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Air Compressor power question

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
Yes, yes, YES! The air compressor circuit itself is not where your problem is. If the 30 Amp breaker is not tripping and the existing wire (in wall and any cord) is not getting hot, then replacing them is not going to stop the lights from dimming unless the lights are on that same 30 Amp breaker. Which I highly doubt.

The problem has to be BEFORE the sub panel where the 30 Amp breaker is.

Do look at the house lights first. If they do not dim, then all is OK up to the main panel in the house.

As a first and simple thing to do you can turn the breaker in the main panel that feeds the shop panel. Then tighten up the connections from that breaker to the line to the shop panel. Also, while the power is off, tighten the connections where that line enters the shop sub panel. You could tighten the other connections in that sub panel at the same time.

If the problem still exists after that, check the line from the main panel to the sub panel for damage or splices. Repair any problems that may show.

If all that fails, call a good electrician.

PS: LED and CFL bulbs can have unknown characteristics, like slow turn off times. Good old incandescent bulbs, which have no electronics in them, are best for the checks.



You have a voltage drop.

Simple ohms law.

Current through a resistance causes a drop in voltage across the resistance.

Digital voltmeter is too slow.

Your lights however, AR working fine for this.

Do lights dim in house?

No?

Lights on sub panel dim so voltage drop between main panel and shop panel.

If yes, then drop between supply and main panel.

Often the main breaker can get oxidized where it connects to buss.

Depending on your panel, you may have access to the wire on the meter side of main breaker.

Carefully attach a test light to this connection and ground other side.

When you fid the spot that does not dim as much then inspect it.

You can use your light, one side to each side of breaker, any light indicates problem.

We have 7.5 hp single phase with proper unloader, lights indicate compressor starting but just tiny amount.

Sent from my SM-G781V using Tapatalk
 

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
As for this being a single phase issue....believe me, plenty of three phase users spend lots of time and money over resolving voltage dip issues. Start a 40,000HP motor across the line and you can be sure you'll get a lesson in voltage stability.
 

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Location
Minnesota
when it starts I get a bit of momentary light dimming from the startup power surge.

Normal. No issue. Keep running it.

Switching to a general purpose capacitor-start motor rather than the usual miniaturized definite-purpose compressor-duty motor may allow the unit to start faster and thus reduce the duration of the phenomenon but locked rotor amperage will still be comparable so you'll still get just as much severity of dimming.

Guessing you're not running your lights off of a 30A compressor circuit so the most likely culprit in your case is a combination of long, small feeder conductors for your sub-panel and long distance from a potentially small pole-type transformer.

Same phenomenon occurs when I start my 10 horse, though to a lesser extent than it sounds like you're describing. This is because I'm fed damn near right off of a 1200A meter bank and 225kVA pad-mount transformer.

Conversely, if you're in a rural/low-density area with for instance a 200A service, then it's more likely you've either got your own small 14-20kVA transformer or are sharing something to the tune of a 24-30 with two to four other houses. Much greater transformer impedance and sharper voltage drop characteristics than the situation I described above.

Bottom line:

Voltage drop when starting big motors on a residential service is normal. Leave it alone and let 'er buck.
 

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Location
Minnesota
Associated question. Shop has a 100 amp panel. 5HP single phase compressor motor on it's own breaker. When it runs the over head lighting flickers just enough to be noticeable. Did it with high output flourescents and still does it after I converted to LED's. Any thoughts? Sorry if this is a hijack...Bob

Flywheel on the pump is too small.

Same thing happens when I run my 1/2 horse Brunner 542, which has no flywheel at all and cranks at a measly 400 RPM.

On the other hand my big 10 horse Quincy 244 with it's 90 pound flywheel spinning at 900 RPM causes exactly zero flicker.

You'll also get more flicker with a vee-type pump as well as opposed to an inline twin. The loading is much more cyclical with single crankpin vee-type pumps.

Will bet anything your flicker is in perfect tune with the thumping of the pump on your unit.

