Measurement method of insulation resistance of three phase a
Release time：2020-05-08 15:49Number of views：Originate：
1、 Practical tasks
(1) Test whether the insulation resistance of three-phase asynchronous motor meets the requirements;
(2) According to the given circuit to be tested, measure and record the insulation resistance according to the use method.
During the use of electrical equipment, due to heat, pollution, humidity or aging, the insulation resistance will be reduced, which may lead to electric leakage or short circuit of equipment and electric shock accident. In order to ensure the normal operation and personal safety of the equipment, it is necessary to regularly check the newly purchased and newly placed equipment that has been put into use for more than three months or the equipment and circuits in use. If any hidden danger is found, it shall be eliminated in time. Follow the manufacturers of three-phase asynchronous motor to see how to measure the insulation resistance of three-phase asynchronous motor.
Correct selection of instruments
To measure the insulation resistance of three-phase asynchronous motor, megger (digital or pointer) shall be selected. The selection principle of megger is as follows
(1) Select the rated voltage level of the megger according to the voltage of the tested circuit or electrical equipment;
For lines or equipment measuring 500 V and below, select 500 V or 1000 V megohmmeter;
For lines or equipment with rated voltage higher than 500V, megger of 1000V or 2500V shall be selected.
For insulators, busbars and other high-voltage equipment or lines, two thousand five volts or five thousand volts megohmmeter shall be selected.
(2) According to the insulation resistance requirements of the tested line or electrical equipment, select the rated range of the megger (for example, if the insulation of the line is not less than 1000 megohm, the range of the megger shall be greater than 1000 megohm);
Check before use (take pointer table as an example)
(1) Check whether the shell is intact; whether there is a certificate of conformity or regular inspection certificate;
(2) Check whether the pointer is bent;
(3) Check whether the lead connection is correct (l-lead is red, ground e-lead is black, shielding g), whether it is damaged, and whether the probe chuck insulation is good;
(4) Check whether the rotating handle is normal;
(5) Whether the voltage is suitable (500V or 1000V for low-voltage line or equipment, and more than 2500V for high-voltage line or equipment).
Short circuit test and open circuit test
(1) Short circuit test: short circuit the two probes, slowly turn the handle, and the dial pointer shall point to zero;
(2) Open circuit test: open two probes, quickly turn the handle (120 RPM), and the dial pointer points to ∝.
Note: if the pointer cannot reach the zero position or ∨, it can only be used after being adjusted by a professional organization, otherwise the measurement results will be affected.
When measuring the insulation resistance between the motor phase and the ground (shell), the e end (black wire) is connected with the motor shell, the L end (red wire) is connected with one end (U-shaped shell) of the tested winding, V-shaped shell and W-shaped shell, for example.
Phase earth (enclosure) wiring
When measuring the interphase insulation resistance of the motor, e-terminal (black wire) and l-terminal (red wire) are respectively connected to one end (U-V; Northwest University; V-W)
During each measurement, turn the handle from slow speed to fast speed until the speed reaches about 120 rpm, keep the handle speed uniform and stable, generally rotate for 1 minute, and read after the pointer is stable.
After the measurement, the connecting wire can only be removed after the megger stops rotating, the tested object is grounded and discharged.
Result judgment or reading
Read directly. The insulation resistance required for low-voltage equipment or line shall not be less than 0.5m.
Pay attention when using
(1) The tested equipment or line must be in power off state;
(2) It is forbidden to measure the insulation resistance during lightning or near high-voltage equipment. The insulation resistance can only be measured when the device is not charged and there is no induced current.
(3) During vibration test, no one is allowed to work on the tested equipment.
(4) The watch wires should not be twisted together, they should be separated.
(5) Before the shaking table stops rotating or before the tested equipment is discharged, it is forbidden to touch the shaking table by hand. When removing the wire, do not touch the metal part of the wire.
(6) When measuring the handle, it should be placed firmly to prevent inaccurate reading due to shaking and uneven shaking speed, especially when measuring large-scale equipment, the instrument will burn down due to power back;
(7) During measurement, absorption ratio R60 / R15 ≥ 1.3 shall be considered for large capacity equipment, and finally discharge.
(8) During the measurement, if the pointer points to the "0" position, indicating the internal short circuit of the tested equipment, stop the handle immediately.
(4) Finishing after measurement
After use, place it in a ventilated and dry place (in the cabinet).
It is easy to remember and understand.