×
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

Load Calculations with ASD's

Load Calculations with ASD's

Load Calculations with ASD's

(OP)
I've noticed that design consultants may ignore the ASD input current ratings when summing loads for ampacity calcs, voltage drop, etc.  Instead, they only include the motor FLA values as though the motor was actually connected across-the-line and not ASD-driven where the ASD is wasting significant KW.  Is this method correct, or should the ASD input current ratings be used instead of motor FLA when adding loads?  Surprisingly, the NEC doesn't address this issue.

RE: Load Calculations with ASD's

I know of no normal operating scenario where the drive would be wasting "significant" amounts of KW.

In fact, worst case is about 2% of motor maximum load KW.  That's hardly significant and is why motor KW is used.

RE: Load Calculations with ASD's

When a Variable Frequency Drive is controlling a motor, the performance is no longer as a traditional motor load.

The motor currents from starting to full load are now controlled by the driver and are different as compared to the current take from the power supply.

The Driver-motor is now the load. To be stricter the efficiency of the driver must be considered while calculating the line loads.

RE: Load Calculations with ASD's

If you have many VFD's consider the extra load due to the drivers efficiency.

RE: Load Calculations with ASD's

Any loss of efficiency due to a VFD would be more than off set by the gain in energy savigs by the very use of the VFD at frequent use of less than rated speed.

If VFD were to add to losses than normal starters, they wouldn't be so popular after all.. not that it is the only reason a vfd is applied.

But it is true that the drive has some losses and can be separately accounted for, but using the nameplate current is not the way because that simply lists its maximum capabilty  not the current it will draw all the time.

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