Know How

Insulation Resistance

The insulation resistance test takes place with complex theoretical view at a replacing display, consisting of 4 basic components.

These are:

C = capacitor between both poles
Rs = surface resistance
Rpi+Cpi = equivalent of the polarization index
Riso = insulation resistance

Why?

The test object has an insulation resistance between two test points. This is the resistance Riso. This resistance is usually quite high and is between some 100M§Ù up to 10T§Ù.

In addition the test object also has a capacity between both test points. This is either formed between the windings to be tested or from the windings to the body. Between the windings it is usually lower than from the winding to the body. Capacities occur between uninsulated metal surfaces. The bigger the surface or the lesser the distance of the metal surfaces towards each other the bigger is the capacity. With in the stator the windings form one metal surface and the sheet packets the other. The bigger the stator (size) the bigger become the surfaces and thus the capacity.

An electric product often has bare uninsulated electrical leads. This is for example valid for a DC motor. At the surface of the leads electrically conducting dust may be deposited and/or humidity condensed. This results in a kind of surface resistance. In the ideal case this should be unlimited but sometimes it can become extremely low (< 1M§Ù).

Electrical insulating material like for example resin and so on have dipole similar molecules. Thus they show a +/- polarization. When connecting a high-voltage DC all molecules adjust to the electrical field. Only after the end of a time this polarization is finished. this should be simulated by the RC element.

Which currents arise from this?

After having connected the test voltage a total current (Iges) arises which consists of 4 single currents.

Total current = Iges = Ic + Is + Ipi + Iiso

Via the ohmic law the total insulation resistance is calculated.

This is: Rgiso = Uhv/Iges

Due to the fact that both currents Ic and Ipi become zero after a time this time has to be waited for for a detailed evaluation of the insulation resistance.

The charging of the capacitor is done fast and finished after a few seconds in most of the cases. This aim is to charge the capacitor within max. 30 seconds. After this time ic is 0:

The charging of the polarization capacity takes considerably more time. In the worst case this could take up to 10 minutes at an electric motor. Among such conditions it thus could take more than 10 minutes until the real insulation can be detected.

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