Even for products of the same specification, the insulation resistance tested at different time periods will be quite different. The editor of Dalan Motor today exchanged and shared with you the detection of the insulation resistance of the motor for the hydraulic oil pump.
1. Factors affecting the insulation resistance
(1) Ambient temperature and humidity
The insulation resistance of motors of general materials decreases with the increase of ambient temperature and humidity. Relatively speaking, surface resistance (rate) is more sensitive to ambient humidity, while bulk resistance (rate) is more sensitive to temperature. The resistance value of a general medium at 70 ℃ is only 10% of that at 20 ℃. Therefore, when measuring the insulation resistance, the temperature and humidity at which the sample and the environment are in equilibrium must be indicated.
(2) Test time
When a certain DC voltage is used to pressurize the material to be tested, the current on the material to be tested does not reach a stable value instantaneously, but a decay process. At the moment of pressurization, a large charging current flows, followed by a relatively slow absorption current for a long time, and finally a relatively stable conductance current. The higher the measured resistance value, the longer the time to reach equilibrium. Therefore, in order to correctly read the measured resistance value during measurement, the value should be read after stabilization.
2. Measurement of insulation resistance
(1) Detection method
Use an insulation resistance tester (commonly known as a megohmmeter or shaker) to test.
(2) Detection principle
Add a DC voltage to the dielectric, and after a certain period of time the polarization process ends, the corresponding resistance of the leakage current flowing through the dielectric is called the insulation resistance.
3. Selection of tester
(1) Choose megohmmeter according to rated voltage level. In general, equipment with a rated voltage below 500V should use a 500V or 1000V megohmmeter or shaker; equipment with a rated voltage above 500V should use a 1000V ~ 2500V megohmmeter or shaker.
(2) Resistance range. There are two small black dots on the dial scale of the shaking table. The area between the small black dots is the accurate measurement area.
(3) Evaluation and judgment. When the rated load of the motor works to a stable state, the insulation resistance Rm (unit: MΩ) between the winding and the chassis should conform to the relationship expressed by the following formula. Rm≥U / (1000 + P / 100). In the formula, U is the rated voltage of the tested motor winding in V; P is the rated power of the tested motor in kW.
4. Precautions for the use of megger or shaker
(1) The megohmmeter should be selected according to the voltage level of the equipment. If a megohmmeter with an excessively high rated voltage is used to measure low-voltage insulation, the insulation may be broken.
(2) Before measuring the insulation resistance, the power supply of the device under test should be cut off and short-circuited.
(3) The connection of the megohmmeter should be two separate single wires with good insulation.
(4) Before the measurement, conduct an open circuit and short circuit test on the megohmmeter to check whether the megohmmeter is good.
(5) When measuring, press the shell of the megohmmeter with one hand to prevent the megohmmeter from vibrating. When the hands indicate 0, stop shaking immediately to avoid burning the watch.
(6) If the weather is wet or the insulation resistance of the cable is measured, the shield terminal should be connected to eliminate the effect of leakage current on the surface of the insulation.
(7) It is forbidden to measure the insulation of the equipment with a megohmmeter in the case of lightning or humid weather and in the vicinity of equipment with high voltage.
----- Responsible Editor: Dalan Oil Pump Motor 02-Procurement Consultant
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