×
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

One VFD and four motors

One VFD and four motors

One VFD and four motors

(OP)
Hi
there is idea to connect one VFD  and three (or four) same motors in parallel (motors for axial fans)
Some advises would be appreciated
(search done)
Thanks

RE: One VFD and four motors

Jun2009
A few basics:
1) Use the current of all four motors as the minimum level of the 1 x VFD current rating. It is advisable to add some level of overload factor (typically 10%). So, each motor is (for eg) 5 amps then 5 x 4=20amps then, if you can afford it, *1.1=22amps. So your VFD should have a minimum current rating of 22amps.
2) be mindful of the motor cable length. You VFD manufacturer will advise a maximum cable length when operating motors. Using multiple motors connected to one VFD should take in the total cable length between VFD to Motor 1, then VFD to Motor 2 etc. This is important if you are working towards EMC levels as the EMC compliance of cable length will not be the same as "maximum cable length allowed".
3) It is advised to have individual motor protection (thermal overloads as an example) and these are either wired to the VFD or other means to stop the system if one motor is in overload. A multi-motor application will mean the VFD does not see individual overloads.
4) If stopping/starting motors individually then be aware that starting a motor on the output of a VFD will demand high start-up current so it is generally not advisable unless you have some sequence of switching over motors when VFD is stopped.
5) VFD functions like "flying start" (catching a spinning motor) do not tend to work on multimotors so should be disabled, but check with your VFD supplier.

I'm sure there are a few more to check but that's a few for starters.
  

RE: One VFD and four motors

(OP)
Thank you for answer
2) be mindful of the motor cable length. You VFD manufacturer will advise a maximum cable length when operating motors.
Using multiple motors connected to one VFD should take in the total cable length between VFD to Motor 1, then VFD to Motor 2 etc
What about star  topology?

3) It is advised to have individual motor protection (thermal overloads as an example)
 and these are either wired to the VFD or other means to stop the system if one motor is in overload

 Question is what if failed motor will be cut off line by contactor and  the system continues to work
or all system should be off?Is it possible put in line between VFD and motors some devices switchers contactors and so on?

RE: One VFD and four motors

This would seem to be an approach whose time has come .... and gone.  With the low cost of drives these days, not sure if this generally makes much sense anymore.  

ozmosis has some good advice on how to do it.  I would only add that his recommendation for individual motor overloads is a legal requirement in the US.   

RE: One VFD and four motors

I agree with ozmosis' recommendations except that the drive needs to sized 10% larger in ampacity for each additional motor.  In this case, if four motors total, then 30% oversize on output amps.  If three motors, then 20% oversize.

While I know of no electrical code rules regarding this, in addition to separate thermal overloads for each motor, once you get to three or more motors, I would advise time-delay fusing in each motor lead as well.  This is because the drive ampacity is getting so much higher than the individual motor FLA's that a fire could conceivably be started in one motor and the drive not fault on output overcurrent or output short circuit.

Most drive manufacturers will not permit loads (motors) being pulled off an active drive output but not switched back in.  You need a contactor that has an adequate interrupt flash rating.  You must stop the drive first to reconnect any or all motors or the drive will fault on overcurrent due to the motor inrush.

RE: One VFD and four motors

Correction:  In last paragragh I meant to say "Most drive manufacturers will NOW permit"

RE: One VFD and four motors

Like others have said, do not try to connect motors when the drive is running. Some people will try to install (or will recommend using) an over sized drive so that motors can be connected while the drive is running. I don't recommend you do this.

 

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