Know How

The voltage distribution across the individual coils during the surge test varies significantly depending on the size.

Slow Pulse Rise
For smaller machines, e.g., 4 KW, the voltage profile across the winding is linearly distributed. For large machines of 100 KW and greater, the impulse voltage at the beginning of the winding is significantly higher than after the first third. An example with a surge test between V and Y illustrates this.

Fast Pulse Rise
With a fast pulse rise, the impulse voltage at the beginning of the winding is significantly higher than after the first third. This is typical for operating the drive with a frequency converter. An example with a surge test between V and Y illustrates this.

The following image shows the voltage profile on a 4 KW machine. It was measured across a phase from U to X. The phase had 6 coils, each with 30 turns. The measurement ground point was connection X.

The following image shows the voltage profile on a 100 KW machine with different impulse rise times. It was measured across a phase from V to Y. The phase had 9 coils. The measurement ground point was connection Y. A clear correlation between the rise time of the surge test and the voltage distribution within the winding is also discernible.

At the beginning of the winding, the voltage is very high. Depending on the impulse rise time, the voltage in the subsequent turns and coils drops disproportionately fast. The graph illustrates this in principle. A precise analysis of the voltage profile shows that this can vary significantly from Motor to Motor.

It can generally be said that the beginning of a winding is more stressed in practice than the rest of the winding. With the current use of modern frequency converters, the stress increases proportionally with the flank steepness of the IGBTs in the frequency converter. And the flank steepness will continue to increase to reduce losses in the converter. However, this means an ever-increasing stress on the winding.