The relationship between the measuring speed of the megohmmeter and the measured insulation resistance value
Regarding the relationship between the measuring speed of the megohmmeter and the measured insulation resistance value, the uniform speed of the measuring megohmmeter will not affect the measurement of fixed high-value resistance. When using a megohmmeter to measure the insulation resistance DC resistance tester, will be different.
Generally speaking, as long as the speed of the megohmmeter is uniform, it will not affect the measurement of fixed high-value resistance.
Because judging from the measuring mechanism of the megohmmeter, it is composed of a magnetoelectric ratio meter and a DC hand-operated generator.
The voltage of the generator is determined by the internal rotation speed. The faster the rotor rotates (that is, the faster the magnetic field in the coil becomes faster), the higher the voltage generated. Since the reading of the megohmmeter reflects the ratio of the generator voltage and current, when the voltage changes, the current passing through the current coil of the megohmmeter also changes proportionally, so the measured resistance reading remains unchanged.
However, the situation when measuring insulation resistance with a megohmmeter and a DC resistance tester is different.
Because the leakage current through the insulating medium is related to the level of the applied voltage, especially the insulation with local defects, it can only be reflected when the voltage reaches a certain position. If the rotation speed is too slow and the voltage is too low, the measurement result will be high.
In addition, uneven speed also affects the measurement results. If it changes from fast to slow, the voltage charged at both ends of the insulating medium at a fast speed will be higher than the terminal voltage of the megger generator, causing a current backflow phenomenon, which will make the measurement results higher.
It can be seen that the megohmmeter should keep the rated speed rotating evenly, which is generally 120 revolutions per minute. It can have a change of ±20%, but it should not exceed ±25% at most.
In order to ensure that the rotation speed of the megohmmeter is not too high, some megohmmeters are equipped with a speed regulator in the cranking mechanism. It uses the effect of inertial centrifugal force. When the shaking speed is too fast, the speed regulator will automatically turn on the generator. When the rotor is separated from the crank handle, the rotor speed will slowly decrease. When it drops to a certain speed, the regulator will return the generator rotor and crank handle to their original positions.
