An asynchronous motor is checked with a multimeter in three steps with the power completely off: the links are removed, the resistance of each phase winding is measured and compared, and then you check for continuity between the windings and the frame. This finds an open or shorted winding; weakened insulation only shows up on a megger.

Step-by-step measurement

  1. Safety: Isolate the motor supply at the panel, lock the switch and have a qualified electrician confirm the cables are dead. Never measure on a live circuit.
  2. Remove the links: Take out the star or delta links in the terminal box; otherwise the windings stay connected and the reading misleads.
  3. Winding resistance: Set the multimeter to the lowest ohm range and measure between U1-U2, V1-V2 and W1-W2. The value depends on motor power; what matters is that the three values are close to each other.
  4. Phase-to-phase check: With the links removed there must be no continuity between U1-V1, V1-W1 and U1-W1. Continuity means a short between windings.
  5. Leakage to frame: Between each terminal and the earth screw on the frame the multimeter should show an open circuit.

How to read the results

  • If one winding shows an open circuit, that winding is broken.
  • If one winding reads clearly lower than the others, there may be a turn-to-turn short in that phase.
  • If there is continuity to the frame, the motor must not be run under any circumstances.
  • If the values are balanced and there is no continuity to the frame, the motor is most likely electrically sound; the problem may be in the supply, controls or mechanics.

Even when the multimeter test is clean, the insulation may be weak; for a firm verdict have a megger insulation test done. If the measurement shows the motor needs replacing, the asynchronous electric motors page lists models of the same power and speed. Send a photo of the nameplate and the terminal box on WhatsApp and we will choose the right equivalent for your machine together.