3 replies to this topic

[boby]

How to choose incoming cable capacity for a VFD ?

[marke]

Hello boby

Select the cable such that the thermal current rating of the cable is greater than the VFD rating times 1.3 and you should be OK.

Best regards,
Mark.

[Davemell0]

Hello boby

Select the cable such that the thermal current rating of the cable is greater than the VFD rating times 1.3 and you should be OK.

Best regards,
Mark.

Mark

I normally do not have a reason to question any of your responses however, I am used to dealing with this problem somewhat differently. The VFD rating is generally based on the output, but because most vfd's have a better power factor than the motors they are driving, the input current is around 10 % less than the output current. (Note this applies to voltage source drives only) So I would normally expect to size a vfd feeder based on the input current to the vfd *1.25. I would appreciate it very much if you could indicate why you provided the reccomendation above, instead of a reccomendation based on the vfd input current.

Thanks

Dave

[marke]

Hi Dave

most vfd's have a better power factor than the motors they are driving

The displacement power factor is certainly better than the motors that are being driven, but the distortion power factor can be a separate issue.
If the VFD is fited with a DC Bus choke or AC Line reactors, the THDi could be around 35%, but if the VFD does not have either reactors, the THDi could be in the order of 85%.
The harmonic currents cause additional heating in the cables, fuses, isolators and other supply components. Where the VFD is fed by a transfomer, it must be upsized also.

Best regads,
Mark.