Insulation resistance, absorption ratio and polarization index test:
Generally, manual voltmeters, electric voltmeters, and digital voltmeters are selected. The basic principle of the megohmmeter, whether it is an analog type or a digital type, is based on the same principle, that is, the currents flowing through the two coils generate different directions of rotational torque in the same magnetic field. The indication of the pointer or the digital display is related to the following formula: a = f (I1/I2), where the corresponding voltage coil, the circuit resistance is fixed, that is, during the measurement, the rotational speed should be constant to make the output voltage stable.
Its terminal connectors include L high-voltage terminal, E ground terminal, and G shielding terminal. The shielding terminal G is directly connected to the negative pole, and the surface leakage current flows back through it directly without passing through the measuring mechanism. Therefore, it is called “shielding”.
As described earlier, when a direct current voltage is applied to the insulating medium, the currents passing through are Ic, Ia, and Ig. Ic is the capacitive current, which completes charging quickly, while Ia is the absorption current, which takes a longer time for polarization. Therefore, the absorption ratio is used to confirm the situation of large-capacity, complex multi-layer insulation, that is, K = R60 / R15. In recent years, with the continuous increase in transformer capacity, the phenomenon of incorrect judgment of the insulation resistance of its windings has increased. For example, if its absorption ratio is <1.3, but it operates well. The reason is that the attenuation time of its absorption current is too long. Therefore, the polarization index is specified to be used for measurement, that is, R10min / R1min.
For Ig, there are both volume and surface distinctions. Therefore, when the equipment surface is dirty or when the insulation is damp or cracked, the current it conducts will increase dramatically. Thus, measuring the insulation and its absorption ratio can conveniently determine the overall dampness condition of the equipment. For example, the process of measuring the insulation during the withstand voltage test of the Zhanjiang Aoli oil generator and the situation of measuring the leakage current during the lightning arrester test can clearly illustrate this issue.
When testing insulation, the selection of the equipment is generally based on the voltage level of the tested equipment. Different voltage-level digital megohmmeters are used accordingly. According to the commissioning regulations, it is as follows:
250V – Motor temperature measuring element, 500V – Generator rotor winding insulation test, 1000V – Motor bearing insulation resistance, 2500V – General motor, transformer, 5000V – Large-capacity transformer, water-cooled multimeter – Insulation test for water-cooled generators. However, there are exceptions. For instance, the AC withstand test for the rotor of a shunt-wound generator stipulates that a 2500V multimeter or a digital megohmmeter can be used instead.
Post time: Mar-27-2026