Soft start on air compressor

[kwired]

My tech checked the lights, and they are tapped for 208. Customer refuses to have unloader checked, ( I think they do their own maintenance since it is a rental repair facility) so I’m just going to quote them a soft start and upgrade the wire and conduit size and be done with it. All I can do at this point.

If unloader is failing to "unload" a soft starter may not start it. It very possibly won't have necessary torque to even get it rolling until it is applying at least two thirds of full voltage. At that point current may be too high for too long and soft start may trip on overload, and you will probably still have fairly significant voltage drop that might still take out the lights.

 

[ptonsparky]

But when the complaint is an electrical problem and you determine the real cause is a mechanical issue - you need to be right or you look bad in the end. Many times had motor overloads trip and had to find the mechanical issue and explain why it trips the overload, even if it is as simple as you can't run/feed the item that fast and/or with that much material in/on it.

So many times.

 

[tom baker]

I have had some experience with softstarts on a motor - 100 hp fire pump _ that started against a full load/ head of water. It would not soft start. Other soft starters I had started against no load/head and worked fine. A soft start reduces the voltage which increases the current, if the current is already high on start up it won't work.
A VFD would work. But I would check the unloader, run some inrush tests for amps and you will know.
And it may be a issue with the HPS lamps and or ballast. There are many different types of ballasts, The less expensive ones can have issues with low voltages, when I designed street lighting systems I used a mag regulator ballast. check what kind of ballasts are installed
See page 18 and 19 of this publication https://hydroponics.net/productdocs/HID_Pocket_Guide.pdf

 

[Jraef]

I've done countless (well, maybe a couple hundred) air compressors on Soft Starters, it works fine. You will likely end up at about 300-350% Current Limit, (assuming you have one with that capability) and set the ramp time as low as it will go so that it goes into Current Limit immediately. Should get to speed in under 10 seconds that way.

But to the other points, IF it is a bad unloader, the Soft Starter will make things WORSE. So let your customer know in advance that if you put in the Soft Starter and the motor stalls, it was a bad unloaded all along and not your fault. Do compressor people lie about that? You BET they do; they want to blame anything wrong with the compressor on something else.

Get a good soft starter, not a cheap one. The cheap ones only put SCRs on 2 of the phases, the third phase is a piece of bus bar. That is TERRIBLE for the motor, but on light loads like centrifugal pumps you pass though the ramping so fast, nobody notices. On a compressor, it will wreck the motor in short order.

 

[hillbilly1]

I've done countless (well, maybe a couple hundred) air compressors on Soft Starters, it works fine. You will likely end up at about 300-350% Current Limit, (assuming you have one with that capability) and set the ramp time as low as it will go so that it goes into Current Limit immediately. Should get to speed in under 10 seconds that way.

But to the other points, IF it is a bad unloader, the Soft Starter will make things WORSE. So let your customer know in advance that if you put in the Soft Starter and the motor stalls, it was a bad unloaded all along and not your fault. Do compressor people lie about that? You BET they do; they want to blame anything wrong with the compressor on something else.

Get a good soft starter, not a cheap one. The cheap ones only put SCRs on 2 of the phases, the third phase is a piece of bus bar. That is TERRIBLE for the motor, but on light loads like centrifugal pumps you pass though the ramping so fast, nobody notices. On a compressor, it will wreck the motor in short order.

I’m quoting this one,

https://www.grainger.com/mobile/product/SCHNEIDER-ELECTRIC-SCHNEIDER-ELECTRIC-1-or-3-WP5937423/_/N-19f9Z1yyldslZ1z0e8jh/Ntt-soft+start+3+phase?EndecaKeyword=soft+start+3+phase&NLSCM=5&R=55WN34&breadcrumbCatId=1000445&fromPidp=true&nls_boost_words=3+phase&picUrl=%2F%2Fstatic.grainger.com%2Frp%2Fs%2Fis%2Fimage%2FGrainger%2F6VMD3_AS01%3F%24smthumb%24webparentimage%24&searchRedirect=soft+start+3+phase&sst=subset

I also specified the quote is based upon correctly working compressor. .

 

[hillbilly1]

This is the nameplate on the motor.

 

[hillbilly1]

Update on this problem, finally was able to get the poco engineer out, before he came out, he said their records shown two sets of 350 kcmil feeding this service. Once we met, and I shown him the single 2 1/2" conduit coming into the ct cabinet, he decided the records were wrong! LOL! He also noticed they had 25 kva transformers feeding the 800 and a 400 amp service on the other building. His supervisor wouldn't let him open the ct cabinet, so he would have to get a meter man or line crew out to see what is actually in the cabinet. He was also concerned about the length of the other service, as it was around 600' away.

 

[Jraef]

Update on this problem, finally was able to get the poco engineer out, before he came out, he said their records shown two sets of 350 kcmil feeding this service. Once we met, and I shown him the single 2 1/2" conduit coming into the ct cabinet, he decided the records were wrong! LOL! He also noticed they had 25 kva transformers feeding the 800 and a 400 amp service on the other building. His supervisor wouldn't let him open the ct cabinet, so he would have to get a meter man or line crew out to see what is actually in the cabinet. He was also concerned about the length of the other service, as it was around 600' away.

What? A utility having bad records of what is installed? Unheard of! (JK)

I had one once where we couldn't get a 400HP pump to start, even with a soft starter. I poked around and found the utility transformer (padmount), it was tiny. Called PG&E and they said it was 750kVA, I told them no way, I KNOW what a 750kVA transformer looked like. Long boring story later, turned out to be 75kVA, their records were wrong. Hey, what's a zero between friends, right?

 

[hillbilly1]

What’s really interesting, is there is two sets of 350’s, and two sets of 4/0’s going up the pole, but we don’t know where they go! The next building over is 480 with its own bank of transformers, and to the left is nothing but an open field.........