Motor insulation is made up of various insulating materials. The ratio of the direct voltage applied to the motor insulation to the steady leakage current flowing through it is defined as the insulation resistance. A high insulation resistance in the motor indicates good insulation, while a drop in insulation resistance indicates that its insulation has become damp or has aged and deteriorated
Therefore, the insulation resistance of the equipment is the simplest and most basic method to check its insulation condition. For equipment with large capacity or whose insulation resistance barely the requirements, the absorption ratio should also be measured to judge its insulation condition.
1. Motor insulation resistance
The insulation resistance of the motor stator winding to the housing, the cold insulation resistance value of the motor stator winding to the housing shall be greater than 20 MΩ.
2. Insulation resistance requirements for motor controllers
Insulation resistance of the drive motor controller is specified in section 5.2.7.3 of the standard GB/T 184881-2015 “Technical Conditions for Electric Vehicle Drive Motor System Part 1″.
The cold and hot insulation resistances between the power terminals and the housing, the signal terminals and the housing, and the power terminals and the signal terminals of the motor controller shall all be greater than or equal to 1 MΩ.
3.What are the cold and hot insulation resistances?
The insulation performance of materials at different temperatures is different. In high temperature environment, the insulation performance of electrical materials will be greatly affected, because high temperature will changes in the molecular structure of materials, thus affecting the electrical properties of materials. Generally, the insulation performance is better at low temperature, and the insulation performance becomes worse at high. The insulation performance measured at room temperature is the cold insulation resistance, and the insulation performance measured at high temperature is the hot insulation resistance.
The cold-state insulation resistance shall be measured under the condition that the difference between the temperature of the inside of the product (such as motor winding, internalitance of the electronic control, etc.) and the temperature of the surrounding air shall not be greater than ± 3 ℃. Cold state usually refers to the standby state the motor controller, motor and generator, when the temperature inside the product is basically the same as the temperature of the surrounding air.
The hot insulation resistance value measured immediately after the motor is stopped is the lowest insulation resistance value of the motor (the internal temperature of the product reaches the highest), and this resistance value is hardly affected by the ambient temperature, so it can be used to judge the true insulation resistance condition of the motor, motor controller, etc.
The insulation resistance value measured at cold state is affected by the ambient temperature, and the insulation resistance value measured at different ambient temperatures is different, so it is to be used as the basis for judging the real insulation resistance of the motor. When measuring the insulation resistance value at cold state, the temperature is generally in the range of 10-30 ℃, and the corresponding insulation resistance range of such a wide temperature range is also relatively wide, and the insulation resistance of the general motor controller at 25 ℃, RH60% cold state can reach more than 300 MΩ.
Hot insulation resistance can put the product into a high-temperature box, set Tmax as the highest temperature that can be reached inside the product, fully heatimmersed T time, and then detect the hot insulation resistance of the product in the box, the insulation resistance value measured at this time is the lowest insulation resistance value of the, and also can reflect the insulation resistance value of the product when it is running on the vehicle.
4. How to measure insulation resistance?
The common method is to use an insulation pressure tester, set to the insulation resistance test mode, with the voltage set to DC 500V, time 60s, duration 60s, and monitor the insulation resistance value. The resistance value tested after stability is the insulation resistance.
The general requirements for motor controllers are as follows: hot insulation resistance > 1MΩ, cold insulation resistance > 20MΩ
5. What factors lead to such a large difference in insulation resistance?
The insulation resistance of all media decreases with increasing temperature.
For the motor controller, its insulation resistance measuring terminal is mainly high voltage to the shell, and the insulation between high voltage and shell mainly relies on the insulation column and safety Y capacitor, among which the safety capacitor has the greatest impact. Because the capacitance and the leakage current are in direct proportion, the resistance of the capacitor is inversely proportional to the capacitance, that is, the higher the capacitance, the lower the insulation resistance. The larger the insulation resistance indicates that the leakage current under DC voltage is smaller.
Generally speaking, the larger the insulation resistance value, the higher the accuracy of the circuit.
6. What are the hazards of low insulation resistance?
In new energy vehicles, the working voltage of the motor and motor controller is relatively high, it can reach 750V DC, and some can above 1200V DC, and due to the inductive characteristics of the motor load, the instantaneous voltage may be as high as 2000V or more If the insulation resistance of the motor controller or motor is lower than the specified value, it may cause leakage, and there is a possibility of a safety accident at this time, so resistance is one of the mandatory inspection items for electronic control products.
7. Summary
Insulation resistance is divided into cold insulation resistance and hot insulation resistance, which is a method of naming, but in fact there is no clear regulation value for and hot. Generally, hot insulation resistance refers to the insulation resistance value measured immediately after the vehicle stops running for a period of time, when the insulation resistance value is relatively small and to the resistance value at the time of actual operation.
So, it is better to measure insulation resistance before the product is taken offline, or immediately after the product has undergone thermal insulation resistance testing, when it is still in the process of aging or after high-temperature aging, as the measured resistance value can more accurately reflect the true insulation characteristics of the product.
Post time: Sep-15-2025