# VFD or Soft Start for this?



## JRaef (Mar 23, 2009)

It depends.

Unless there is some value in varying the speed of the compressor, you don't need a VFD. Whether or not there is value in that is something the compressor supplier would need to get involved in, not all compressors are created equal and on some, varying the speed can actually damage them. As the Electrician you don't want to step into responsibility for that I don't think.

But a soft start would generally be a good idea, it's done all the time on all sorts of compressors, mainly because of the reduction in wear and tear on the mechanical components and the fact that the Across-the-Line starter will wear out relatively quickly on a compressor of it cycles on and off a lot. The reduction in voltage drop from starting current is a bonus. 

But if this is a compressed air system in a 24/7 facility where the compressor starts once a month just for PM and runs non-stop with an unloader system, then even the soft starter may be difficult to justify.


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## LBC Jesse (Apr 26, 2012)

Thanks! Maybe I should sit back and let them tell me what they need.. I reckon I always try to play the "nice guy" and help them save money or save their equipment, I appreciate your input


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## John Valdes (May 17, 2007)

I agree with JReaf regarding how often the machine will be stopped and started.
But I would not like an across line contactor on this size motor unless its medium voltage. 
The soft start can ramp start and stop, which is good for the machine and its mechanical parts. Across line starting is hard on equipment.
Soft starts have become much more reasonable in cost allowing them to be used on all sizes of motors.
In fact, some soft starts can be less money than the contactor arrangement. Using a wye delta configuration for starting, it would be crazy not to get a quote on the soft start.


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## LBC Jesse (Apr 26, 2012)

He finally replied that the compressor will run 24/7 - obviously turning off and on as air supply is required through out the new wing being built (compressor will be feeding multiple workstations requiring the air)..He didnt tell me what type of comperessor it is, so until it can be verified and approved by engineer (or qualified electrician) my quote is based on only the info provided and I retain right to withdraw from bid... 
And yep.. price of the enclosed soft start with bypass was cheaper than i thought (under $5k)


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## JRaef (Mar 23, 2009)

LBC Jesse said:


> He finally replied that the compressor will run 24/7 - obviously turning off and on as air supply is required through out the new wing being built (compressor will be feeding multiple workstations requiring the air)..He didnt tell me what type of comperessor it is, so until it can be verified and approved by engineer (or qualified electrician) my quote is based on only the info provided and I retain right to withdraw from bid...
> And yep.. price of the enclosed soft start with bypass was cheaper than i thought (under $5k)


What brand / model is it? Seems cheap, just be careful and read the specs carefully. Not all soft starters are created equal either. Some companies keep the cost down by only putting SCRs on 2 of the 3 phases, the 3rd phase is just a piece of bus bar straight through. Nothing illegal about it, but it is NOT good for the motor and it puts the entire system at risk of failure later (but that's a long story). I just point it out because even though we all know there is no free lunch, we sometimes fail to realize that reduced price lunches are reduced for a reason too.


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## LBC Jesse (Apr 26, 2012)

Opal MS6.. http://www.opalstarters.com/sites/default/files/B_OPAL%20MS6-2011.pdf
Im open for ANY suggestions.. I carry a very lucrative product liabilty policy on evrything i sell.. Id hate to be putting my money on a fat yella chicken..

also, Im quoting a supply house, so I didnt put alot on it, he still has to mark it up etc..


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## JRaef (Mar 23, 2009)

LBC Jesse said:


> Opal MS6.. http://www.opalstarters.com/sites/default/files/B_OPAL MS6-2011.pdf
> Im open for ANY suggestions.. I carry a very lucrative product liabilty policy on evrything i sell.. Id hate to be putting my money on a fat yella chicken..
> 
> also, Im quoting a supply house, so I didnt put alot on it, he still has to mark it up etc..


That's a good product, nothing wrong with it at all. Heavy duty, built right (albeit Canadian :whistling2, you should have no trouble.


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## varmit (Apr 19, 2009)

I would check the compressor specs closely. You don't see many reciprocating compressors these days. Most new compressors seem to be screw compressors that use a VFD, often supplied as part of the compressor assembly. The VFD is analog controlled to maintain the proper air pressure without the old style unloading solenoids.


