A variable frequency drive does not care how far the motor is — until it does. Above a few tens of meters of motor cable, the voltage pulses at the inverter output start reflecting back from the cable end, and two problems appear: voltage spikes at the motor terminals that stress insulation, and reflected-charge currents that heat the drive. The G120X answers both with two separate tools — output chokes or du/dt filters, and DC terminals — and knowing which one your pump room needs is a wiring-stage decision, not a commissioning-stage one.
What long cables actually do
With each PWM pulse, cable capacitance charges and discharges. On long runs the reflected wave can nearly double the peak voltage at the motor terminals and forces the inverter output stage to recharge the cable every pulse. The symptoms are honest and measurable: motor winding heating, higher drive losses, and in bad cases bearing currents.
Three fixes, in order of cost
First, an output reactor or du/dt filter slows the voltage rise and tames the reflection — the standard fix for cable runs beyond the catalog limit. Second, shielded motor cable with a proper 360° gland at both ends reduces the coupling path. Third, DC terminals: the 75 kW full-feature drive with DC terminals lets you add external components — a brake resistor, a common DC bus, or a dedicated du/dt choke sized to the cable — without opening the drive. The unfiltered basic-I/O variant with DC terminals serves the same trick on simpler boards.
DC terminals also enable shared bus pumping: several drives on one DC link recover braking energy from a decelerating pump into an accelerating one. In a multi-pump station that is free efficiency.
How long is "long" for a G120X?
The unrestricted cable length depends on frame size and whether an output filter is fitted — the shipped configuration assumes unscreened cable up to a modest length and screened cable beyond it. Treat the catalog value as a design constraint at ordering time: if your run exceeds it, order the DC-terminal variant and budget the choke.
Do I need du/dt for a 30 m run to a standard IE3 motor?
Usually no — within the catalog limit a screened cable and sound grounding suffice. The exception is a motor without reinforced insulation, or a run routed through a hot panel where margin matters. When in doubt, the du/dt choke is cheap insurance against the most expensive failure mode: a winding.
