How To Test Three - Phase AC Motors

The basic steps in ascertaining the health of a three-phase AC motor are given below:
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Continuity Test
(e) AC Motor Winding Resistance Test
(f) Insulation Resistance Test
(g) Running Amps Test

General Inspections
For the three-phase motor, do the following:

(1) Check the appearance of the motor. Check for burnt, damage to body or cooling fan or shaft.
(2) Manually rotate motor shaft to examine bearing condition. Look out for smooth and free shaft rotation. If shaft rotation is free and smooth, bearing is possibly in good condition, otherwise consider replacing, repair or carry out further diagnosis.
(3) As with all testing and inspections, the motor name plate provides valuable information that will help to ascertain the true health of the motor. Examine the name plate thoroughly and compare values of running amps test (see below) with name plate value

Earth Continuity and Resistance Test
With a multimeter, measure the resistance between motor frame (body) and earth. A good motor should read less than 0.5 ohms. Any value greater 0.5 ohms indicate trouble with the motor. Further troubleshooting maybe required

Power Supply Test
For three phase motors, the expected voltage for a 230/400V system is 230V phase to neutral and 400V between each of the three phase supply lines. Check that the correct voltage is applied to the motor using a multimeter.  Ensure the terminal for power supply is in good condition. Check the connection bar for terminal (U, V, and W). For three phase motors, connection type is either Star (Y) or Delta

AC Motor Winding Continuity Test
Using a multimeter, check the continuity of motor winding from phase to phase ( U to V, V to W , W to U ).Each phase to phase must have a continuity if winding is OK. If any particular phase fails the continuity test, your motor is probably burnt.
Please see how to identify three phase windings for proper winding identification. U, V, W is a European winding designation.

AC Motor Winding Resistance Test
Check the motor winding resistance or ohms reading using a multimeter or ohmmeter for phase to phase terminal ( U to V,V to W ,W to U ).The ohms reading for each winding must be the same (or nearly the same). Remember that the three phases have identical windings or nearly so!

Insulation Resistance Test
Insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail. For a three phase motor, insulation resistance is usually measured between each motor winding or phase and between each motor phase and motor frame (earth) using an insulation tester or megger. Set the voltage setting of the insulation resistance tester to 500V.  Check from phase to phase (U to V, V to W, W to U). Check from phase to motor frame (earth) (U to E, V to E , W to E ). Minimum test value of motor insulation resistance is 1 Meg Ohm (1 MO). See how to measure insulation resistance of Electric Motor

Running Amps Test
With the motor running, check the full load amps (FLA) with a suitable meter or preferably a clamp on meter and compare with the name plate FLA. Deviations from rated FLA could signify problems with the motor under test.