What is the difference between the principle of measuring resistance with a shaking table and a multimeter
What is the difference between the principle of measuring resistance with a shaking table and measuring resistance with a multimeter
A tramegger, also known as a megohmmeter, is mainly used to measure the insulation resistance of electrical equipment. It is composed of components such as an AC generator voltage doubling rectifier circuit and a meter head. When the shaking table is shaken, a DC voltage is generated. When a certain voltage is applied to the insulation material, an extremely weak current will flow through the insulation material, which consists of three parts: capacitive current, absorption current, and leakage current. The ratio of the DC voltage generated by the shaking table to the leakage current is the insulation resistance. The test of using the 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, and thus discover equipment defects. The rated voltage of a megohmmeter includes several types such as 250, 500, 1000, and 2500V, and the measurement range includes several types such as 500, 1000, and 2000M Ω
Insulation resistance tester, also known as megohmmeter, shake meter, or Megger meter. The insulation resistance meter mainly consists of three parts. The first is a DC high voltage generator, which is used to generate a DC high voltage. The second is the measurement circuit. The third is display.
(1) DC high-voltage generator
To measure insulation resistance, a high voltage must be applied at the measurement end, which 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** Type of hand cranked generator. At present, about 80% of the megohmmeters produced in China use this method (the name of the shaking table comes from)** The method is to boost the voltage through a mains transformer and rectify it to obtain high DC voltage. The method commonly used for commercial megohmmeters. The third method is to use transistor oscillation or specialized pulse width modulation circuits to generate DC high voltage, which is commonly used in battery and mains insulation resistance meters.
(2) Measurement circuit
The integration of the measurement circuit and display part in the megohmmeter mentioned earlier. It is completed by a current ratio meter head, which consists of two coils with an angle of about 60 °. One coil is parallel to the voltage at both ends, and the other coil is in series in the measurement circuit. The deflection angle of the pointer on the meter head is determined by the current ratio between the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance value, the greater the coil current in the measurement circuit, and 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 happens to be at the location 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 relatively large. When the measured resistance is small or 0, the current flowing through the current coil is large, and the coil has deflected to a location with lower magnetic flux density, resulting in a relatively small deflection angle. This achieves non-linear correction. The resistance value displayed on the head of a typical megohmmeter needs to span several orders of magnitude. But when using a linear ammeter directly connected in series to the measurement circuit, it is not 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 high resistance occurs, there is no shunt, resulting in resistance values reaching several orders of magnitude.
