Criteria for setting the switching frequency (C00018) of the drive controller

Question:
Which aspects are to be considered when setting the switching frequency (C0018) of the drive controller?

Answer:
When setting the switching frequency a compromise between low noise emission and optimized heat losses in the drive system (drive controller and motor) has to be found. Generally, switching frequencies of 16 kHz are only selected when noise optimised operation is necessary (e. g. in case of a stage drives in theatre technology).

The increase of the switching frequency results in a:

• reduced noise development  => human beings notice this less intensively
• enlargement of the switching losses of the semi power conductors (IGBT) => drive controller is heated more intensively => derating of the leakage currents
• increasing of the leakage currents (number of pulses)
• reduction of the motor current's harmonic wave content => reduction of the motor temperature

The reduction of the switching frequency has the opposite results.

Note for frequency inverters:
Basically, the ratio switching frequency / maximum output frequency should not fall below the value of approx. 25:1 (e. g. switching frequency = 8 kHz and the maximum output frequency = 320 Hz).
If sine filters or sine interference filters are used, the minimum required switching frequency of 4 kHz (in case of sine filters) or 6 kHz (in case of sine interference filters) has to be considered. If values fall below this minimum frequency the drive controller could be damaged caused by resonances. For servo controllers this is not relevant, as in case of current-controlled systems normally no filter measures can be applied between motor and drive.

If Residual Current protective Devices (RCDs) are used, due to an increased switching frequency the triggering of the RCD can be prevented, since - thanks to the frequency response of the RCD - higher frequencies of the leakage currents are energized above average. Consequently, it will become less sensitive to the occurring leakage currents.

Further information:

• 'Bearing current reduction, possibilities' see Document ID 200403711
• 'Earth leakage current, drive controller and RCDs type B or type B+ ' see Document ID 201005329