×
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

Powering Soft-Start with a VFD

Powering Soft-Start with a VFD

Powering Soft-Start with a VFD

(OP)
I am wondering if anyone knows of any issues that could arise by powering a soft-start with a VFD.

Basically, we need to use the VFD as a phase converter to supply a 3 phase actuator that has SCRs.

Has anyone run into anything like this before?

RE: Powering Soft-Start with a VFD

The output of most VFDs is PWM: Pulse Width Modulation. PWM and SCRs will not get along. You might try using a rotary converter to go from single phase to three pahse.

RE: Powering Soft-Start with a VFD

Bad plan. Can you bypass the SCRs and wire direct?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Powering Soft-Start with a VFD

(OP)
I think we are going with djs suggestion in this case.  The SCRs are internal to the actuator and we would prefer not to bypass them.

RE: Powering Soft-Start with a VFD

It's done all the time. Most portable demo units that soft starter manufacturers send around with their salesmen are doing just that very thing.

The three tricky parts however, are;
1)You need a good sine wave filter on the output side of the VFD, because the Soft Starters have a line sensing circuit that determines the zero-cross points of the 3 phases in order to set up the gate firing sequence. Non-sinusoidal outputs can confuse them.

2) You need to have a contactor on the output of the VFD that only closes when the VFD is at full frequency, otherwise the Soft starter is getting low frequency to the gate firing control board and that can mess you up as well (but it depends on the soft starter).

3) You need to power up the soft starter control circuitry from the LINE SIDE of the AC supply, before the VFD. But on some soft starters, this may interfere with proper operation because they may have a system that detects line loss if there is control power but no line power. If that's the case, then the control circuit power needs to be interlocked through an aux of the same contactor mentioned above.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Powering Soft-Start with a VFD

jraef;  Can you turn ON the VFD and once it's ON close a switch between the VFD and the SS?  Another words not have to somehow monitor the VFD output?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Powering Soft-Start with a VFD

Yes, in fact it's usually automatic by using the output of the VFD programmed for "At Frequency" to close the contactor only after it is at 60Hz (or 50 as the case may be).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Powering Soft-Start with a VFD

I think you would cause the vfd to trip out and might just cause it to fail if you have a contactor close during the vfd running.  Best practice is to close the contactor before the VFD is in run mode.

RE: Powering Soft-Start with a VFD

Hi dude.  This would be a scalar drive that just puts out a frequency and has no knowledge nor cares what the motor might be doing.   AND it's just feeding the SS to start with.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Powering Soft-Start with a VFD

Right. Normally you would NOT want to do that, but in this particular case, the soft starter will not conduct until AFTER the VFD output reaches it, and then it will be reduced current. So in this specific instance, it's OK to close that downstream contactor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

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