# Add softstart to std 3ph compressor motor?



## dronai (Apr 11, 2011)

How many HP is that compressor ? Sounds like the wire may be undersized


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## bkmichael65 (Mar 25, 2013)

Just google the size of motor you want a soft start for and you should get thousands of results. I'm not sure about the prices. You may be looking at a couple hundred dollars depending on the size of the motor


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## Wirenuting (Sep 12, 2010)

Why is it starting so hard?
Is this a new problem?

Have you checked the compressors mechanical parts? ie: unloader valve?

Why size & type of compressor?


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## dronai (Apr 11, 2011)

Wirenuting said:


> Why is it starting so hard?
> Is this a new problem?
> 
> Have you checked the compressors mechanical parts? ie: unloader valve?
> ...


 Would that be unusual to softstart a compressor ? I wouldn't think they are much more than 10 HP


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## macmikeman (Jan 23, 2007)

Not unusual. I've softstarted plenty of refrigeration motors.


If there is a Carrier store in your town check there, you can pick one up for a bit over a hundred bucks from them.


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## Wirenuting (Sep 12, 2010)

dronai said:


> Would that be unusual to softstart a compressor ? I wouldn't think they are much more than 10 HP


We have up to 30 hp air compressors and never used a soft start. 
They should be starting unloaded with no air in the heads. A few here have delayed load, 2 secs or so, but those are bigger units down in our power house, steam plant. They are left over from our coal buring days. 

Maybe a peek at the Quincy compressor site might give some soft start insure options.


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## John Valdes (May 17, 2007)

You can use one like this if the motor is under 75 HP. Small frame, dip switch programming and din rail mount. Great prices too.

http://www.weg.net/us/Products-Services/Drives/Soft-Starters/Soft-Starter-SSW-05-0.75hp-75hp


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## JRaef (Mar 23, 2009)

Second only to pumps, compressors are the next biggest market for soft starters, I've doe hundreds, been involved in thousands. But still, I have to go with wirenutting's first post on this; before you spend any money on electrical solutions, eliminate all possible mechanical issues first. 

For example the unloader valve he mentioned. If you have a sticking or stuck unloader valve, adding a soft starter is going to make it WORSE, not better, then you have spent money needlessly. So again, was this something that has always been the case, or was it fine, then changed? Or is it new? What size is it? If it is 10HP and causing a VD that shuts down a CNC machine, something is seriously wrong and I doubt a soft starter will help. But if it's 350HP, then there needs to be a lot more attention paid to your solution than just throwing parts at it, i.e. your transformer may be too small, and a soft starter may not help either.

Also, before applying a soft starter to a recip compressor, check with the manufacturer of the compressor. Some use "splash" lubrication of the piston rings and if you set the current limit too low to where it takes too long to accelerate, you can lose them.

Secondary note on soft starters in general:
The one depicted above should NOT be used on an air compressor (no disrespect intended John, I just happen to have spent a lot of time with soft starters and know more than I should about their inner workings). Although they never say it in their sales literature, the manual clearly shows that this is a "2-phase" soft starter design, a more and more common type being sold now. It's not that it is for a 2 phase system, it means it only has SCRs (the "soft start" parts) on 2 of the 3 phases, the 3rd phase is a piece of bus bar. Aside from the increased risk of a single shorted SCR causing a burned up motor (it usually requires at least 2 in opposing phases), this design results in the output being severely unbalanced, which causes disproportionate heating in the motor per unit of work performed as it is ramping, when the current and heating effects on the motor are worst. It's a long complicated story on how and why, but it is very well known. Nevertheless, some soft starter mfrs use this just to make them cheaper and smaller. This type of design is fine if you are starting a pump or a fan, where the load torque requirement and duty cycle is low and acceleration is fast, you would never see any appreciable motor stress. But on any kind of heavy load or high duty cycle, such as an air compressor, this type of soft starter is going to significantly shorten the motor life. At the same time, it also makes the motor produce LESS torque per unit of current, which in this case may end up making it take longer to accelerate, which may make the VD experience worse.


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## Cow (Jan 16, 2008)

The very first thing I would do is check voltage at the service when the compressor starts. If it's dipping pretty bad here, I'd take note of the service size, compressor HP, and utility transformer size and then call the utility.

Utilities have special requirements when motors get over a certain size, one of our power companies over here states 3HP single phase and 35HP 3 phase is the max without consulting a power company engineer.


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## micromind (Aug 11, 2007)

Before we get too hot and heavy here, is it single or 3Ø?

Also, it an actual 3HP, or advertised as such (as in cheap box store compressors)?

If it's single phase, is it 120? and if so, can it be changed to 240?

What about the CNC router? Single or 3Ø, and what voltage?

What is the service? Single phase, 3Ø wye or 3Ø ∆?

Yes, lots of questions, but it'll get us on the right track.


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## JRaef (Mar 23, 2009)

micromind said:


> Before we get too hot and heavy here, is it single or 3Ø?
> 
> Also, it an actual 3HP, or advertised as such (as in cheap box store compressors)?
> 
> ...


I think you may have misread the OP...


> I have a situation where a shop has a std 3ph piston air-compressor.


