This compact and portable instrument measures the no-load transformer ratio, i.e., ratio of high voltage winding to low voltage winding of a transformer in a bridge circuit and at low flux density in a core. Besides ratio, the ratiometer is used to measure percentage deviation of ratio, phase angle deviation and excitation current. It can also be used as a transformer comparator. When balanced, the ratio of the transformer is read directly in numerical form from the dials on the front panel. The balancing procedure is simplified by an internal synchronous and phase sensitive detector, which can be made sensitive to either the ratio or the phase angle adjustments. The traditional variable ratio bridge operation is cumbersome, the reading is not intuitive, and the necessary conversion is required, and the test result only reflects the one-phase transformation ratio. The instrument overcomes the shortcomings of the traditional variable ratio bridge test, and completes the three-phase ratio test at one time, and the test speed is fast and the accuracy is high, which greatly improves the test efficiency