Industrial Control Wiring, AC Drives, and 3 Phase Motors
AC Motor Basics

AC Motor Troubleshooting: Resistance, Amps, and Megger Tests Explained

Complete & Continue Next Lesson Learn More

How to Determine if Your AC Motor is Good or Bad: Resistance, Amperage, and Megger Tests

Before replacing an entire motor, it is crucial to perform thorough diagnostics to confirm if the motor windings have failed. While the insulation resistance test (often called "megging") is essential, simple tests using a standard multimeter can often tell you everything you need to know.

In this video, we provide a complete walkthrough of the three key tests for both 3-phase and single-phase AC motors: Amperage Draw, Resistance, and Insulation Resistance. We demonstrate the differences between a healthy motor and one with unbalanced windings or failed insulation.

We highly recommend establishing a baseline for your good motors before they fail. By tracking the following metrics, you can quickly identify issues in the future:

1. Amperage Check (Under Load)

  • What to look for: All three phases must have closely balanced current draw.

  • Sign of a bad motor: A significant imbalance in amps between phases often indicates an issue with the motor windings.

2. Resistance Check (De-Energized)

  • What to look for: The resistance (ohms) between all winding terminals (T1-T2, T2-T3, T1-T3) should be equal or nearly equal.

  • Sign of a bad motor: A substantial difference in resistance confirms an imbalance in the windings, making the motor a candidate for replacement.

3. Insulation Resistance (Megger Test)

  • What to look for: Use a dedicated insulation tester (megohmmeter) to apply high voltage (typically twice the motor voltage) and check resistance from each winding lead to the motor ground/frame.

  • Sign of a bad motor: A low reading (near zero) indicates a breakdown in the insulation, meaning the windings are shorted to ground.

Get the Right Tools for the Job

Accurate testing relies on having reliable meters. In the video, we utilize professional tools to show how to get precise readings for your diagnostic work: