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 the 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 motor

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
  • 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). The 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 whether the condition of an electrical motor is 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 which are as follows: Common (C), Start (S), and Run (R). To identify the correct terminal connection the following procedure applies:

  1. The highest resistance reading is between the start and run terminals
  2. The middle resistance reading is between the start and common terminals.
  3. The lowest resistance reading is between the run and common terminals.

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