How to test an electric motor?


  • Check the appearance of the motor; Check for body damage or damage to the cooling fan blade or shaft
  • Manually rotate the shaft to check the bearing condition; Check for free & smooth rotation.
  • Note the motor data from the motor NAMEPLATE
  • Earth Continuity: Use your ohmmeter to verify the resistance between earth and the motor frame is less than 0.5 Ω
  • Power supply– correct voltage (230 volts per line), 415 v between Ll to L2, L2 to L3, and, L3 to L1

Three Phase:

  • Ensure the terminal for the power supply is in good condition. Check the connection bar for the terminal (U, V, W ). Connection type – STAR OR DELTA
Three-phase motor
  • Confirm the power supply VOLTAGE for the electric motor. 230/400.
  • Using the multimeter, check the continuity of winding from phase to phase (U to V, V to W, W to U). Each phase to phase must have continuity if the winding is OK
  • Check the motor winding ohms reading using a multimeter or ohmmeter for the 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)
  • Insulation resistance of motor winding using Insulation tester meter set to the 500 Volt scale (1000v DC). 1. Check from phase to phase (U to V, V to W, W to U) and 2. check from phase to earthing (U to E, V to E, W to E). Minimum test value of the electric motor is 1 Meg Ohm (1 MΩ)
  • With the motor running, check the running amps of the motor using a Clamp on the meter. Compare to the FLA on the nameplate of the motor.
  • If every step is completed, decide the condition of an electrical motor either OK or NEED TO REPAIR

Single Phase

Check the motor winding ohms reading using a multimeter or ohmmeter. (C to S, C to R, S to R). The reading for starting to run should be equal to C to S + C to R.

Single-phase motor

Correct electrical terminal identification

There are three-terminal connections on a hermetically sealed motor compressor and are as follows: Common (C), Start (S), and Run (R). To identify the correct terminal connection the following procedure applies:

  • The highest resistance reading is between the start and run terminals
  • The middle resistance reading is between the start and common terminals.
  • The lowest resistance reading is between the run and common terminals.

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