In electric vehicle maintenance, industrial equipment operation & maintenance, and traction motor after-sales troubleshooting, motor fault diagnosis is a frequently used and core fundamental skill. On-site misjudgment of motor conditions often leads to repeated part replacement, ineffective commissioning, and even customer complaints.
This article organizes a standardized testing procedure for asynchronous motors. Only two common tools — a 500V megohmmeter and a digital multimeter — are required. Through two core tests, technicians can comprehensively evaluate the insulation integrity and conduction status of motor windings.
Electrical faults of motors fall into two main categories: insulation faults and winding faults.
- Insulation faults: Degradation of insulating layers between windings and ground, or between different winding phases, causing abnormal current leakage or short circuits. These faults can be directly identified by measuring insulation resistance with a megohmmeter.
- Winding faults: Internal open circuits or inter-turn short circuits inside windings. Such faults do not affect overall winding-to-ground insulation, so they cannot be detected by a megohmmeter. A multimeter is required to measure DC resistance and continuity for diagnosis.
Based on the above principles, the logic of the two-step detection method is as follows:
Step 1: Use a megohmmeter to inspect insulation faults;
Step 2: Use a multimeter to inspect winding integrity faults.
The two tests complement each other and form a basic evaluation system for the electrical condition of the motor.
Step 1: Insulation Resistance Test with Megohmmeter
Pre-Test Preparation
Fully disconnect the three-phase power wires U, V, W from the controller side, ensuring the test circuit only contains the motor’s internal windings. If connected to the controller, the measured insulation resistance will include leakage current from the controller power devices, rendering readings invalid for reference.
Select an unpainted, rust-free bare metal area on the motor housing as the grounding contact point. If the housing paint is intact, sand off a section to create reliable metal contact.
A 500V megohmmeter is recommended for this test. This voltage class fits routine insulation inspection for low-voltage asynchronous motors; excessively high test voltage may impose unnecessary electrical stress on intact insulation.
Test Item 1: Winding-to-Ground Insulation
Connect the megohmmeter’s L (Line) terminal to any phase wire, and the E (Earth) terminal to the motor metal housing.
Crank the megohmmeter at a steady speed (approximately 120 revolutions per minute), and record the reading once the pointer stabilizes. Measure three groups of data sequentially: U-phase to ground, V-phase to ground, W-phase to ground.
Judgment Criteria:
- Insulation resistance ≥ 0.5 MΩ: Insulation qualified
- Insulation resistance between 0.2 MΩ and 0.5 MΩ: Substandard insulation, likely dampness; re-test after drying treatment
- Insulation resistance close to 0 Ω: Winding breakdown to ground, severe earthing fault; motor must not be put into operation
Test Item 2: Phase-to-Phase Insulation
Connect the two terminals of the megohmmeter to two different phase wires, and measure insulation resistance sequentially for U-V, V-W, V-U phase pairs.
The pass standard is also ≥ 0.5 MΩ. If insulation resistance between any two phases approaches 0 Ω, the inter-phase insulation has broken down, indicating an inter-phase short circuit in windings; the motor needs to be scrapped.
⚠️ Safety Warnings: The megohmmeter outputs high voltage. Discharge motor windings both during and after testing. Do not connect megohmmeter probes to weak-current circuits such as encoders, Hall sensors, or temperature sensors — high voltage will permanently damage sensitive electronic components.
Step 2: Winding Resistance & Continuity Test with Multimeter
Insulation tests only reflect the insulation condition between windings and external components, and cannot reveal internal winding conduction issues. Measuring winding DC resistance is mandatory to verify winding integrity.
DC Resistance Measurement of Three-Phase Windings
Switch the multimeter to the low-resistance range, then measure DC resistance of three winding pairs: U-V, V-W, W-U.
Judgment Criterion: Resistance values of the three phases shall be nearly identical, with a deviation no greater than 5%.
Example: If all three readings are around 5.7 Ω with allowable deviation, the windings are free of obvious open circuits or inter-turn short circuits.
- Infinite resistance reading for one phase → Winding open circuit
- Significantly lower resistance for one phase → Potential inter-turn short circuit
- All three resistances close to 0 Ω → Severe winding short circuit with fused conductors
Note: High-power traction motors inherently feature very low winding DC resistance (usually several ohms or less than 1 ohm). Use the multimeter’s low-resistance range and ensure tight contact between probes and terminals to minimize measurement error from contact resistance.
Winding Continuity Check
Apart from quantitative resistance measurement, use the multimeter’s continuity mode for qualitative verification. If no buzzer sounds between two phases, an open winding circuit can be confirmed without precise resistance measurement.
Supplementary Inspection: Mechanical Condition Assessment
After completing electrical tests, inspect the motor’s mechanical performance. Rotate the motor shaft manually and observe the following conditions:
- Smooth rotation with no jamming or metallic scraping noise → Bearings in good condition
- Periodic binding during rotation, or obvious axial/radial shaft play → Worn or damaged bearings
- Shaft completely locked and unable to rotate → Seized bearings or rotor-to-stator rubbing
Unaddressed bearing faults will further cause rotor eccentricity and winding abrasion, eventually triggering electrical failures. Therefore, even if all electrical tests pass, the motor shall not be commissioned if mechanical defects exist.
Limitations of This Detection Method
Limitations of the Megohmmeter
A passing megohmmeter reading only confirms no winding earthing or inter-phase short circuits — it cannot rule out minor inter-turn short circuits. Mild short circuits between adjacent coils within a single phase do not compromise overall winding-to-ground insulation, making them undetectable by megohmmeter testing.
Typical operational symptoms of hidden inter-turn short circuits: Passing insulation test, balanced three-phase resistance, yet abnormal temperature rise during motor operation, insufficient output torque, and frequent overcurrent protection tripping on the controller. In such cases, further diagnosis via no-load current testing, temperature rise tests, or vibration spectrum analysis is required.
Limitations of the Resistance Measurement Method
Balanced three-phase resistance only eliminates obvious inter-turn short circuits and winding open circuits. Extremely minor inter-turn short circuits (e.g., shorting of only one or two coils) produce negligible resistance variations that may fall below the multimeter’s measurement precision and go undetected.
Therefore, the accurate conclusion is: Balanced resistance means no obvious winding faults detected, rather than absolute perfect winding condition. If actual operating performance contradicts test results, additional troubleshooting must be carried out; a final verdict cannot rely solely on resistance testing.
