Current Transformer Tester – Principle and Structure
Application Background
In power supply and distribution systems, current levels vary widely, ranging from a few amperes to tens of thousands of amperes. To facilitate accurate measurement by instruments and meters, current must be converted to a more uniform level. Additionally, working voltages on supply circuits are relatively high, making direct accurate measurement risky for instruments, meters, and personnel. Therefore, the current transformer tester is designed to provide both current transformation and electrical equipment isolation.
Current Level Conversion
Most analog current digital display instruments operate at the ampere level; therefore, the secondary current of most current transformer testers is also at the ampere level (e.g., 5 A). However, the data signals sampled by electronic computers are generally at the milliamp level (0–9 V, 4–50 mA, etc.). Consequently, a small current transformer tester (with secondary current at the milliamp level) is used as an interface between the current transformer tester and the electronic computer's sampled data signal, enabling secondary current conversion.
Structure
The current transformer tester consists of a primary electromagnetic coil, a secondary electromagnetic coil, an iron core, an insulating layer support, and a terminal block. The iron core is made of laminated ferrite cores. The primary electromagnetic coil is connected in series with the main power circuit. Based on the measured current I₁, it generates alternating magnetic flux in the iron core, inducing a corresponding secondary current I₂ (rated voltage: 5 A) in the secondary electromagnetic coil. Ignoring excitation regulator losses, I₁n₁ = I₂n₂, where n₁ and n₂ are the turns of the primary and secondary coils. The transformation ratio is K = I₁/I₂ = n₂/n₁.
Safety Isolation
Since the primary electromagnetic coil is connected to the main power circuit, it must be insulated with materials consistent with the primary circuit's working voltage to ensure both secondary circuit safety and personal safety. The secondary circuit consists of the secondary electromagnetic coil, the instrument panel, and the current electromagnetic coil of the automotive relay, connected in series.
Classification
Current transformer testers can be broadly divided into two categories: those for accurate measurement and those for protection purposes.