One VFD and four motors

[Jun2009]

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

 

[ozmosis]

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.

 

[Jun2009]

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?

 

[dpc]

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.

 

[DickDV]

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.

 

[DickDV]

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

 

[LionelHutz]

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.