VFD as soft starter

nightex

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Jun 2007
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Hi

Typically soft starters are used only for short periods of time, thats why I want to use VFD as soft starter. I want to soft start 10 ventilators separately with one drive. Idea is to wind up motor with drive up to 50Hz and then release it for free spin and then connect motor to mains, after releasing one motor VFD is ready to start another motor. All process should be controlled with simple plc.

In this case I believe I need more contactors, contactor falut protection, etc

Purpose of this thought is to save bearings, reduce power peaks with minimum investment.

So I want to ask is it possible at all, what sort of problems I can experience with this system??
 
You can accomplish this with what is called synchronous transfer, where you start a unit, synchronize the VFD output with the utility power waveform, and transfer to across the line. The VFD is then available to start another unit.

When all is said and done this is usually only economical on large motors and medium voltage, particularly when the VFD is used to modulate one of the units. In your application I guess I don't see how this would be more economical than simple soft starts for each unit.
 
So this is possible :) Curious to see this kind of system in reality. This was only my contemplation of possibilities I doubt this kind of system will be used in our factory. In reality we run 18 ventilators of power 22kW each. After few years of operation all contactors begining to wear out, we replacing circuit breakers C63 with D63 dunno why they sometimes trip, must be because voltage drops during simultaneous ventilator starts.
 
I don't see why you don't use softstarters with up-to-voltage bypass contactors. You are never out of sync with the line and when you get up to full speed, the contactors go closed. No more heat and long life for the SCR's.

No variable speed but what's not to like? You aren't using variable speed!
 
Rereading your post, it seems you want to start these ten fans sequentially. Why is that? Inrush current? System shock?

You could do the same thing with a simple softstarter, doing your own transferring and sequencing with a small PLC.

Why are you trying to do this?
 
I would re-read what Tom wrote.

"... this is usually only economical on large motors and medium voltage ..."

Though it is possible, is it practical? 22kw is NOT a large motor. However, if possible you should really not be starting them all at the same time. But even then, you're only looking at 10 of them - unless you have a really, really badly undersized electrical distribution system, it should tolerate this.

Why do the C63 breakers trip? A tripping breaker indicates a problem - and just replacing it does not necessarily fix the problem. In reality, a D-curve breaker is the right breaker for a motor load, so that's the right thing to do. But I also suspect that you're borderline on the amp rating. What is the FLA on your motors??

Now, back to your original premise - can you do this? Yes, technically, you can. But, just because you can doesn't mean you should. This would become a very complex system for somebody to troubleshoot. This extra complexity would more than be offset by the extra time required to recover from the first problem. A simple softstart on each of those ventilator motors is really the way to go...
 
Yes you are right there is no variable speed in this application and VFD functionality is not necessary. I could use powerful soft starter capable to start each motor in different intervals, I just thought that soft starters was not designed for frequent switching.
 
OZEE said:
I would re-read what Tom wrote.

"... this is usually only economical on large motors and medium voltage ..."

Though it is possible, is it practical? 22kw is NOT a large motor. However, if possible you should really not be starting them all at the same time. But even then, you're only looking at 10 of them - unless you have a really, really badly undersized electrical distribution system, it should tolerate this.

Why do the C63 breakers trip? A tripping breaker indicates a problem - and just replacing it does not necessarily fix the problem. In reality, a D-curve breaker is the right breaker for a motor load, so that's the right thing to do. But I also suspect that you're borderline on the amp rating. What is the FLA on your motors??

Now, back to your original premise - can you do this? Yes, technically, you can. But, just because you can doesn't mean you should. This would become a very complex system for somebody to troubleshoot. This extra complexity would more than be offset by the extra time required to recover from the first problem. A simple softstart on each of those ventilator motors is really the way to go...
Click to expand...
I suspect voltage drop thats why they trip. I dont remember FLA now, but I suspect about ~46A.
 
Last edited:
Look at these: http://www.abb.us/product/ap/seitp3...160068c70c.aspx?productLanguage=us&country=US

The shynchronous transfer application I did was for 1200hp 4160 VAC motors, and we used the VFD to modulate flow. I think soft starters are better for you application. I wouldn't try to gang them - use one per blower and a simple PLC to alternate them and start one at a time. I imagine you are using some kind of zone control on your composting anyway, so you should be able to stagger starting without too much process impact.
 
The complexity I mentioned doesn't show up in your picture. You need to sync so you catch the transfer at the right place of the sine wave. That's not necessarily an easy task... A self-contained softstart with built-in bypass contactor is just too easy for this application. Whether you use a soft-start or a VFD, they have essentially the same limitations for starting cycles/hour. That's more a function of the cooling of the SCRs, which is basically the same either way. So I guess it comes back to the previously asked question: "How often?" For 22kW, I wouldn't worry about it unless it's more than maybe 10x/hour.
 

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