Transformer Turns Ratio Test

Turns Ratio Test is one of the most important tests of a transformer that checks whether the ratio between its primary and secondary coils or windings is correct. This test helps in determining the quality of the transformer and ensures that the transformer is working properly.

What is Turns Ratio Test?

Turns ratio test checks whether the primary and secondary coils of the transformer are wound in the correct ratio. If the ratio is not correct, the transformer will not be able to transform voltage properly.

Why Turns Ratio Test important?

Turns Ratio Test is important because it helps to verify the turns ratio between the primary and secondary windings of a transformer.

1. Ensures Correct Voltage Ratio: The main function of a transformer is to change the voltage according to a specific ratio. If the turns ratio is not correct, then the output voltage will also be incorrect.
2. Helps to identify manufacturing defects: If any turns are missing in the primary or secondary winding, it can be detected through this test.
3. Necessary for maintenance: Over time, problems can occur in the windings. This test can detect problems in old or damaged transformers.
4. Verifying the functionality of the transformer: If the turns ratio is correct, it means that the transformer is still working properly.

In short, the turns ratio test is an important quality control test, which ensures that the transformer is working as designed or not.

How Turns Ratio Works

Number of Turns in Primary Coil / Number of Turns in Secondary Coil. This refers to how many times the wire is wound in the primary and secondary coils.

Let’s explain with an example:

Suppose 100 turns in the primary coil (wire is wound 100 times), 50 turns in the secondary coil

Then:

Turns ratio = 100:50 = 2:1

Now:

if 220V voltage is given in the primary, then in the secondary we get:

220V / 2 = 110V

Relationship between Primary and Secondary Coiling

Primary and Secondary Coiling Turns Ratio test (or TTR test – Turns Ratio Test) is an important test used to determine the proportional relationship between the windings of a transformer. In simple Bengali, it means:

Turns Ratio = Number of Coils in Primary Winding % Number of Coils in Secondary Winding

Number of Coils in Primary Winding

This ratio shows the relationship between the input voltage of the transformer and the output voltage.

Example:

Let’s assume a transformer:

The primary winding has 1000 turns

The secondary winding has 100 turns

Then Turns Ratio = 1000:100 = 10:1

This means that if 1000V is given to the primary, then 100V will be obtained from the secondary.

Why Turns Ratio Test Need

Transformer Performance Verification:

Purpose of the test:

1. To determine the turns ratio between the primary and secondary windings of the transformer.
2. To find out if there is any deviation between the manufactured ratio and the design ratio.
3. To identify internal short circuits or connection errors in the transformer.
4. To provide assistance in performance verification.

Test Procedure

1. The transformer should be completely disconnected from the power.
2. All terminals should be properly cleaned and connected.
3. The primary and secondary terminals of the turn’s ratio meter should be properly connected.
4. The ratio meter reading should be recorded by switching on the device.
5. The test should be done on each phase (A-B, B-C, C-A) (for three phase transformers).

Acceptable Range

If the turns ratio result is within ±0.5% of the design ratio, it is considered acceptable. If it is different, there may be a fault in the transformer.

Role of Turns Ratio in Fault Detection:

1. Fault Detection through Turns Ratio Test: When an internal fault occurs in a transformer (such as a winding short circuit), the ratio of the primary and secondary windings changes. A sudden change in this ratio indicates a fault.
2. Insulation Damage Detection: If the insulation is damaged anywhere in the winding, then an anomaly is seen in the flow of current, which results in a difference in the turns ratio.
3. Detection of Turn-to-Turn Short: When a turn-to-turn short occurs, some coils become ineffective and the ratio ratio decreases. This is a clear indication of fault detection.
4. Maintenance prediction: Regular monitoring of turns ratio can identify problems early, allowing for early maintenance or repair of the transformer.

Turns Ratio Testing Equipment and Tools

Turns Ratio Meter (TTR)

• This is the main instrument used to measure the turns ratio.
• Usually, an automatic (digital) TTR is used.

Power Supply

• Even if the transformer is not turned on, a low voltage source is required for the test (the TTR itself provides this source).

Connecting Cables / Test Leads

• Safe and reliable cables are required for the test setup.

Insulation Gloves and Safety Equipment

• Safety equipment is essential during any electrical testing.

Multimeter (optional)

• For auxiliary measurements.

Test Record Sheet or Laptop (for Data Logging)

• To record the test results.

Test Steps:

• Turn off and discharge the transformer: It is essential to disconnect all charges and connections.
• Connect the TTR device: Connect the primary leads A and B of the TTR to the two ends of the primary winding and the secondary leads A and b to the two ends of the secondary winding.
• Turn on the test: The TTR itself applies a low voltage and determines the ratio.
• Note the results: Record the ratio shown on the display and compare it with the design.

Turn Ratio Test Safety Instructions

Preparation and Permissions:

• Obtain all relevant permissions and approvals before starting the test.
• Ensure that the transformer is completely de-energized and earthed.

Personal Protective Equipment (PPE):

• Wear a safety helmet, insulated gloves, safety glasses, and safety shoes.
• Use an arc flash suit as required.

Test Equipment and Connections:

• Verify that the equipment is working properly before setting up the test.
• Ensure that the test leads are connected correctly.
• Incorrect connections can cause serious accidents.

Precautions During Test:

• Keep anyone away from the test device or transformer during the test.
• Monitor the instrument screen during the test and stop the test immediately if anything unusual occurs.

Voltage Application:

• Apply the test voltage within the specified range.
• Extreme caution is required when applying high voltage.

After the test is complete:

• Disconnect all connections and store the equipment.
• Document the test results and keep records.

Emergency:

• Disconnect power immediately in the event of any accident or emergency.
• Know the location of the nearest fire extinguisher and first aid kit.