Current Loop in VFD

[ecenus]

I am putting a current limit loop in the VFD for my 125hp 3-phase AC Induction Motor.

My scheme is to get the current feedback from my DSP board. If the current is over a certain limit level, say 300A, I will lower my commanded frequency f so that the voltage drops. (I use v/f scalar control). However, current is rising so fast, even if I lower the voltage, the current still makes my system trip out as it rises too fast...

Anyone here has any experience in setting up such current limit loop with v/f control scheme?

Thanks a lot in advance!

 

[Skogsgurra]

Hello ecenus!

You seem to have a "persistent" problem with current in your application. Several of your posts are about that.

In this post, you say that you find out that the current is high and then you lower the frequency. That may work, but isn't the way it usually is done. And if you plan to do it this way, you need a rather fast frequency control followed by an even faster voltage control.

Is this a custom design for a very specific purpose where a standard VFD doesn't work? Could you, perhaps, show us your diagrams and some code (or pseudo-code)? It will be a lot easier to understand your difficulties if we may have a look at them.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

What is making the current rise so fast? Usually, if you are keeping the motor running at synchronous speed the current will not suddenly change. You should be able to keep ahead of it with your controller.

Oh, what's the rated current of the motor?

 

[ecenus]

Thanks, again!