Motor insulation resistance test must be clear
Insulation resistance detection is the most common and unavoidable project of any motor manufacturer and user. The national standard GB755, G14711, GB12350 and other mandatory standards stipulate the specific requirements of motor insulation resistance test. Today Ms. participated in a basic conclusion to share with you.
Testing equipment and selection principle
The meter for measuring the insulation resistance is an insulation resistance meter, commonly known as a megohmmeter, which has two types: hand-powered (usually called a shaker) and electronic ("high-impedance"). The specification of the megohmmeter is determined by the rated voltage value it emits. There are four kinds of 250V, 500V, 1000V and 2500V commonly used in motor testing.
●Measure the insulation resistance of the motor winding
When measuring the insulation resistance of the motor winding, it should be selected according to the rated voltage of the motor. For motors of different voltage levels, the megohmmeter of different specifications should be selected. The selection rules are shown in the following table.
●Measure the thermal resistance of the thermal element
When measuring the insulation resistance of a thermal element (such as PTC, PT100) embedded in the winding, the lowest voltage level, that is, a megger of 250V specification should be selected.
Measuring method and specific requirements
According to the standard, the motor insulation resistance test should be carried out in the actual cold state and the hot state respectively. In the process of drying the winding core of the motor, it is the thermal insulation resistance test; during the inspection test, it can be carried out only in the actual cold state.
● For the different windings of the motor, if their two wire ends have been led out of the motor casing, the insulation resistance of each winding to the casing and the windings should be measured separately.
● When the insulation resistance test, the windings that do not participate in the test should be reliably connected to the casing.
● If the motor windings (such as three-phase windings) are connected in a Y shape or a delta shape, only the insulation resistance between the winding and the casing can be detected; the insulation resistance between the winding phase and the phase can no longer be measured.
● Before using the Megohmmeter, check the status compliance: Check whether the instrument and the lead wire are normal under the condition that the lead wire is shorted and separated.
Take the manual megohmmeter as an example. If the lead wire is shorted, the pointer of the watch should be "0" when the handle is shaken. Otherwise, if the two lead wires are disconnected, the pointer of the watch should point to infinity when the handle is shaken. ".
● After the hand-operated megohmmeter is used, the two joints should be short-circuited. Because the megohmmeter is the generator principle, the voltage it emits will cause different degrees of damage to the human body. Similarly, after the measurement, the measured winding should be discharged before the measuring line is removed.
● When measuring with a megohmmeter, the L end is connected to the winding, and the E end is connected to the casing; for the megohmmeter of the manual power generation, the rotation speed should be kept at about 120r/min; the reading should be read after the meter pointer is stable.
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