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Cable length between VFD and Motor

The variable frequency drive cable doesn't do much for motor insulation, it is for motor bearing protection (I won't speak to the effectiveness of such cable). Many years of experience says that if you are going to spend the money on the VFD cable, have it installed properly. Regardless, always attempt to keep the cable distance from the VFD to motor short as possible (30m or less). If it gets longer, discuss with your motor and VFD manufacturer. There are many differences between both VFD and motor designs, let alone the system voltage used.

Let's start with long cable runs between the variable frequency drive and motor with regards to proper VFD operation; the capacitance from the cables (phase to ground) can create enough current to ground where there can be nuisance ground fault trips. You will find the limits are different between VFD sizes as well as manufactures. The smaller the variable frequency drive, the shorter the limit. For drives around 15KW and smaller the typical limit is around 100 meters where larger drives are good to about 300 meters. These limits are only around ground currents (where the VFD would not properly operate), there are other considerations such as meeting EU EMC limits, damaging motor bearing currents and motor insulation stress. For motor insulation protection, there are several considerations. When possible, specify motor insulation to be at least 1400V. It is not uncommon to add not only inductance, but a LRC filter. They are commonly referred to as du/dt filters. You will typically find two types: one being enough to handle most applications but for cable lengths over 200 meters, a sinus filter that provides nearly sinusoidal VOLTAGE output. This will do a good job with reducing motor insulation stress, bearing currents and allow much longer cable runs. Of course there are always financial considerations, the reason we find several common filter options.

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