
Vfd
#1
Posted 07 September 2009 - 06:21 AM
#2
Posted 07 September 2009 - 09:41 AM
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 Empson | administrator
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#3
Posted 26 September 2009 - 04:00 PM
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
#4
Posted 27 September 2009 - 07:16 AM
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.
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
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