An insulation resistance test is a useful way to check whether an electric motor winding is dry and electrically separated from the frame before startup. It is often called a megohmmeter or “megger” test, but it is not a complete motor health assessment. A safe test requires the motor to be isolated from the supply, drive, cables, and connected equipment, and the result must be interpreted using the manufacturer’s instructions.
What an insulation resistance test measures
A megohmmeter applies a controlled DC test voltage and measures the resistance of the insulation system. The reading indicates how much leakage current passes through the winding insulation to ground or between conductors. Higher resistance generally indicates better separation, but a single high reading does not prove that every part of the winding is healthy.
Insulation resistance can be affected by moisture, dirt, temperature, surface contamination, aging, and the test connection. It is best used as a repeatable trend: compare readings taken with similar test conditions and record the temperature, test voltage, duration, and connection points.
When should a motor be tested?
Common occasions include:
| Situation | Reason for testing |
|---|---|
| Before first commissioning | Confirm the winding and installation are suitable before energizing the motor. |
| After long storage | Detect moisture or contamination that may have entered during storage. |
| After flooding or washdown | Check whether water has reduced the insulation condition. |
| After rewinding or repair | Provide a baseline before returning the motor to service. |
| During a maintenance program | Track changes over time and identify deterioration before failure. |
A test is also useful when a motor shows unexplained tripping, a low-insulation alarm, or a change after relocation. If the test result is abnormal, do not repeatedly energize the motor in the hope that the problem will disappear.
Safety and preparation come first
Insulation testing is electrical work and should be performed by a qualified person using the site’s lockout and electrical-safety procedures. Disconnect and isolate all sources of energy, verify the circuit is de-energized, and secure the driven machine. A megohmmeter can leave a charge on a winding after the test, so the windings must be discharged against the frame as specified by the test equipment and motor instructions.
Disconnect the motor cable from the variable frequency drive before testing. Do not apply a megohmmeter to a drive output, encoder, temperature sensor, brake, surge protection device, or other electronic accessory unless its manufacturer explicitly allows it. ABB drive documentation specifically instructs users to disconnect the motor cable from the drive and measure the motor separately.
Basic test approach
- Read the motor manual, nameplate, and drive instructions to identify the permitted test voltage and connection method.
- Record the motor identity, winding temperature, ambient conditions, and whether the motor is clean and dry.
- Isolate the motor, disconnect supply cables and sensitive accessories, and verify zero voltage.
- Connect the tester as instructed, commonly between a phase conductor and the motor frame or protective earth; test phase-to-phase only when the procedure requires it.
- Apply the specified test for the specified duration, then record the stabilized reading and test conditions.
- Discharge the winding, remove the test leads, restore all connections, and inspect the terminal box before re-energizing.
The exact arrangement can change for star-connected, delta-connected, parallel-wound, multi-speed, or inverter-duty motors. Follow the motor manufacturer’s procedure instead of assuming that every motor is tested in the same way.
How to interpret the result
There is no single pass/fail number that applies to every motor. Published guidance differs with winding voltage, motor design, temperature, insulation system, and the purpose of the test. For example, ABB documentation gives different reference criteria for different motor and drive applications. Use the value specified for the actual motor, and ask the manufacturer or a qualified test professional when the result is near a limit.
Temperature matters. ABB notes that insulation resistance can change significantly with winding temperature, including a reduction as temperature rises. If two readings are taken at different temperatures, they may not be directly comparable. Record the temperature and apply the correction method required by the manufacturer or the applicable testing standard.
A declining trend is often more useful than one isolated number. A sudden drop, a large difference between phases, or a low reading after moisture exposure deserves investigation. The winding may need cleaning or controlled drying, followed by a repeat test. Drying should be performed with an approved method that will not damage insulation, bearings, seals, or paint.
What this test cannot prove
Insulation resistance mainly assesses leakage to ground or between separated conductors. It may not reveal every turn-to-turn weakness, partial-discharge problem, mechanical defect, rotor fault, or bearing issue. If the application is critical or the readings are inconsistent, additional tests such as winding resistance, surge comparison, polarization-index evaluation, vibration analysis, or thermal inspection may be appropriate.
Record the test as part of motor maintenance
Keep the motor model, serial number, test instrument, test voltage, duration, temperature, phase connections, readings, and corrective actions in the maintenance record. Relate the result to the motor’s insulation class and temperature rise, and review it with the broader checks in our guides to motor maintenance, motor windings, and reading a motor nameplate.
Used safely and consistently, insulation resistance testing turns a one-time measurement into useful condition information. The goal is not to chase the largest possible number; it is to detect moisture, contamination, and deterioration early enough to make a controlled repair decision.