Measurement results and compliance determination
The insulation resistance of the motor in the hot state is lower than that of the cold insulation resistance. In the standard, the minimum limit value of the insulation resistance is specified for different motors.
1 hot insulation resistance
● General purpose motor insulation resistance. The insulation resistance value measured in the thermal state of the motor winding shall not be less than the value of equation (1).
In formula (1):
RM—the insulation resistance of the motor winding, in MΩ;
U——the rated voltage of the motor, in V;
P—The rated power of the motor; the unit of DC motor and AC motor is kW, kVA for AC generator, and kvar for reverse camera.
The mandatory standard GB14711 stipulates that if the value calculated by equation (1) is less than 0.38 MΩ, 0.38 MΩ should be used as the minimum correction limit value.
● Evaluation criteria for insulation resistance of small power motors. In GB5171, the insulation resistance of the low-power motor windings should be ≥1MΩ in the hot state and not less than 20MΩ in the normal state.
● Tropical type motor insulation resistance assessment standard. GB12351 stipulates that the insulation resistance of the tropical motor winding should not be less than the following specified values in the hot state;
(1) Low power motor: 0.5 MΩ.
(2) In addition to low-power motors, wet-tropic AC motors with frame numbers below 630 and wet-tropic DC motors with armature diameters of 990 mm and below (referred to as small and medium-sized wet-tropic motors).
1) The motor with stator voltage ≥3000V and the motor with induced voltage ≥110V-3000V are rated as IP22 and above to IP44 (referred to as protective motor), twice the value of formula (1), but the minimum is 0.38 MΩ.
2) The rated voltage of 110V~3000V is the motor with the protection degree of lP44 or above (referred to as closed motor), and the value of formula (1) is 3 times, but the minimum is 0.38MΩ.
(3) Small and medium-sized dry tropical motors: The value is obtained according to formula (1), but the minimum is 0.38 MΩ.
2 cold insulation resistance
●General purpose motor
GB14711 stipulates that for low-voltage motors (AC 1000V and below, DC 1500V and below), it should be no less than 5MΩ. For high-voltage motors, it should be specified in the corresponding technical conditions. In general, the higher the temperature, the smaller the insulation resistance. The cold and hot insulation resistance conversion formulas are given below for reference.
RMC-cold insulation resistance evaluation value in formula (2), MΩ;
Te—motor insulation heat classification temperature, such as Class B insulation 95 ° C, Class F insulation 115 ° C;
T—winding temperature (°C) during measurement, generally using ambient temperature;
U—rated voltage of the motor, V.
●Evaluation criteria for small power motors
In GB/T5171, the insulation resistance of the low-power motor winding should be ≥20MΩ in the cold state.
3 insulation resistance absorption ratio
The absorption ratio KM is a measured value of RM60s when the test voltage is applied for 60 s when the insulation resistance is measured. The ratio of the measured value RM15s when applied for 15 s is in accordance with the formula (3).
4 insulation resistance polarization index (PI)
The polarization index (Pl) is the ratio of the measured value RM10min when the test voltage is applied for 10 minutes to the measured value RM10min when the test voltage is applied for 10 minutes, and is in accordance with the formula (4).
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