Insulation resistance testing is a common and unavoidable task for any motor manufacturer and user. National standards such as GB755, G14711, and GB12350, among other mandatory standards, specify the detailed requirements for motor insulation resistance testing. Today, Ms. Can has compiled a basic summary to share with everyone.
Testing equipment and selection principles
The instrument used to measure insulation resistance is called an insulation resistance meter, commonly known as a megohmmeter. There are two types: hand-cranked (usually called a megohmmeter) and electronic ("high-resistance meter"). The specifications of a megohmmeter are determined by its rated voltage value. Four commonly used voltage ratings for motor testing are 250V, 500V, 1000V, and 2500V.
●Measure the insulation resistance of the motor windings
When measuring the insulation resistance of motor windings, the appropriate megohmmeter should be selected based on the motor's rated voltage. Different megohmmeters of different specifications should be selected for motors of different voltage levels. The selection rules are shown in the table below.
●Measuring the insulation resistance of a thermistor
When measuring the insulation resistance of thermistors (such as PTC and PT100) embedded in the winding, a megohmmeter with the lowest voltage rating, i.e., 250V, should generally be selected.
Measurement methods and specific requirements
According to the standard, the insulation resistance test of a motor should be carried out under both actual cold and hot conditions. During the drying process of the motor's winding core, the insulation resistance is tested under hot conditions; however, during the actual cold condition, the test can be conducted only.
● For different windings of the motor, if both ends of their wires are brought out outside the motor housing, the insulation resistance of each winding to the housing and between each winding should be measured separately.
● During the insulation resistance test, the windings not involved in the test should be reliably connected to the casing.
●If the motor windings (such as three-phase windings) are connected in a Y or Δ configuration, only the insulation resistance between the windings and the casing can be tested; the insulation resistance between the winding phases cannot be measured.
● Before using a megohmmeter, its condition should be checked: check whether the meter and the leads are normal with the leads shorted and open.
Taking a hand-cranked megohmmeter as an example, if the leads are short-circuited, the pointer should return to the "0" position when the handle is turned. Conversely, if the two leads are disconnected, the pointer should point to infinity "∝" when the handle is turned.
● After using a hand-cranked megohmmeter, the two terminals should be short-circuited to discharge it; because the megohmmeter operates on a generator principle, the voltage it generates can cause varying degrees of harm to the human body. Similarly, after measurement, the tested winding should be discharged before disconnecting the measuring leads.
● When using a megohmmeter, connect the L terminal to the winding and the E terminal to the casing; for megohmmeters with hand-cranked generators, the rotation speed should be maintained at around 120 r/min; the reading should be taken after the instrument pointer has stabilized.
Measurement results and compliance determination
The insulation resistance of a motor in a hot state is lower than that in a cold state. The standard specifies different minimum insulation resistance values for different motors.
1. Hot insulation resistance
●Insulation resistance of general purpose motors. The insulation resistance value measured under the hot state of the motor windings should not be less than the value of formula (1).
In formula (1):
RM—Insulation resistance of motor windings, measured in MΩ;
U—Rated voltage of the motor, in volts (V);
P—Rated power of the motor; the unit for DC motors and AC motors is kW, for AC generators it is kVA, and for synchronous condensers it is kvar.
The mandatory standard GB14711 stipulates that if the value calculated by formula (1) is less than 0.38 MΩ, then 0.38 MΩ shall be used as the minimum correction limit.
●Insulation resistance test standard for small power motors. GB5171 stipulates that the insulation resistance of the windings of small power motors should be ≥1MΩ when hot and not less than 20MΩ when under normal conditions.
● Insulation resistance test standard for tropical motors. GB12351 stipulates that the insulation resistance of the windings of tropical motors should not be less than the following specified values when hot;
(1) Small power motor: 0.5MΩ .
(2) Except for small power motors, AC motors with frame size below 630 and DC motors with armature diameter of 990mm and below in humid tropical regions (hereinafter referred to as small and medium-sized humid tropical motors).
1) For motors with stator voltage ≥3000V and motors with induced voltage ≥110V-3000V and outer casing protection level of IP22 and above to IP44 and below (hereinafter referred to as protective motors), the value obtained by formula (1) is twice the value, but the minimum is 0.38MΩ .
2) Motors with a rated voltage of 110V~3000V and an enclosure protection level of LP44 or above (referred to as enclosed motors) have a value of 3 times that required by formula (1), but the minimum is 0.38MΩ .
(3) Small and medium-sized dry tropical motors: according to the value obtained by formula (1), but the minimum is 0.38MΩ .
2. Cold insulation resistance
●General Purpose Motors
GB14711 stipulates that for low-voltage motors (AC 1000V and below, DC 1500V and below), the insulation resistance should be no less than 5MΩ. For high-voltage motors, this should be specified in the corresponding technical conditions. Generally, the higher the temperature, the lower the insulation resistance. The following formulas for converting insulation resistance between cold and hot states are provided for reference.
In formula (2), RMC is the cold insulation resistance test value, in MΩ;
te—Motor insulation thermal classification temperature, such as Class B insulation 95℃, Class F insulation 115℃;
t—Winding temperature (°C) during measurement, generally using ambient temperature;
U—Rated voltage of the motor, V.
● Evaluation criteria for small power electric motors
GB/T5171 stipulates that the insulation resistance of the windings of small power motors should be ≥20MΩ when cold.
3. Insulation resistance absorption ratio
The absorption ratio KM is the ratio of the measured value RM60s when the test voltage is applied for 60s to the measured value RM15s when the voltage is applied for 15s. It is considered qualified if it meets 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 1 minute. It is considered qualified if it meets the formula (4).