Can install a constant-voltage transformer ($$$) for your lights if it really bothers you that much. Or upgrade to a much heavier pump with a massive flywheel like a QR-25. Otherwise look into non-dimmable LED bulbs, as those are more often resistant to fluctuations in line voltage.

Or just ignore it. That's what I do.
 

bbaley

Plastic
Joined
Jan 21, 2018
Location
SW Washington, in the sticks sorta
Normal. No issue. Keep running it.

Switching to a general purpose capacitor-start motor rather than the usual miniaturized definite-purpose compressor-duty motor may allow the unit to start faster and thus reduce the duration of the phenomenon but locked rotor amperage will still be comparable so you'll still get just as much severity of dimming.

Guessing you're not running your lights off of a 30A compressor circuit so the most likely culprit in your case is a combination of long, small feeder conductors for your sub-panel and long distance from a potentially small pole-type transformer.

Same phenomenon occurs when I start my 10 horse, though to a lesser extent than it sounds like you're describing. This is because I'm fed damn near right off of a 1200A meter bank and 225kVA pad-mount transformer.

Conversely, if you're in a rural/low-density area with for instance a 200A service, then it's more likely you've either got your own small 14-20kVA transformer or are sharing something to the tune of a 24-30 with two to four other houses. Much greater transformer impedance and sharper voltage drop characteristics than the situation I described above.

Bottom line:

Voltage drop when starting big motors on a residential service is normal. Leave it alone and let 'er buck.


Thanks -
I suppose my biggest concern is when running other equipment, for example the plasma cutter (the purpose for the air compressor).

Between the machines, at running max load I think it would be just under 50A (22.5 + 25-ish), so if plasma cutter was running at full amps and the compressor started, and it is only a 100A sub panel...
 

bbaley

Plastic
Joined
Jan 21, 2018
Location
SW Washington, in the sticks sorta
Yes, yes, YES! The air compressor circuit itself is not where your problem is. If the 30 Amp breaker is not tripping and the existing wire (in wall and any cord) is not getting hot, then replacing them is not going to stop the lights from dimming unless the lights are on that same 30 Amp breaker. Which I highly doubt.

The problem has to be BEFORE the sub panel where the 30 Amp breaker is.

Do look at the house lights first. If they do not dim, then all is OK up to the main panel in the house.

As a first and simple thing to do you can turn the breaker in the main panel that feeds the shop panel. Then tighten up the connections from that breaker to the line to the shop panel. Also, while the power is off, tighten the connections where that line enters the shop sub panel. You could tighten the other connections in that sub panel at the same time.

If the problem still exists after that, check the line from the main panel to the sub panel for damage or splices. Repair any problems that may show.

If all that fails, call a good electrician.

PS: LED and CFL bulbs can have unknown characteristics, like slow turn off times. Good old incandescent bulbs, which have no electronics in them, are best for the checks.

Thanks - yes, I am trying to find an electrician to come and look. they appear to be as available as hen's teeth right now near me.
Trying to get educated so I understand what possible solutions, problems, non-problems are.

Thank you all for information !
 

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Location
Minnesota
so if plasma cutter was running at full amps and the compressor started, and it is only a 100A sub panel...

Still not an issue. Modern electrical installations and overcurrent protective devices (when implemented correctly) are designed to be tolerant of harmless, momentary overloads. I.E. motor start-ups.

You could probably start your air compressor just fine with 70 amps of existing load already on that panel.

TripCurve.jpg
 

bbaley

Plastic
Joined
Jan 21, 2018
Location
SW Washington, in the sticks sorta
If the problem still exists after that, check the line from the main panel to the sub panel for damage or splices. Repair any problems that may show.

The line from mains panel (@house) to shop sub panel was just installed 4 years ago (I hired electrician), and is buried in conduit, with no splices. There has been no digging or disturbance of that area.

