×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Feeder for VFD

Feeder for VFD

Feeder for VFD

(OP)
Chapter 430, Part X of NEC requires that feeders for VFD be rated at 125% of their rated input.  I would like to know if I'm interpreting this correctly.  If I have a VFD rated at 400A, the feeder for this VFD will have to be sized for 500A (400*1.25)? Ahead of the VFD, the feeder for the motor will be sized for the particular motor? Can I use the Nameplate of the motor to size the feeder instead of the tables?

RE: Feeder for VFD

You've used the word "feeder" a few too many times I think. The feeder conductors goping TO the VFD must now be sized for the VFD max. amp rating x 125%, so in your case that would indeed be 500A. The motor leads going FROM the VFD I believe can be sized per the motor FLA, because the VFD is providing thermal OL protection for those conductors. I don't have my NEC here with me, maybe someone else can comment on that last part.

RE: Feeder for VFD

The current between the VFD and the motor is sinusoidal and so  the cable does not need derating. The cable and protection circuits feeding the VFD see a curent with a poor distortion power factor and so they need to be rerated. The shape factor of the current waveform changes with load and VFD design. Low cost VFDs do not include a DC reactor or AC line reactor and so the current waveform is very peaky and the distortion power factor is much lower than those designs that include a reactor of some form. It is not possible to get 180 degree conduction into a three phase rectifier unless it is an active rectifier, so the power factor will always be less than 0.95 unless an active front end rectifier is used. (expensive)
The distorted current on the input of the VFD is the reason for the increased cable size requirements. Th output current should not be distorted so standard ratings can apply.

Best regards,

Mark Empson
http://www.lmphotonics.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources