How to Determine the Polarity of a Current Transformer?

1. DC Method (Instantaneous Battery Method)
This is the most commonly used method on-site, offering simple operation and accurate results.
Steps:
Connect the positive terminal of a dry cell battery (1.5~3V) to the primary winding L1, and the negative terminal to L2.
Connect the secondary winding K1 to the positive terminal of a pointer-type meter (such as a milliammeter), and K2 to the negative terminal.
Observe the pointer deflection the moment the switch is closed:
If the pointer deflects in the positive direction, then L1 and K1 are of the same polarity (subtractive polarity);
If it deflects in the reverse direction, the polarity is incorrect (additive polarity).
Principle: This method utilizes the law of electromagnetic induction to determine the polarity by observing the direction of the induced current generated by a momentary DC pulse.
2. AC Method (Voltage Comparison Method)
Suitable for transformers with smaller transformation ratios; attention should be paid to safety voltage.
Steps:
Short-circuit the primary winding L2 and the secondary winding K2.
Apply an AC voltage of 1-5V to the secondary side, and measure the primary voltage U1, secondary voltage U2, and the voltage U3 between L1 and K1.
If U3 = U1 - U2, it is subtractive polarity; if U3 = U1 + U2, it is additive polarity.
Note: This method is not suitable for transformers with a transformation ratio exceeding 10, as the U2 value will be too small to distinguish.
3. Instrument Method
Using equipment such as a current transformer calibrator, the polarity can be directly determined through a polarity indicator. This method is suitable for laboratory or high-precision applications.
Polarity Marking and Subtractive Polarity Principle
Current transformers usually use subtractive polarity marking, meaning that when the primary current flows into L1, the secondary current flows out of K1, and their directions are opposite. This design ensures that the magnetic fluxes generated by the primary and secondary windings cancel each other out in the core, ensuring normal measurement and protection functions. If the polarity is reversed, it may cause the meter to reverse or the protection device to malfunction.