×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

VFD Increase Motor Rated Current results in Lower RPM?

VFD Increase Motor Rated Current results in Lower RPM?

VFD Increase Motor Rated Current results in Lower RPM?

(OP)
Hoping someone could help me understand this situation I'm having.

We have a MOVITRAC LT VFD connected to a SEW Eurodrive DFT71D2 induction motor.

Originally, the Motor Rated Current was incorrectly set on the VFD as 1.8A. At this time, we were getting a measured RPM of 400RPM @ 25Hz.
It was then noticed that the Motor Rated Current should be set on the VFD to 3.05A. Once we changed the setting we got a measured RPM of 276RPM @ 25Hz.

Can anyone please help me understand why increasing the Motor Rated Current results in the RPM decreasing for the same frequency setting. I thought that the Motor Rated Current is only for ensuring safe operation of the motor. I.e. Motor will trip at a higher rated current if set to 3.05A.

(Note: Measured RPM is from after the gearbox).

Thank you!

RE: VFD Increase Motor Rated Current results in Lower RPM?

The motor rated current is used for many things, heck, almost everything! In your case I believe you were snared by the excitation current. The drive calculates the amount of current needed to magnetize the rotor. By grossly understating the the motor rating the drive grossly under-excited the motor. This actually can cause a motor that's not fully loaded to speed up considerably. I believe this is what's happened.

BTW: That's called "field weakening".

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

RE: VFD Increase Motor Rated Current results in Lower RPM?

Field weakening is a DC motor effect, Keith.
BK21; What is your synchronous RPM at 25 Hz?
An induction motor will approach synchronous speed.
Is your load overhauling and driving the motor?
That may cause greater speed with less current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VFD Increase Motor Rated Current results in Lower RPM?

Yes, it is a DC motor effect, as I described it is also an AC motor effect.

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

RE: VFD Increase Motor Rated Current results in Lower RPM?

I think it's backwards.
A weak excitation field will allow an induction motor to slow down more under load, then of course, when the load is removed there will be a greater speed increase.
But.
The motor speed will not exceed the synchronous speed.
A DC motor has a speed that roughly corresponds to synchronous speed;
That is the speed where the motor is slightly over-driven so that the motor is neither motoring nor generating, similar to an induction motor at synchronous speed.
Weakening the field in an induction motor does not speed it up, nor raise the synchronous speed but allows more slip RPM at a given load.
Weakening the field in a DC motor raises the base speed.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VFD Increase Motor Rated Current results in Lower RPM?

Ah ok. Noted. Thanks.
I see VFD overspeeding say setting to speed of a 230V60Hz motor/drive to an output speed of 70Hz. Frequently called "field weakening" but by virtue of violating the motor's V/Hz rating.



In the op's case they've told the VFD the motor is smaller than it is. So the VFD is going to reduce the its output to supply lower current since it expects a lower excitation requirement. The only way to send a lower current is to send a lower voltage. It would seem to me that the motor would see that lower voltage as a lower V/Hz because the same Hz is still showing up.


Of course there are also torque-boost, compensation, and PID functions that might be involved with the current limit. And what mode the VFD is in... Perhaps FOC or vector could be a mind-twister about what mis-setting the current limit would result in.

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

RE: VFD Increase Motor Rated Current results in Lower RPM?

Thank you for the information Keith.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VFD Increase Motor Rated Current results in Lower RPM?

BK21 - If you want an answer it would help if you could post the rated rpm and frequency of the motor and the gearbox ratio. For example, the motor might be 1450rpm @ 50Hz.

You need to ratio the motor speed for the applied frequency and apply the gearbox ratio to see which output speed makes sense.


Quote:

The motor rated current is used for many things, heck, almost everything! In your case I believe you were snared by the excitation current. The drive calculates the amount of current needed to magnetize the rotor. By grossly understating the the motor rating the drive grossly under-excited the motor. This actually can cause a motor that's not fully loaded to speed up considerably. I believe this is what's happened.

No, an AC induction motor will not speed up when using a VFD and maintaining the same output frequency simply by lowering the V/Hz ratio. The only time it could happen is if the motor is being over-driven and no longer creates enough torque to hold back the load. You can sometimes run to a higher frequency and speed by not maintaining the V/Hz ratio, but that requires increasing the frequency as well, not just applying a lower V/Hz ratio to the motor.

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


Resources

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