Variable-speed drives are used in industrial systems for a wide range of tasks - for example in machine tools, pumps, compressors, conveyor systems, but also in non-fixed devices. As a rule, these drives are supplied from three-phase TN networks.
A complete drive system consists of an EMC mains filter, the frequency inverter and the motor. An equally important component, which is often neglected when considering the system, is the shielded cable between the inverter and motor. This can often be 200 meters or longer. For safety reasons, the drive systems are connected to the mains via residual current circuit breakers.

RCDs (residual current devices) do not differentiate between an actual residual current and a leakage current. If the sum of all system-related leakage currents is too high, the RCD will trip.

© TDK

A major problem with variable-speed drives is an operational leakage current, which is generated in particular by the inverter. Its magnitude depends on the interference suppression capacitors and the parasitic capacitances to earth, the commutation of the B6 rectifier circuit and the switching processes of the power semiconductors. In many applications, the sum of the leakage currents exceeds the tripping threshold of the residual current circuit breaker.

Conventional residual current circuit-breakers for variable-speed drives, for example, have a tripping threshold of 30 mA for currents in the frequency range below 100 Hz, which increases significantly in the range above 100 Hz. Figure 2 shows the tripping characteristic of a typical residual current circuit-breaker, with a limit of 300 mA for frequencies above 1 kHz.

Figure 2: Leakage current in the frequency range (red) compared to the RCD tripping threshold (blue).

© TDK

Variable leakage currents generated by the switching processes in the inverter can cause the 300 mA threshold to be exceeded. In the example shown, this is the case at 2.7 kHz. Stationary leakage currents, on the other hand, which are generated by the commutation of the B6 rectifier circuit, already occur at a significantly lower frequency of 100 Hz to 1 kHz, where the trigger threshold already assumes significantly lower values. Specifically, the 150 Hz component is around 90 mA, which always triggers the residual current circuit breaker. Finally, there are also transient leakage currents, such as those that occur when the mains voltage is switched on and off.

The disadvantages of previous approaches

To date, there has been no comprehensive solution to counter the various causes of leakage currents. Attempts are often made to vary the total capacitance to earth in the system. For example, the 150 Hz component of the leakage current can be reduced by switching off the filter capacitor in the inverter. However, EMC compatibility is then no longer guaranteed in many cases. If, on the other hand, the capacitance of the Y capacitors in the EMC filter is reduced, the 50 Hz leakage current decreases, but the clock-frequency leakage current component also increases.