Compressor start draw problem - Capacitor?

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Old 11-27-16, 06:29 PM
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Compressor start draw problem - Capacitor?

Hi All,

So I have a separate garage about 50 ft from my house which is wired with one 15 amp circuit. I plan on running a 100 amp service at some point, but until I do, 15 amps is all I have.

I just purchased a Harbor Freight 21 Gallon 2.5 HP compressor. I really didn't even think about it's electrical requirements before I bought it, but it says it requires 14 amps.

If I plug it directly into the outlet, and it's not too cold out there, it will dim all the lights in the garage and then kick over. After testing it, I moved it up to my loft (by myself ) , where it is no longer in reach of an outlet. I got a heavy duty 10 ft extension cord and plugged it in, and the day I installed it, it worked ok.

The next day the temp dropped about 25 degrees and it now no longer will start. The lights dim and it chugs a bit and then it trips it's own breaker.

I'm assuming it needs just a little more push to get it going and was wondering if I would be able to build a capacitor "box" to give it just a little bit of a kick at start up.

I was thinking about taking a bunch of 40uf ac running capacitors and wiring them in parallel into box with a 5-15 plug on one end and a 5-15 receptacle on the other.

I have searched for a pre-built solution to solve this problem and haven't found anything, but found a few loose "rules" saying that you should have anywhere from 60 to 100uf for each hp, so that would be about 4-7 40uf caps in parallel, but I don't know if this application would be relevant to those rules.

So, does anybody have any comments, advice, or suggestions on this endeavor before I blow the money on the parts? Whether or not anybody thinks it would work at all, a suggestion on size and number of caps to use, or any other words of wisdom? I'm thinking 40uf caps because they seem to be the best bang for the buck available. Would there be any disadvantage to putting more caps than required just to make sure it has a good margin available?

Thank you so much for any input any of you may have!
 
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Old 11-27-16, 06:38 PM
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Welcome to the forums! You're basically going to have to run more power to the unit. It will take a 20 amp circuit to handle the compressor's start requirements. Short cutting won't get it.
 
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Old 11-27-16, 06:47 PM
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I'm sure a 20 amp circuit would be ideal, but it lists the electrical requirements as 14 amps and it only has a 5-15 plug, if it required a 20 amp circuit wouldn't have a 5-20 plug on it?

What are you basing the 20amp requirement on?

And thank you!
 
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Old 11-27-16, 07:23 PM
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A motor draws three or more times the run amps when it starts.
 
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Old 11-27-16, 08:20 PM
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Capacitors in parallel will not hold any type of charge that would be useful for AC motor starting. It would be completely different for DC.

A starting capacitor is put into series with the start winding and acts as an electronic switch.
 
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Old 11-27-16, 08:57 PM
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430.21 (A) states that the conductors for a single motor shall be rated at least 125% of the motor current. 14 amps X 1.25 = 17.5 amps
Run 12/2 on a 20 amp breaker.

Also, read this from the Owner's manual:
cold Weather Operation
Premium quality 30-weight, non-detergent air compressor oil (sold separately) is recommended for use with this compressor. Start compressor
in heated area if outdoor temperatures drop
below 32° F. If this is not practical, drain out the old pump oil and use SAE 10W Non-detergent Air Compressor Oil in the pump crankcase instead whenever the compressor’s temperature will fall below 40°. Do not use multi-viscosity oil (such
as 10W-30), they leave carbon deposits on pump components and lead to accelerated failure. Heavy operation may require heavier viscosity oil.
4. If uncertain which oil to use for this compressor, please call Harbor Freight Tools customer service at 1-888-866-5797 for assistance.
5. change the compressor oil after the first hour of use to remove any debris.
 
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Old 11-28-16, 10:06 AM
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Well I'm not really using it as a "starting capacitor", I'm much more of a DC guy than AC, but it's my understanding that a starting capacitor is to provide power for a motor during startup because the motor in question doesn't have enough phases to provide full rotation at startup. (This is a really bad explanation I know but that's sort of my limited grasp).

What I'm hoping for is just a bit of a extra stored charge for a second or 2 to get it over the initial hump. Are you saying that a capacitor in AC doesn't store energy like it would in DC?
 
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Old 11-28-16, 10:10 AM
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I'll have to check the wiring on the garage to see what was run, I doubt it's 12/2 however. Still, if it requires or even if it's a good idea for it to have a 20 amp circuit, why wouldn't they have put a 5-20 plug on it? Isn't it a code violation to put a 15 amp receptacle on a 20 amp circuit?

Yes, I read that, my garage is insulated and it rarely if ever gets below 32, and if it did, I wouldn't be out there using the compressor anyway

Actually I've discovered that purging the tank allows it to easily start. I read a review on HF.com from somebody with the exact issue stating that there is an issue with something he called a blowback valve on top of the head where he took it out and lubed it up and it completely resolved his issue, I will have to look into that.
 
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Old 11-28-16, 12:14 PM
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!5A duplex outlets are allowed by the NEC to be on a 20A circuit. The true 20A outlet is only different by having the one sideways blade. The pass through current rating on the 15A outlets are the same. So it's really a waste of money to use actual 20A outlets unless you have equipment that has a 20A plug with the sideways blade. It's rare to see any 120V tools that have the 20A plug on them.
 
 

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