Differences in Resistance Measurement Principles Between Megohmmeter and Multimeter
A megohmmeter, also known as a shaking table, is mainly used to measure the insulation resistance of electrical equipment. It is composed of components such as the voltage doubling rectifier circuit of the AC generator and the meter head. When the shaking table is shaken, a direct current voltage is generated. When a certain voltage is applied to the insulation material, an extremely weak current will flow through it, which is composed of three parts: capacitive current, absorption current, and leakage current. The ratio of the DC voltage generated by a shaking table to the leakage current is called the insulation resistance test. The test of using a shaking table to check whether the insulation material is qualified is called the insulation resistance test. It can detect whether the insulation material is damp, damaged, or aged, thereby discovering equipment defects. The rated voltage of a megohmmeter is 250, 500, 1000, 2500V, etc., and the measurement range is 500, 1000, 2000M Ω, etc
The insulation resistance tester is also known as a megohmmeter, megohmmeter, or megohmmeter. The insulation resistance meter mainly consists of three parts. It is a DC high voltage generator used to generate a DC high voltage. **It's a measurement circuit. The third is display.
(1) DC high voltage generator
To measure insulation resistance, a high voltage must be applied at the measuring end, which is specified in the national standard for insulation resistance meters as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V
There are generally three methods for generating high DC voltage. **Type of hand cranked generator. At present, about 80% of the megohmmeters produced in China use this method (the name of the shaking meter comes from). **The method is to increase the voltage through a mains transformer and rectify it to obtain high DC voltage. The method commonly used for a commercial megohmmeter. The third method is to use transistor oscillation or dedicated pulse width modulation circuits to generate DC high voltage, which is commonly used in battery type and mains type insulation resistance meters.
(2) Measurement circuit
In the previously mentioned megohmmeter, the measurement circuit and display part are combined into one. It is completed by a current ratio meter head, which consists of two coils with an angle of 60 ° (about). One coil is connected to both ends of the voltage, and the other coil is connected in series in the measurement circuit. The deflection angle of the meter pointer is determined by the current ratio between the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance value, the larger the coil current in the measurement circuit, and therefore the greater the deflection angle of the pointer. Another method is to use a linear ammeter for measurement and display. In the current ratio meter head used earlier, due to the non-uniform magnetic field in the coil, when the pointer is at infinity, the current coil is located exactly where the magnetic flux density is strong. Therefore, although the measured resistance is large, the current flowing through the current coil is very small, and the deflection angle of the coil will be large. When the measured resistance is small or zero, the current flowing through the current coil is large, and the coil has been deflected to a place with lower magnetic flux density, resulting in a relatively small deflection angle. This achieves non-linear correction. The resistance value displayed on the meter head of a typical megohmmeter needs to span several orders of magnitude. But when using a linear current meter directly connected in series into the measurement circuit, it is no longer possible. At high resistance values, the scales are all squeezed together and cannot be distinguished. In order to achieve nonlinear correction, nonlinear components must be added to the measurement circuit. Thus achieving a shunt effect at low resistance values. When the resistance is high, no shunt is generated, resulting in a resistance value display of several orders of magnitude.