Will check/tighten connections at sub panel.
Will check house lights for dimming at compressor startup - however no indication of that up to this point.
 

bbaley

Plastic
Joined
Jan 21, 2018
Location
SW Washington, in the sticks sorta
Still not an issue. Modern electrical installations and overcurrent protective devices are designed to be tolerant of harmless, momentary overloads. I.E. motor start-ups.

You could probably start your air compressor just fine with 70 amps of existing load already on that panel.

ok - thank you for putting up with my ignorance and questions. much appreciated
 

CITIZEN F16

Titanium
Joined
May 2, 2021
A 100 amp panel is plenty....if you are 50 feet from the 134kv step down transformer that is 50 feet away from the 1.1GW nuclear generating plant that does nothing but feed your shop.

A 100 amp panel is not enough...if you are 2 miles from a 600v step down transformer being fed by a 134kv transformer that is 4 miles away that is 190 miles from the 25MW hyrdo plant that also feeds a small town.

In layman's terms it depends on where you are in the food chain, or how many farms are upstream from your's pulling water from the river. My first shop was in a rundown area in a tract home's garage. I had a 125 main service fed to a 100 amp sup panel in the shop. If machines were running the compressor would not dim the lights when it kicked on.

Same compressor on a property with 200 amp main and 125 to the shop, the compressor only will dim the lights. The difference the tract home shop was a stone's throw from a substation, I only had a half dozen people in front of me. Now I am second to last on the end of the line out in the woods with 50-100 people in front of me with the homes a lot further apart.
 

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
"Flywheel on the pump is too small."


When I read this I thought 'bullshit' then it caused me to re-read the OP's original statement. He is not saying the lights flicker ON START but rather while the compressor is RUNNING. That's a whole other issue.

If it is a running issue, then yes, increasing flywheel size will help. That's because there are current pulsations due to the reciprocating load posed by the compressor. More flywheel is the most direct way to dampen the pulsations which would reduce flicker. Keep in mind, more flywheel will also slow down the start of the compressor/motor so you can't over-do it. If you hang a big enough flywheel on it, there will be minimal current pulsations but the motor cage will melt before you get it started....
 

bhigdog

Stainless
Joined
Jul 20, 2005
Location
Eastern PA
"Flywheel on the pump is too small."


When I read this I thought 'bullshit' then it caused me to re-read the OP's original statement. He is not saying the lights flicker ON START but rather while the compressor is RUNNING. That's a whole other issue.

If it is a running issue, then yes, increasing flywheel size will help. That's because there are current pulsations due to the reciprocating load posed by the compressor. More flywheel is the most direct way to dampen the pulsations which would reduce flicker. Keep in mind, more flywheel will also slow down the start of the compressor/motor so you can't over-do it. If you hang a big enough flywheel on it, there will be minimal current pulsations but the motor cage will melt before you get it started....

Thanks guys. Reciprocating load makes sense. That the flicker is in sync with the compressor load should have been obvious to me. Easiest fix is just to ignore it....Bob
 

Modelman

Titanium
Joined
Sep 12, 2007
Location
Northern Illinois
Conversely, if you're in a rural/low-density area with for instance a 200A service, then it's more likely you've either got your own small 14-20kVA transformer or are sharing something to the tune of a 24-30 with two to four other houses. Much greater transformer impedance and sharper voltage drop characteristics than the situation I described above.

What we often fail to realize is the power company doesn't have to follow NEC when sizing their conductors. The NEC sizing tables we are familiar with are for conductors enclosed in conduit or raceway where limiting heat is important; wire strung on poles doesn't have that limitation, so smaller wire is used. Of course, smaller wire has more voltage drop.

When I lived in a tiny town out in the country I upgraded the house to a 200A service, mainly because I wanted an underground service across the drive, to remove the danger of a truck snagging my overhead service drop. I had to provide, and pay for, 0000 aluminum from the meter base, under the driveway and up the pole, it looked like garden hose coming out of the field guard, where the power company spiced it to the piddly little wire they had on the pole line; looked like #2. My well pump would make the lights flicker before the service upgrade; there was no improvement after. It wasn't my service or panel causing the problem, but about 500' of #2 between the transformer and my service.

