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With a long cable, a filter should be considered. The inverter output is not a smooth sine wave but a series of very fast-rising voltage pulses. As the cable gets longer, these pulses reflect at the motor terminals and peak voltages above the supply voltage can occur. These peaks stress especially the insulation of the first turns of the winding. The capacitance of a long cable also draws extra current from the inverter.
| Solution | What it does | When to consider it |
|---|---|---|
| dv/dt filter | Slows the rate of voltage rise and reduces the peak voltage at the terminals. | When the cable approaches or exceeds the length the inverter allows without a filter. |
| Sine filter | Converts the output into a near-sinusoidal wave; the gentlest result for winding, bearings and noise. | Very long cables, several motors, an old motor whose insulation condition is unknown. |
| Output reactor | Limits current rise and capacitive current. | Medium cable lengths; according to the inverter manufacturer's recommendation. |
The cable length allowed without a filter varies with the inverter model, the carrier frequency and whether the cable is shielded; this value should be taken from the inverter catalogue. Stating the cable length in the order allows us to assess whether extra measures are needed on the motor side.
A sine filter causes some voltage drop, so a small reduction in motor torque may be seen; this is also taken into account in the selection.