High Voltage Insulation Resistance Tester
Darui Electric recommends:
High Voltage Insulation Resistance Tester, English name: High Voltage Insulation Resistance Tester. This instrument is suitable for measuring the insulation resistance of various transformers, motors, cables, switches, electrical equipment and insulating materials.
Product Structure
1. The high voltage insulation resistance tester mainly consists of three parts.
The first is the DC high voltage generator, used to generate a DC high voltage.
The second is the measurement circuit.
The third is the display.
(1) DC high voltage generator
To measure insulation resistance, a high voltage must be applied to the measurement terminal. This high voltage value is specified in the national standard for insulation resistance testers as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V…
There are generally three methods to generate DC high voltage. The first is the hand-cranked generator type. About 80% of the products produced in China use this method (the origin of the name "megger"). The second method is to step up the voltage through a mains transformer and rectify it to obtain DC high voltage. This is generally the method used by mains-powered megohmmeters. The third method is to use a transistor oscillator or a dedicated pulse width modulation circuit to generate DC high voltage, which is generally the method used by battery-powered and mains-powered insulation resistance testers.
(2) Measurement circuit
In the aforementioned megger (megohmmeter), the measurement circuit and the display part are combined into one. It is accomplished by a cross-coil ohmmeter head, which consists of two coils with an angle of 60° (approximately) between them. One coil is connected in parallel across the voltage, and the other coil is connected in series in the measurement circuit. The deflection angle of the meter pointer depends on the current ratio of the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance, the greater the current in the coil connected in series in the measurement circuit, and thus the larger the deflection angle. Another method is to use a linear ammeter for measurement and display. In the cross-coil ohmmeter head used earlier, due to the non-uniform magnetic field in the coil, when the pointer is at infinity, the current coil is exactly at the place with the strongest magnetic flux density, so even if the measured resistance is large and the current flowing through the current coil is very small, the deflection angle of the coil will be relatively large. When the measured resistance is small or zero, the current flowing through the current coil is large, and the coil has already deflected to a place with smaller magnetic flux density, so the resulting deflection angle will not be very large. This achieves non-linear correction. Generally, the resistance display of a megohmmeter head needs to span several orders of magnitude. However, when a linear ammeter head is directly connected in series in the measurement circuit, it won't work. The scale at high resistance values is all squeezed together and cannot be distinguished. To achieve non-linear correction, non-linear elements must be added to the measurement circuit, thereby producing a shunting effect at small resistance values and no shunting at high resistance values, so that the resistance display can span several orders of magnitude. With the development of electronic technology and computer technology, digital display meters are gradually replacing pointer-type instruments.
Digital measurement technology for insulation resistance has also been developed, among which the voltage ratio bridge circuit is one of the better measurement circuits. The voltage ratio bridge circuit consists of a voltage bridge and a measurement bridge. The signals output by these two bridges are respectively converted through A/D conversion and then converted into digital values for display through a microcontroller.
Note: The above information and technical parameters are compiled and released by Yangzhou Darui Electric Co., Ltd.