Dennis
 

bikemutt

Aluminum
Joined
Mar 5, 2022
You aren't going to do much to help your issue. Is it that bad you can't live with it?

If you were really going all-out, you'd use a VFD to start the compressor motor. There would be essentially no inrush then. But the controls and cost would be 'real'.

Going with a VFD would also mean replacing the motor with a 3-phase; at 5hp you are correct, the cost would be 'real', lol.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
What we often fail to realize is the power company doesn't have to follow NEC when sizing their conductors. The NEC sizing tables we are familiar with are for conductors enclosed in conduit or raceway where limiting heat is important; wire strung on poles doesn't have that limitation, so smaller wire is used. Of course, smaller wire has more voltage drop.

When I lived in a tiny town out in the country I upgraded the house to a 200A service, mainly because I wanted an underground service across the drive, to remove the danger of a truck snagging my overhead service drop. I had to provide, and pay for, 0000 aluminum from the meter base, under the driveway and up the pole, it looked like garden hose coming out of the field guard, where the power company spiced it to the piddly little wire they had on the pole line; looked like #2. My well pump would make the lights flicker before the service upgrade; there was no improvement after. It wasn't my service or panel causing the problem, but about 500' of #2 between the transformer and my service.

Dennis

We have a 1200A 208V 3 phase service at our shop. What they don't say is that if you draw 1200A the voltage will drop to 183 volts.

In some areas they'll refuse to upgrade your service capacity, not because of the capacity of the wires, but because the resistance is so high you won't have enough voltage left.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Going with a VFD would also mean replacing the motor with a 3-phase; at 5hp you are correct, the cost would be 'real', lol.

Surpluscenter.com has surplus 3 phase motors, and great deals pop up often. I have no affiliation but I bought a brand new 182 frame 5hp 240V 3 phase TEFC motor for $180.

Also, reprogrammed VFDs exist that connect an additional phase to the start capacitor terminal. The ones I used were from gohertz and they worked great, though for the small speed range we needed we did not actually rewire the motor and ran it as normal single phase.
 

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
In addition to changing the motor, you'd have to come up with some sort of control that would, essentially, sense the lower pressure limit and then turn the VFD on. After it booted up, you'd then start the compressor motor. Once the pressure had reached its upper limit, you'd turn the motor off then turn off the VFD. Not very practical for a compressor that starts and stops several times in a short period. YOu could leave the VFD on all the time, but that's also not ideal for all the usual reasons. There might be some control on the market already that do this...but again, not gonna be low cost.
 

bikemutt

Aluminum
Joined
Mar 5, 2022
Surpluscenter.com has surplus 3 phase motors, and great deals pop up often. I have no affiliation but I bought a brand new 182 frame 5hp 240V 3 phase TEFC motor for $180.

Also, reprogrammed VFDs exist that connect an additional phase to the start capacitor terminal. The ones I used were from gohertz and they worked great, though for the small speed range we needed we did not actually rewire the motor and ran it as normal single phase.

I've heard it's possible to have a VFD run a single-phase motor, when I was looking though, that sort of thing sounded risky at best so I passed on it. If the goal is to control startup inrush current I suppose it might be something to consider. That price on a 3-phase 5hp is outstanding! I paid that for a new 1hp from Grainger.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
In addition to changing the motor, you'd have to come up with some sort of control that would, essentially, sense the lower pressure limit and then turn the VFD on. After it booted up, you'd then start the compressor motor. Once the pressure had reached its upper limit, you'd turn the motor off then turn off the VFD. Not very practical for a compressor that starts and stops several times in a short period. YOu could leave the VFD on all the time, but that's also not ideal for all the usual reasons. There might be some control on the market already that do this...but again, not gonna be low cost.

Every VFD I've seen has some configurable DC input, and the pressure switches on compressors just open and close a contact when they need to be on or off. Just leave the VFD powered (very low current consumption) and have a switch in series with the pressure switch to the enable input on the VFD.
 








 
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