ratings, usually 0–300V or 0–600V AC rating.
- Ammeter: For low current range, usually 0–5A or 0–10A AC rating.
- Wattmeter: For low power measurements, usually LPF (Low Power Factor) type Wattmeter.
- Connection wires and terminal blocks: For making correct and safe connections.
- No-load transformer: Tested transformer (for OC Test, voltage is given only to the primary side, the secondary is open).
- Insulating gloves and safety equipment: For safety.
Transformer Open Circuit Test Steps
Required Equipment:
- Voltmeter
- Ammeter
- Wattmeter
- Variac or adjustable voltage source
- Transformer (for testing)
Step 1: Connection Preparation
- Use the low-voltage (LV) winding of the transformer for open circuit test.
- Keep the high-voltage (HV) winding open, do not connect any load.
- Apply voltage gradually to the LV winding through the variac.
Step 2: Equipment Connection
- Connect an ammeter and a wattmeter in series to the LV winding.
- Connect the voltmeter across the two ends of the LV winding.
Step 3: Increase the voltage gradually
- Use the variac to increase the input voltage gradually until it reaches the rated voltage.
Step 4: Taking Readings
When the rated voltage is reached, take the following readings:
- Voltage (V) – from the voltmeter
- Current (I₀) – from the ammeter
- Watts (W₀) – from the wattmeter
Precautions and safety measures
Precautions and safety measures to be taken while performing the open-circuit test of the transformer:
- Check the equipment: Check in advance whether the voltmeter, ammeter, and wattmeter are working properly. Make sure that the insulation of the connections is correct.
- Test on the low-voltage side: Usually Open Circuit Test is done on the low-voltage (LV) side of the transformer. Because it is relatively safe and metering equipment is easily available.
- Keep the high-voltage side open: The high-voltage (HV) side of the transformer should be kept completely open (open). There should be no load or connection on this side.
- Ensure correct wiring: Take care not to connect the wires incorrectly while connecting. Make sure that the grounding is done correctly.
- Wear safety equipment: Use safety gloves, insulated shoes, and eye protection. There should be no water or wet environment around the transformer.
- Avoid touching during the test: Make sure that no one touches the meter or transformer wires while the device is running.
- Have an assistant while testing: Do not test alone, but have a colleague so that help is available in case of emergency.
- Test according to the voltage limit: The supplied voltage should not exceed the rated voltage of the transformer.
- Turn off the power and disconnect the connection after the test: Turn off the power and disconnect all connections safely after the test.
Open Circuit Test is used to determine the core loss of a transformer without load. In this test, nominal voltage is applied to the primary winding, and the secondary is left open. The iron loss, no-load current, and power factor of the transformer can be determined from the readings obtained from the test.
This test is usually done on the low-voltage side (LV side) because the current is higher and the measurement is easier. The test results indicate how effectively the transformer is working and what its efficiency may be.
