The relationship between the shaking speed of a megohmmeter and the measured insulation resistance value
Regarding the relationship between the speed of megohmmeter shaking and the measured insulation resistance value, the even speed of megohmmeter shaking will not affect the measurement of fixed high value resistance. However, the situation of using a megohmmeter to measure insulation resistance with a DC resistance tester will be different.
The shaking speed of a megohmmeter
The speed of megohmmeter shaking measurement generally does not affect the measurement of fixed high resistance as long as the speed is uniform.
Because from the measurement mechanism of the megohmmeter, it is woven from a magnetoelectric ratio meter and a DC hand generator.
The magnitude of the generator voltage is determined by the internal rotation speed, and the faster the rotor rotates (i.e. the faster the magnetic field in the coil increases), the higher the voltage generated. As the reading of the megohmmeter reflects the ratio of generator voltage to current, when there is a change in voltage, the current passing through the current coil of the megohmmeter also changes proportionally, so the measured resistance reading remains unchanged.
However, the situation of using a megohmmeter to measure insulation resistance with a DC resistance tester is different.
Because the leakage current through the insulation medium is related to the level of applied voltage, especially for insulation with local defects, it can only be reflected when the voltage reaches a certain level. If the speed is too slow and the voltage is too low, it will cause the measurement results to be too high.
In addition, uneven speed also affects the measurement results. If the speed changes from fast to slow, the voltage charged at both ends of the insulation medium at high speed will be higher than the terminal voltage of the megger generator, resulting in current backflow, which will cause the measurement results to be higher.
It can be seen that the shaker should rotate uniformly at the rated speed, generally set at 120 revolutions per minute, with a variation of ± 20%, but no more than ± 25%.
In order to ensure that the rotational speed of the tramegger is not too high, some trameggers are equipped with a governor in the crank mechanism, which uses the inertial centrifugal force. When the shaking speed is too fast, the governor will automatically separate the generator rotor from the crank, and the rotor speed will slowly drop down. When the speed drops to a certain speed, the regulator will restore the generator rotor and the crank back to their original positions.
