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  • How to test 3-Phase motors



    Three phase motors can operate on either 220 Volts or 440 Volts alternating current. It all depends on how the motor connection is set up.


    Delta Connection (220 VAC)


    Star Connection (440 VAC)

    Most 3-phase motors will have six terminal posts inside the junction box (See photo below). Three of the terminals will be the input terminals, the other three send off power to the internal windings.



    This setup is typical for most motors. Notice in the above photo that the connections do not connect directly across the junction.

    Test 3-Phase Motor

    Step 1- Disconnect input & output wires

    Once you are certain that source power is shut off, unhook the three input wires usually labeled L1, L2, and L3 off the input terminals. Now unhook the output side wires off the remaining three terminals.

    Step 2- Test each terminal to ground

    They are most commonly labeled U1, V1, and W1, and U2, V2, and W2. Meter each terminal contact to the case metal. A normal meter reading should be infinite Ohms from any terminal to ground. If the reading shows close to zero Ohms, then the terminal is shorted and the motor will need replaced or repaired.

    Step 3- Test the terminal junction

    To check the junction, measure resistance across at the same letters. So from U1 diagonally across to U2 you should read some amount of resistance or possibly near continuity. Repeat this check on all three junctions. All three three pairs should produce near the same resistance reading. See photo below.



    Step 4- Test the input terminals to each other

    Ohm from U1 to V1 first, then check U1 to W1, and lastly V1 to W1. The reading should be open, or infinite Ohms since nothing is hooked up to the terminals.

    Step 5- Test each phase winding to ground

    Separately measure the resistance from each output winding wire to the case ground. The wires must be disconnected to perform this test. The reading should be open, or infinite Ohms. If you read low resistance or continuity, then that phase's winding is shorted to ground and must be repaired.

    Step 6- Test each phase winding to each other

    Measure resistance from each output winding wire to each other. This will be three separate tests. The resistance between the windings will vary depending on your particular motor, but should read very close to the same on all three tests. If you read near zero Ohms between the windings, then the windings are shorted together and it is time to purchase a new motor.

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