With the rapid development of electric vehicle, as the core driving component, the performance and reliability of motor are directly related to the vehicle's endurance, safety and driving experience.
Electric vehicle motors usually work in high voltage environment (such as 400V or 800V system), so the insulation performance is very important. Insulation aging, dampness or damage may lead to motor efficiency decline, short circuit or even safety accidents, affecting the normal operation of the vehicle.
Therefore, measuring the insulation resistance of the motor regularly with a megohmmeter is a key step in preventive maintenance. This not only helps to extend the life of the motor, but also improves driving safety.
Based on the general motor measurement method and combined with the characteristics of electric vehicle, we will introduce the correct use of megohmmeter in detail, from preparation to result judgment, to help you ensure that the motor is in the best condition.
Using megohmmeter to measure the motor is an important method to judge its insulation performance
01 Preparations and Safety Measures
Before measurement, adequate preparation and safety measures are the basis to ensure smooth operation and personnel safety.
Tool Selection
Select proper megohmmeter according to rated voltage of motor. For common 380 V motors, a 500 V megohmmeter shall be selected; For high voltage motors (e.g. above 3 kV), use a 1000 V or 2500 V megohmmeter.
Safe Power-off
Ensure that the motor has been completely disconnected from the power supply, and hang a "No closing" warning sign at the power switch to prevent accidental power-on.
Megohmmeter Self-check
Before measurement, check whether the megohmmeter is normal with open circuit test and short circuit test.
Equipment Discharge
Short all terminals of the motor to the enclosure (ground) with wires to fully release the possible residual charge in the motor winding.
Important: The discharge process takes at least 2-3 minutes to prevent electric shock or damage to the megohmmeter. Clean dust, oil or moisture in the terminal box of the motor to ensure accurate measurement.
02 Detailed Explanation of Measurement Steps
The measurement is mainly divided into two parts: winding insulation resistance to ground and winding insulation resistance between phases.
Measurement of Insulation Resistance to Ground
Measuring the insulation resistance of the winding to the enclosure (ground) is the key to check whether there is a risk of leakage in the motor.
- Wiring: reliably connect the "E" end of the megohmmeter to the exposed part of the metal shell of the motor, and connect the "L" end to each winding terminal of the motor in turn.
- Shaking measurement: shake the megohmmeter handle at a uniform speed of about 120 rpm for 1 minute.
- Record: Record the insulation resistance value of each winding to the ground.
Measurement of Phase-to-Phase Insulation Resistance
Measure the insulation resistance between each phase winding of the motor to judge whether there is a short circuit between phases.
- Disconnection: Ideally, the connection tabs between the motor windings should be removed so that each phase winding can be measured separately.
- Wiring: Connect the "L" and "E" terminals of the megohmmeter to the two different phase winding terminals of the motor in turn.
- Shaking measurement: shake the handle at the speed of 120 rpm, and read the resistance value after the pointer is stable.
- Record: record the insulation resistance values of all phase-to-phase combinations.
03 How to Judge the Measurement Result
After the measurement, you need to judge the insulation state of the motor according to the insulation resistance value.
| Measurement Results | Explanation of Meaning | Deal with Suggestions |
|---|---|---|
| Insulation resistance ≥ 0.5 MΩ for 380 V motors | The insulation performance is basically qualified. In theory, it meets the minimum requirements, but in practice, the higher the resistance value, the better. | It can be preliminarily considered that the insulation is good, and it needs to be judged comprehensively in combination with other tests. |
| Insulation resistance < 0.5 MΩ | Poor insulation, possible moisture, dust, or partial insulation damage. | Cleaning, drying, or further servicing is required. |
| Insulation resistance is 0 | The winding may be short-circuited directly to the casing (short circuit to ground) or severely short-circuited between phases. | Energization is strictly prohibited. The short circuit point must be found and the winding must be repaired or replaced. |
| Serious unbalance of three-phase resistance | Possible turn-to-turn short circuit or partial insulation damage. | Further DC resistance measurement with a multimeter is required for confirmation. |
Summary of acceptance criteria:
- All measured values (to ground and between phases) shall not be lower than minimum standard (e.g. 0.5MΩ for 380V motor).
- The insulation resistance of three-phase winding shall be basically balanced, and the difference shall not exceed 10%.
04 Measurement Precautions
When using a megohmmeter to measure a motor, there are some details that require special attention to ensure accurate measurement and personal safety.
- Influence of environmental factors: Humidity and temperature have a great influence on the measured value of insulation resistance. High ambient humidity will cause the measurement to be low.
- Selection of megger voltage: Be sure to select the appropriate megger voltage according to the rated voltage of the motor.
- Discharge after measurement: After the measurement is completed, the winding must be short-circuited to the ground for at least 2-3 minutes before operation or disconnection.
- Comprehensive judgment of motor status: megohmmeter can only measure insulation resistance. Acceptable insulation does not mean that the motor is completely normal.
05 Common Problems and Solutions
In practice, you may encounter some typical problems:
- Low insulation resistance (but above 0.5MΩ): It usually indicates that the motor may be affected with moisture, carbon brush powder or oil accumulated inside. The treatment method includes drying and internal cleaning.
- If the insulation resistance is 0: it indicates that the winding is directly grounded and short-circuited or water enters the junction box seriously. It is necessary to check the short circuit point and repair or replace the coil.
- The pointer swings or the reading is unstable: it may be caused by uneven speed of the megohmmeter, poor contact of the test line or defects in the internal insulation of the motor.
Measuring motor insulation by megohmmeter is not only the basis of traditional motor maintenance, but also the core link of electric vehicle motor maintenance.
In electric vehicle, high-voltage motors require higher insulation, and regular inspection can effectively prevent failures and improve energy efficiency and safety.
As a car owner or technician, this skill can help you identify potential problems early and avoid costly repairs. Remember that insulation measurements are only one part of a comprehensive diagnosis, combined with other tests such as DC resistance and no-load current checks, to fully assess motor health.