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Voltage drop on air compressor

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W

Woodlandeer

Member
Location
Grass Valley, CA
A new air compressor was installed, and it requires no less than 208 "Y" 3 phase volt.
The main service was tested at 199 to 201 "Y" 3 phase. just that drop in voltage is causing problems with the air compressor.

Question: What is the best way to increase the "Y" 3 phase voltage up 8+ more volts.

Thanks,
Brian
 
W

Woodlandeer

Member
Location
Grass Valley, CA
Voltage drop with air compressor

Voltage drop with air compressor

A little about the building being served. It is part of a complex of commercial units occupied by product manufactures, retail/wholesale sales supplies etc.

Each unit has from one to five 120/208V 200A elec panels, the feeders are 3/0 copper.

The unit I'm referring to is less than 100 ft away from the switchgear (PG&E Main) and is supplied by a 200A 3 phase "Y" CB. Didn't think that a voltage drop on the lines could be a problem, because the voltage is within NEC specs, but I could be wrong.

I also didn't think that three buck/boost transformers were the right solution for the compressor, hoping for a different approach to boost the voltage up just 8 or so volts.

The compressor sits about 30 ft away from the elec panel and is supplied by 3 #10 THHN conductors and one #10 grounding conductor in a fuseable disconnect, fused at 50A CL.

Compressor requirements are: 208V 28.8A and the control transformer is on the 208V tap, of the several taps that are available, inside the unit.

The measurements I took were from the: CB, the load side of the disconnect and the contractor block (inside the unit). everything had the same voltage (199-201 volts line to line, 118-121 volts line to ground) Amps were from about 21A to 27A running load. I had my (true RMS) meter on high/low record for a few minutes on each test.
 
W

Woodlandeer

Member
Location
Grass Valley, CA
voltage drop with air compressor

voltage drop with air compressor

Just a note.
I will be gone for a few days playing Old-Time music at a campout.
But please still post your suggestions, and I will respond when I return.
Again thank you,
Brian
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I am guessing that this is a 240V compressor being used right at the bottom of its allowed voltage range, and not a compressor intended for 208V. I base this guess on the fact that supplies are allowed to have a tolerance, and motor nominal utilization voltage is always lower than the supply voltage (460V motor on 480V supply).

Another issue is that you are measuring voltage (and voltage drop) while the compressor is running. The current draw is quite a bit higher when the compressor is _starting_ and this means correspondingly greater voltage drop.

IMHO a buck-boost transformer is probably the best approach for this. Properly designed you only need 2 transformers, and because the transformers are only sized for the boosted part of the load, you are looking at 3-5KVA worth of transformers (or a bit larger for starting drop).

The more elegant (but probably more expensive) approach is to change or rewind the motor to match your available voltage. Remember that internally motors are all transformers in some sense, and by changing the coil arrangement or number of turns you can match any reasonable voltage.

-Jon
 
W

Woodlandeer

Member
Location
Grass Valley, CA
Voltage drop solutions

Voltage drop solutions

Back from a campout, played a lot of tunes.

Thank you winnie, very interesting info.

Thanks for all the other replies, still open for suggestions.
Go with the easy and safest way I say.

Brian
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
winnie said:
I am guessing that this is a 240V compressor being used right at the bottom of its allowed voltage range, and not a compressor intended for 208V. I base this guess on the fact that supplies are allowed to have a tolerance, and motor nominal utilization voltage is always lower than the supply voltage (460V motor on 480V supply).

Another issue is that you are measuring voltage (and voltage drop) while the compressor is running. The current draw is quite a bit higher when the compressor is _starting_ and this means correspondingly greater voltage drop.

IMHO a buck-boost transformer is probably the best approach for this. Properly designed you only need 2 transformers, and because the transformers are only sized for the boosted part of the load, you are looking at 3-5KVA worth of transformers (or a bit larger for starting drop).

The more elegant (but probably more expensive) approach is to change or rewind the motor to match your available voltage. Remember that internally motors are all transformers in some sense, and by changing the coil arrangement or number of turns you can match any reasonable voltage.

-Jon
Click to expand...

I am guessing it is a 208/230 motor, actually. If the OP does some reading, here and elsewhere, I believe he will find several more knowledgeable people that me,who will explain that in reality This is a nominal 230 volt motor. They are really meant for voltages from 208 to 240, but not lower than 208 volts. We should really be using 200 volt motors which is the next lower frame size on 208 volt systems. Putting 208 volts on a 230 motor is what causes the confusion because it will run properly at 208 but won't have any head room for voltage drop from there.
 
tom baker

tom baker

First Chief Moderator & NEC Expert
Staff member
Location
Bremerton, Washington
Occupation
Master Electrician
A motor for a 208 volt system will be labeled "200 volts" a motor for 240 volts system will be "230 volts".
Is the issue starting or running?
Compressors, depending on the design, can be hard to start, does it have an unloader?
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Woodlandeer said:
A new air compressor was installed, and it requires no less than 208 "Y" 3 phase volt.
The main service was tested at 199 to 201 "Y" 3 phase. just that drop in voltage is causing problems with the air compressor.

Question: What is the best way to increase the "Y" 3 phase voltage up 8+ more volts.

Thanks,
Brian
Click to expand...
What are the problems you mentioned?

What is the service load?

Is POCO transformer large enough for the load or is an undersized transformer contributing to voltage drop?

Boosting voltage may only be a band-aid if there is problems with inadequate supply is what I am getting after with this line of questions and can be more problems down the road, especially if more load keeps getting added.
 
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