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## Wirenuting (Sep 12, 2010)

LBC Jesse said:


> He finally replied that the compressor will run 24/7 - obviously turning off and on as air supply is required through out the new wing being built (compressor will be feeding multiple workstations requiring the air)..He didnt tell me what type of comperessor it is, so until it can be verified and approved by engineer (or qualified electrician) my quote is based on only the info provided and I retain right to withdraw from bid...
> And yep.. price of the enclosed soft start with bypass was cheaper than i thought (under $5k)


Even though you didn't spec the compressor and you'll end up selling him the parts he wants regardless as to what might work better.. 
Ask him if he has a large second storage tank for the extra volume of air he will be using. 
The more he can store, the fewer starts the machine will have to endure. 
Also ask about maintenance. Mention in passing that work/rest timing will be important to record for the first 6 months. Based on time/day and work production level, they can use that first history to predict system failure before it happens. 

It's always nice to walk up to a running unit and see the work/rest time written on a cylinder head in china marker.


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## LBC Jesse (Apr 26, 2012)

I want to thank you guys for the HELPFUL replies..
This is EXACTLY why I joined ET... I am no electrician, never will be. Your input greatly improves what service I can provide to my customers. Knowing what you guys out there in the field have to deal with (good and bad), helps me tremendously. If we could all learn from other's experience, take that knowledge and improve to eliminate a future problem, then ET's forums are a 100% success.


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## Wirenuting (Sep 12, 2010)

LBC Jesse said:


> I want to thank you guys for the HELPFUL replies..
> This is EXACTLY why I joined ET... I am no electrician, never will be. Your input greatly improves what service I can provide to my customers. Knowing what you guys out there in the field have to deal with (good and bad), helps me tremendously. If we could all learn from other's experience, take that knowledge and improve to eliminate a future problem, then ET's forums are a 100% success.


I joined ET because I heard there was good free coffee.. 

http://youtu.be/XNcGMEDgmPw


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## ninopena (Jan 13, 2013)

*Compressors*



LBC Jesse said:


> He finally replied that the compressor will run 24/7 - obviously turning off and on as air supply is required through out the new wing being built (compressor will be feeding multiple workstations requiring the air)..He didnt tell me what type of comperessor it is, so until it can be verified and approved by engineer (or qualified electrician) my quote is based on only the info provided and I retain right to withdraw from bid...
> And yep.. price of the enclosed soft start with bypass was cheaper than i thought (under $5k)


 I have worked on large scale projects and have installed a variety of compressor brands, Kaeser's . Sullair and Ingersoll rand. Usually the facility chooses what they prefer and we supply the feeder. 
In the majority of the installation they have been Y start to Delta Run, the compressor start unloaded then once the Delta starter engages the compressor slowly loads up. In other occasion they are equipped with VFD's as designed by the O.E.M. 
Hope this helps..


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## John Valdes (May 17, 2007)

ninopena said:


> I have worked on large scale projects and have installed a variety of compressor brands, Kaeser's . Sullair and Ingersoll rand. Usually the facility chooses what they prefer and we supply the feeder.
> In the majority of the installation they have been Y start to Delta Run, the compressor start unloaded then once the Delta starter engages the compressor slowly loads up. In other occasion they are equipped with VFD's as designed by the O.E.M.
> Hope this helps..


Three NEMA contactors this size are expensive enough to at least get a quote on electronic starting.
Be it VFD or SS.


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## gnuuser (Jan 13, 2013)

LBC Jesse said:


> A customer asked me to quote him a size 5, 270a 200hp line starter yesterday (which I obliged) when I called him back this AM for a follow up, he informed me that this plant is installing new 200hp compressor (which I assume has to be quite pricey) and I'm thinking a line starter would be hell on a compressor that size.
> In yall's expereince, would a soft start (with or without a bypass contactor) or a drive best suit a compressor that size?
> Thanks in advance for input.


 now i may be wrong on this but i think a soft start may be better depending on the initial current draw for the motor
i believe a motor that size has to have a controlled start-up and shut-down


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## peloflex (Jan 18, 2013)

A NEW compressor makes me think of a last generation compressor, meaning one of those splendid plug and play type that comes with its own control box where you have access to a several modes and adjustments (VFD included of course) 
These newer compressor like to run continuously better than start/stop mode and they really don´t care. The user can set it up either way and the control gives you a set of parameters where you can play with your load/unload pressures and restart time intervals. If this is the case you would just need to run your lines from a switch dissconnect box to the compressor´s electrical panel.


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