Know How

The distribution of the high-voltage at the surge test is very different depending on the size. At smaller machines of for example 4kW the voltage process is arranged linearly throughout the winding. At larger machines of 100kW and more the impulse voltage at the winding beginning is considerably higher than after the first third. This is principally shown at the example of a surge test between V and Y.

The following picture shows the voltage process at a 4kW machine. It was tested at a phase from U to X. This phase had 6 inductors with 30 windings each. The ground point of the test was the connection X.

The following picture shows the voltage process at a 100kW machine with different impulse units. It was tested at a phase from V to Y. The phase had 9 inductors. The ground point after the test was the connection Y. A clear connection between the increase time of the surge voltage and the voltage distribution within the winding can be seen.

At the beginning of the winding the voltage is very high. But depending on the rise time of the impulse the voltage decreases overproportionally very fast within the following windings and inductors. The picture shows this in principle. A detailed analysis of the voltage process shows  that this can be very different from motor to motor.

Generally you can say that the beginning of a winding is more charged in practical use than the rest of the winding. At todays use of modern frequency converters the charge increases the higher the slew rate of the IGBT of the frequency converter becomes. And the slew rate will become more and more in order to reduce the losses within the converter. But this means an increasing charge of the winding.