Why is it a Priority to Measure the DC Resistance of the Transformer?

When measuring the DC resistance and turns ratio of a transformer, it is generally recommended to measure the DC resistance first, followed by the turns ratio. This order is advised due to considerations related to transformer structure and electrical principles.

1. Transformer Structure and Electrical Principles

DC Resistance Measurement
DC resistance measurement is performed by applying a direct current (DC) voltage to the transformer windings and measuring the current to calculate the resistance. During this measurement, the windings carry a direct current, but since DC does not produce an alternating magnetic field, there is no magnetization or eddy current loss in the core.

Turns Ratio Measurement
Turns ratio measurement typically uses an alternating current (AC) signal (low voltage) to measure the voltage ratio between the primary and secondary windings, reflecting the turns ratio of the transformer. Because this measurement involves AC characteristics, the transformer's core will be magnetized, potentially causing short-term hysteresis effects.

2. Impact of Measurement Order

Measuring DC Resistance First
No Impact: Measuring DC resistance first will not magnetize the core since DC current does not create a magnetic field in the core. This ensures that the transformer remains in an unmagnetized state, which will not affect subsequent turns ratio measurements.
Accuracy: When the turns ratio is measured afterward, the core is in its normal state, ensuring accurate and stable turns ratio measurements.
Measuring Turns Ratio First
Magnetization Effect: During turns ratio measurement, the core will be magnetized due to the AC signal, possibly leading to residual magnetization, especially in cases where the core quality is suboptimal or the transformer has not been used for a long time. This residual magnetization may temporarily affect the DC resistance measurement.
Temperature Effect: If the turns ratio measurement takes a long time or the signal is relatively strong, the windings may heat up slightly, which could affect the accuracy of the DC resistance measurement.

3. Potential Consequences

Measurement Error: If the turns ratio is measured before the DC resistance, the residual magnetization effect or winding heating may cause the DC resistance measurement to deviate from its true value, potentially leading to incorrect assessment of the transformer's condition.
Temperature Dependency: Since resistance is proportional to temperature, measuring DC resistance first helps avoid errors caused by winding temperature rise during the turns ratio measurement.

4. Recommendation

Considering these factors, it is recommended to measure the DC resistance first, followed by the turns ratio when testing transformers. This sequence ensures the accuracy and reliability of the measurements.

By measuring in this order, you avoid the risk of DC resistance measurement errors due to core magnetization and ensure that the turns ratio measurement is unaffected by external conditions.

The Relationship Between DC Resistance and Turns Ratio

DC resistance and turns ratio are interrelated in a transformer, and there is a certain relationship between them. Specifically, the relationship between DC resistance and turns ratio is as follows:

The Smaller the DC Resistance, the Larger the Turns Ratio

DC resistance is one of the electrical characteristics of a transformer in a static state. It is related to factors such as the number of turns, wire gauge, thickness, and material of the transformer coils. Under the same conditions, smaller DC resistance indicates stronger conductivity of the transformer coils, allowing them to carry larger currents. Therefore, when the DC resistance is smaller, the turns ratio tends to be larger.

The turns ratio is the ratio between the input voltage and the output voltage of a transformer, and it is related to factors such as the shape of the core, material, number of turns, and wire gauge. A larger turns ratio indicates a greater difference between the input and output voltages. Therefore, under the same conditions, a larger turns ratio is typically accompanied by a reduction in DC resistance.

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