The transformer turns ratio tester (also known as automatic turns ratio and phase group tester) is a precision instrument designed for transformer 'health checks'. Its core task is to accurately measure the voltage ratio (turns ratio), winding connection group, and polarity error of transformers. Simply put, the working principle of a transformer is 'primary turns / secondary turns = primary voltage / secondary voltage'. The turns ratio tester verifies whether this ratio matches the design requirements on the nameplate by actually measuring the voltages on the high and low voltage sides.