What is the difference between the principle of measuring resistance with a megometer and measuring resistance with a multimeter?
Megger, also called megohmmeter, is mainly used to measure the insulation resistance of electrical equipment. It is composed of alternator voltage doubler rectifier circuit, meter and other components. When the megohmmeter shakes, it generates DC voltage. When a certain voltage is applied to the insulating material, an extremely weak current will flow through the insulating material. This current consists of three parts, namely capacitive current, absorption current and leakage current. The ratio of the DC voltage and leakage current generated by the megometer is the insulation resistance. The test of using the megometer to check whether the insulating material is qualified is called the insulation resistance test. It can find whether the insulating material is damp, damaged, or aged, and thus find equipment defects. The rated voltage of the megger is 250, 500, 1000, 2500V, etc., and the measuring range is 500, 1000, 2000MΩ, etc.
Insulation resistance tester is also called megohmmeter, megger, megger. The insulation resistance meter mainly consists of three parts. The first is a DC high voltage generator, which is used to generate DC high voltage. ** is the measurement loop. The third is display.
(1) DC high voltage generator
To measure the insulation resistance, a high voltage must be applied to the measuring end. This high voltage value is specified in the national standard of the insulation resistance meter as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V...
There are generally three methods for generating DC high voltage. The first type of hand-operated generator. At present, about 80% of the megohmmeters produced in my country use this method (the origin of the name of the megger). The second is to step up the voltage through the mains transformer and rectify it to obtain DC high voltage. The method generally used by mains-type megohmmeters. The third method is to use a transistor oscillation type or a dedicated pulse width modulation circuit to generate DC high voltage. This method is generally used by battery-type and mains-type insulation resistance meters.
(2) Measuring loop
In the megger (megohmmeter) mentioned earlier, the measurement circuit and display part are combined into one. It is completed with a flow ratio meter head, which consists of two coils with an included angle of 60° (about). One of the coils is parallel to both ends of the voltage, and the other coil is in series with the measurement loop. middle. The deflection angle of the meter pointer is determined by the current ratio in the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance value, the greater the current of the coils in the measurement loop, and the greater the deflection angle of the pointer. . Another method is to use a linear ammeter for measurement and display. Since the magnetic field in the coil is non-uniform in the current ratiometer meter used earlier, when the pointer is at infinity, the current coil is exactly where the magnetic flux density is strongest. Therefore, even though the resistance being measured is large, the current flowing through the current coil Very rarely, the deflection angle of the coil will be larger at this time. When the measured resistance is small or 0, the current flowing through the current coil is large, and the coil has been deflected to a place where the magnetic flux density is small, and the deflection angle caused by this will not be very large. In this way, non-linear correction is achieved. Generally, the resistance display on the megger head needs to span several orders of magnitude. But it won’t work when a linear ammeter is directly connected in series to the measurement loop. At high resistance values, the scales are all crowded together and cannot be distinguished. In order to achieve nonlinear correction, nonlinear components must be added to the measurement loop. This achieves a shunt effect when the resistance value is small. No shunt is generated when the resistance is high, so that the resistance value display reaches several orders of magnitude.