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## micromind (Aug 11, 2007)

JRaef said:


> I think you may have misread the OP...


I did; I thought it stated 3HP, not 3PH.


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## JRaef (Mar 23, 2009)

micromind said:


> I did; I thought it stated 3HP, not 3PH.


Done that...


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## John Valdes (May 17, 2007)

JRaef said:


> Second only to pumps, compressors are the next biggest market for soft starters, I've doe hundreds, been involved in thousands. But still, I have to go with wirenutting's first post on this; before you spend any money on electrical solutions, eliminate all possible mechanical issues first.
> 
> For example the unloader valve he mentioned. If you have a sticking or stuck unloader valve, adding a soft starter is going to make it WORSE, not better, then you have spent money needlessly. So again, was this something that has always been the case, or was it fine, then changed? Or is it new? What size is it? If it is 10HP and causing a VD that shuts down a CNC machine, something is seriously wrong and I doubt a soft starter will help. But if it's 350HP, then there needs to be a lot more attention paid to your solution than just throwing parts at it, i.e. your transformer may be too small, and a soft starter may not help either.
> 
> ...


No offense taken.
I agree and concur on the design, but you do know this soft start has a built in bypass? I admit I know little about how it works in such a small enclosure.
This soft start (I think) will provide line voltage when the motor is up to speed. At least the literature calls it a true bypass.
I would not use this type of soft start on a motor of sufficient HP and current.
But I would use it for a smaller frame motors and where cost might be the deciding factor.
I sold many of these over the years and never had any issue with the motor or the control.
In most every case I specd this control for the applications you described, including small compressors.


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## JRaef (Mar 23, 2009)

Yes, I know they have a bypass. The issue only applies while ramping anyway. That's why it's usually fine for a pump or fan application because the ramp time is short and the load while ramping is light. The issue is when you have a more severe application, into which I would put air compressors. Not so much for the actual load, but because the duty cycle can be relatively high. That's already stressing the motor, adding to it is asking for trouble.

My biggest objection to the design is in the fact that if you get one shorted SCR, you lose the motor. When you have 6 SCRs, you must short at least two SCRs, and in different phases, in order for there to be an unrestricted current flow to the winding. My experience is that this is very very rare, but one SCR shorting is something that does happen; Murphy rules. In the 4SCR design, since one phase is ALWAYS already a conductor, a single shorted SCR in either of the other two phases completes the path, current flows to the motor and burns it up because there is nothing to stop it. If you put a Line Isolation Contactor ahead of the soft starter, and it has a Shorted SCR Detection Circuit that monitors current flow when it is supposed to be Off, then that works. But it also adds to the size and cost, defeating the reasoning behind the cheaper design in the fiirst place. You will also find that those cheaper 2-phase soft starters don't have that type of shorted SCR detection anyway. They will just have an LED to tell you why the smoke is coming out of the motor. In the case of that Weg soft starter, they don't even give you that, in fact they never mention the risks involved whatsoever, let alone what to do to protect the equipment.


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## John Valdes (May 17, 2007)

JRaef said:


> Yes, I know they have a bypass. The issue only applies while ramping anyway. That's why it's usually fine for a pump or fan application because the ramp time is short and the load while ramping is light. The issue is when you have a more severe application, into which I would put air compressors. Not so much for the actual load, but because the duty cycle can be relatively high. That's already stressing the motor, adding to it is asking for trouble.
> 
> My biggest objection to the design is in the fact that if you get one shorted SCR, you lose the motor. When you have 6 SCRs, you must short at least two SCRs, and in different phases, in order for there to be an unrestricted current flow to the winding. My experience is that this is very very rare, but one SCR shorting is something that does happen; Murphy rules. In the 4SCR design, since one phase is ALWAYS already a conductor, a single shorted SCR in either of the other two phases completes the path, current flows to the motor and burns it up because there is nothing to stop it. If you put a Line Isolation Contactor ahead of the soft starter, and it has a Shorted SCR Detection Circuit that monitors current flow when it is supposed to be Off, then that works. But it also adds to the size and cost, defeating the reasoning behind the cheaper design in the fiirst place. You will also find that those cheaper 2-phase soft starters don't have that type of shorted SCR detection anyway. They will just have an LED to tell you why the smoke is coming out of the motor. In the case of that Weg soft starter, they don't even give you that, in fact they never mention the risks involved whatsoever, let alone what to do to protect the equipment.


Like I said. I sold many of these as soon as they came out with them and sold them for the entire time I worked in this business.
Never had one complaint or return.
I never sold these for high current and multiple starting applications which we both agree are not candidates for this type of control.
I see your point and agree on the compressor and the multiple starts.
I wonder how they worked so well and never gave us any troubles if they suck so bad?
A serious question.


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## JRaef (Mar 23, 2009)

John Valdes said:


> Like I said. I sold many of these as soon as they came out with them and sold them for the entire time I worked in this business.
> Never had one complaint or return.
> I never sold these for high current and multiple starting applications which we both agree are not candidates for this type of control.
> I see your point and agree on the compressor and the multiple starts.
> ...



Well, they don't suck so bad for everything, and since >70% of all AC motors in the US are connected to pumps or fans, it stands to reason that you could sell a lot of them and not run into a problem.


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