During measurement, a low voltage U1 is applied to the primary side of the transformer under test, and a voltage U2 appears on the secondary side. By adjusting the resistance value of R3, the galvanometer can be brought to zero. At this point, the turns ratio k can be calculated using the following formula (R3 is much smaller than R2 and R1, so R3 is ignored):

k = U1 / U2 = (R1 + R2) / R2 = 1 + R1 / R2

To directly read the turns ratio error value, a slide wire resistor R3 can be connected in series between R1 and R2, with one end of the galvanometer placed on the sliding contact. Different resistance values of the slide wire resistor R3 are marked on the bridge panel with corresponding turns ratio errors. This allows for direct measurement of the turns ratio error.

Using the turns ratio bridge method to measure the turns ratio of a three-phase transformer is similar to the dual voltmeter method. It also uses a single-phase power supply, with a switching circuit built into the bridge. For different connection groups, measurements are performed according to different measurement modes (voltage application, short circuit, measurement). Therefore, when conducting turns ratio tests using the turns ratio bridge method, it is necessary to wire correctly and operate properly according to the instruction manual.

The above is the method of measuring transformer turns ratio using the turns ratio bridge method. In the absence of a transformer turns ratio tester, we can use this method to measure the transformer turns ratio.