Contact US

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!

*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

Does PWM Turn Off in a VFD after Reaching Desired Speed?

Does PWM Turn Off in a VFD after Reaching Desired Speed?

Does PWM Turn Off in a VFD after Reaching Desired Speed?

Hello All:

We have a customer that is using a VFD basically as a starter on a motor due to in-rush limitations not allowing them to use a soft-starter only. Once the motor gets up to speed, they no longer utilize the speed variability in the VFD. The motor runs at full load speed.

My question is if that is the case, does the PWM coming from the VFD discontinue or is it there because the voltage and current are still coming through and from the VFD to the motor?

Thanks in advance.

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

It's still there. That's how it makes the output.

"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

No PWM equals no output.

"Why not the best?"
Jimmy Carter

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

I suspect the original poster doesn't know how a VFD works. It takes the incoming AC power, regardless of exact voltage and frequency and waveform, and rectifies it to charge its internal DC bus. Then it chops up the DC (via PWM) to produce a simulation of an AC waveform of the desired frequency and voltage. There is never a direct connection between the incoming AC and the outgoing (chopped/simulated) AC. The fact that the output AC frequency might be the same as the incoming AC frequency is of no significance or concern or relevance. The output of a VFD is always chopped-up DC.

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

A VFD will consume about 4% of the input power at full speed. When it is used only for starting, is it is not usually bypassed by a contactor after starting has completed? Or are there problems involved with switching the VFD output?

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

Mucho problems with trying to switch-out a VFD. You would have to tell the drive to stop via coasting then close around it, while positively physically disconnecting its output, onto the now unsynchronized motor. Bad juju. Live with the losses.

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

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

You cannot have the VFD output and the Bypass connected at the same time, it could (would) blow the transistors in the VFD. So what Keith is referring to is that it must be an "open transition" from VFD to Across-the-Line power to the motor. What can happen though is that during that open transition, the motor is not getting power so it begins to decelerate. Then when the bypass connects, that can mean a significant current and torque spike when it does, of if the transition is too fast and the magnetic fields in the motor are still active, it's like connecting two generators out of synch. I've seen that shear off a 500HP motor shaft.

Some more sophisticated VFDs offer what's called a "synchronous bypass" capability wherein the VFD compensates for the speed drop when transferring to Across-the-Line by slightly over speeding the motor immediately before transfer. But the concept can mean spending significant extra money on a bypass system, so it's generally not used until you get into large drive systems, i.e. hundreds of HP, where the energy savings start to outweigh the added cost. I have heard of people doing t for LV motors, but I have never seen it done because of the costs involved. The few times I have used it were for MV motors and drives, where we have had multiple motors but only one at a time needs to have speed control and the cost of additional VFDs is so extreme that it makes sense economically.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Does PWM Turn Off in a VFD after Reaching Desired Speed?

You might be thinking of a soft starter which has a bypass relay once desired speed is reached.

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! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close