Insulation resistance meter, also known as megohmmeter, shake meter, and Meg meter. The insulation resistance meter is mainly composed of three parts; the first is a DC high voltage generator to generate DC high voltage; the second is a measurement circuit; the third is display.
A special instrument used to measure the maximum resistance value, insulation resistance, absorption ratio and polarization index. Its scale unit is megohm and it has a high-voltage power supply.
The insulation performance of electrical products is one of the important indicators to evaluate the quality of their insulation, which is reflected by the insulation resistance.
We measure the insulation resistance of the product, which refers to the insulation resistance between the live part and the exposed non-charged metal part (shell). According to different products, apply DC high voltage, such as 100V, 250V, 500V, 1000V, etc., and specify a minimum Insulation resistance value. Some standards stipulate that the insulation resistance is not less than 1MΩ for every kV voltage. In household appliance product standards, usually only the thermal insulation resistance is specified, and the insulation resistance value under normal conditions is not specified. The insulation resistance value under normal conditions is determined by the enterprise standards. If the normal insulation resistance value is low, it indicates that there may be some hidden danger or damage in the insulation structure. If the insulation resistance of the motor winding to the casing is low, it may be caused by damage to the insulation of the windings during wire insertion. When using electrical appliances, due to sudden power-on or power-off or other reasons, the circuit generates over-voltage, and breakdown occurs at the insulation damage, causing personal safety or threats.
Structure and composition
(1) DC high voltage generator
To measure the insulation resistance, a high voltage must be applied to the measuring terminal. The high voltage value is specified as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V in the national standard of the insulation resistance meter.
There are generally three methods for generating DC high voltage. The first type of hand crank generator. At present, about 80% of the megohmmeters produced in my country use this method (source of the name of the meter). The second is to boost the voltage through the mains transformer and rectify the DC high voltage. The method used by the general utility type megohmmeter. The third method is to use transistor oscillation type or special pulse width modulation circuit to generate DC high voltage, which is generally adopted by battery type and mains type insulation resistance meters.
(2) Measuring circuit
In the aforementioned shaking table (megohmmeter), the measurement circuit and the display part are combined into one. It is completed with a flow ratio meter head. This meter head is composed of two coils with an angle of 60° (about). One of the coils is connected to both ends of the voltage, and the other is connected to the measuring circuit. middle. The deflection angle of the pointer of the meter head 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 coil current in the measuring 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 meter head of the current ratio meter used above, when the pointer is at infinity, the current coil is just at the place where the magnetic flux density is the strongest, so even though the measured resistance is large, the current flows through the current coil. Rarely, the deflection angle of the coil will be larger at this time. 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 a small magnetic flux density, and the resulting deflection angle will not be too large. In this way, nonlinear correction is achieved. Generally, the resistance display of the megger head needs to span several orders of magnitude. But when the linear ammeter head is directly connected in the measurement loop, it will not work. The scales at high resistance are all squeezed together and cannot be distinguished. In order to achieve nonlinear correction, nonlinear components must be added to the measurement loop. So as to achieve a shunt effect when the resistance value is small. There is no shunt at high resistance, so that the resistance display reaches several orders of magnitude. With the development of electronic technology and computer technology, digital display meters gradually replace pointer meters.
Insulation resistance digital measurement technology has also been developed. Among them, the pressure ratio meter circuit is one of the better measurement circuits. The pressure ratio meter circuit is composed of a voltage bridge circuit and a measurement bridge circuit. The signals output by these two bridges are converted into digital value display through A/D conversion and then processed by a single-chip microcomputer.
Choose
The choice of insulation resistance meter is mainly to measure the voltage value, and the other is the range that needs to be measured and whether it can meet the needs. If the measurement is frequent, it is best to choose the alarm setting function
How to use insulation resistance meter and matters needing attention
Abstract: The insulation performance of electrical products is one of the important indicators to evaluate the quality of their insulation, which is reflected by the insulation resistance. Insulation resistance meter, also known as shake meter, is a special instrument used to measure large resistance and insulation resistance, so we must understand its use and precautions. Keywords: Insulation resistance meter How to use Note
Insulation resistance meter, also known as shake meter or megohm meter, is a special instrument used to measure large resistance and insulation resistance. It consists of a hand-cranked generator and a magneto-electric ratio meter. The hand-cranked generator provides a portable high-voltage measurement power supply with a voltage range of 500-5000V. The magneto-electric ratio meter measures two parts. A current ratio meter that measures the insulation resistance of electrical equipment by generating a reaction torque from electromagnetic force. Based on the measurement results, the insulation of electrical equipment can be easily identified. The rated voltage of commonly used insulation resistance meters is 500V, 1000V, 2500V, etc. Its scale unit is "Megaohm" (MΩ).
1 Use of insulation resistance meter
The insulation resistance meter has three connection terminals, and a terminal marked "line" or "L" (also called phase line) is connected to the conductor of the device under test;
The other terminal marked "ground" or "E" is connected to the shell or ground of the device under test; the third terminal marked "shielding" or "G" is connected to the electrode that needs to be shielded during measurement.
1.1 Insulation resistance meter selection It is necessary to correctly select an insulation resistance meter with a suitable rated voltage. The rated voltage of the insulation resistance meter is selected according to the rated voltage of the device under test. The rated voltage of the insulation resistance meter, that is, the DC voltage of its internal power supply is too high, which may damage the insulation of the device under test during testing; the rated voltage of the insulation resistance meter If it is too low, the measured result cannot reflect the insulation resistance of the electrical equipment under the working voltage. The general regulations stipulate that when measuring equipment with a rated voltage below 500V, an insulation resistance meter of 500~1000V should be used; when the rated voltage is above 500V, an insulation resistance meter of 1000~2500V should be used.
1.2 How to use the insulation resistance meter
①Check whether the position of the pointer "0" and "∞" are correct before use. The inspection method is to open the terminals “L” and “E” first, place the insulation resistance meter in a proper horizontal position, and shake the handle to the generator rated speed (usually 120r/min), the pointer should point to “∞ "Position. If it cannot reach "∞", it means that the lead insulation for the test is poor or the insulation resistance meter itself is damp. Use a dry and clean soft cloth to wipe the insulation between the "L" terminal and the "E" terminal. If necessary, place the insulation resistance meter on the insulating pad. If it does not reach the "∞" value, replace the test lead. Then short-circuit the "L" and "E" terminals, shake the generator slightly, and the pointer should point to the "0" position. If the pointer does not point to zero, it means that the test lead is not connected well or there is a problem with the insulation resistance.
②The test lead of the insulation resistance meter should be a multi-strand flexible wire with good insulation. The leads of the "L" and "E" terminals should be separated independently to avoid entanglement to improve the accuracy of the test results.
③When testing the insulation, keep the insulation resistance meter at rated speed, generally 120～150r/min. At the beginning of the test, connect the "E" terminal lead to the shell of the device under test and the ground. After turning the crank to the rated speed, connect the lead of the "L" terminal to the test pole of the device under test, and wait for the pointer to stabilize (Generally 1min), read and record the resistance value. During the whole test process, the rotation speed of the rocking handle should be kept at a constant constant speed to avoid sudden speed and slowness. At the end of the test, first disconnect the "L" terminal lead from the test pole of the device under test, and then stop the cranking handle. In doing so, the main purpose is to prevent the capacitance of the device under test from recharging the insulation resistance meter and damaging the needle.