Testing of EV AC Electric Motor Phase Resistance Balance Test of The Windings


Test to establish that the three coils in the electric motor are free of defect and current leak.

Should there be a leak it will damage the inverters.

Before replacing a defective inverter a thorough investigation must be undertaken to establish the reason for the inverter failure.

The procedure to test the motors is as follows.

A fair bit has already been stripped off the housing that contains two electric AC motors. This is to give us access to the (in the top right) two sets of connectors to the individual motors. These both have three windings.

The motors are tested individually one at a time.

Leads are connected running between the test computer and the coil terminals.

It is absolutely critical that the colour code be observed. The technician now commands the test to begin.

Above is screen shot of the computer screen while testing. Please note the colour of the coils match these of the test leads.

After the test is completed the following screen shows the result of the test

As there are two motors in this car each need to be tested.

The test leads are transferred to the other set of winding terminals and the test commences

This is a different size motor so one would expect different readings. The critical factor is the very small tolerance between the three windings. These readings are in Milli Ohms. The three green squares tell us that the test result is positive for this issue.

What you are seeing here is the end result of the testing. As each coil is tested the readings are displayed one after the other.