# Help with air compressor replacement motor



## Investigator (Jul 9, 2019)

Hope this is the right place, if not feel free to move it where it belongs.

Kind of long story......I'm building an air compressor.  I got a great deal on a new 80 gallon compressor tank.  I already have a plan, using a harbor freight air compressor pump.  The pump manual says it requires a 5 hp motor. Here's where the questions come in.

I know a "real" 5hp motor should be pulling around 24 amps.  But, most of the motors being sold as 5hp actually come out to be about 3.5 with a max load of 5hp. those motors are considerably cheaper.  I also know that many of the 5hp motors on factory compressors are rated a bit high.  So my question is does the compressor pump manufacturer take that into account when specking a 5hp motor as needed?  Would a "SPL" rated 5hp motor do just fine for my application?

My use will be limited, I doubt it will run more than an hour or two a week.  I don't spray paint cars, or have lots of air tools.  I plan to have a sand blasting cabinet soon, but it will be used rarely.  Basically, I have just a hobby shop need, although it will be running my fogless mist-cooler for both lathe and mill.

So, thoughts, suggestions?  And does anyone have a 5hp motor they need to get rid of?


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## matthewsx (Jul 9, 2019)

Are you including an unloader? Seems like that would make it easier to startup with a somewhat weaker motor.

That being said, a HF pump probably will work just fine with the motors you're looking at.

John


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## JimDawson (Jul 9, 2019)

Investigator said:


> I also know that many of the 5hp motors on factory compressors are rated a bit high. So my question is does the compressor pump manufacturer take that into account when specking a 5hp motor as needed? Would a "SPL" rated 5hp motor do just fine for my application?



Well, no.  But the compressor would not put out the rated CFM because you would have to turn it slower.  So assuming you used one of the 3 HP (''5HP'') motors, you want to use a smaller pulley on the motor and turn it at about 60% of its rated speed so you don't overload the motor.

I am powering mine with a 3 phase motor and a VFD, single phase input.  That way there are no start windings, capacitors, or centrifugal switch to burn up, and as a bonus you can soft start it to reduce start load.


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## JimDawson (Jul 10, 2019)

I found this helpful information here https://www.vivekengineers.net/details/Knowledge-Base/Compressed_Air-Rules.pdf

*Top 10 Compressed Air "rules Of Thumb"*


1. CFM delivery per Horse Power at 100 PSIG:
For "home owner" type of air compressors---2 to 2.5 CFM per HP
For Industrial Air-cooled 2-stage air compressors----3.5 CFM per HP
For Small Vane & Screw air compressors (25 HP or less) 4 CFM per HP
For large Piston, Screw & Centrifugal air compressors--4.5 to 5 CFM per HP
NOTE: THE MORE CFM PER HP - THE LESS ENERGY USED.

2. Air Receiver Size needed for these types of inlet control:
Modulating Control---------------0 to 1 gallon per CFM
On-Line/Off-Line-----------------3 to 4 gallons per CFM
Stop-Start/ Variable Speed-------4 to 6 gallons per CFM
NOTE: THE MORE AIR STORAGE - THE LESS ENERGY USED

3. Amperage per Horse Power:
115 Volts------- 1 phase-------- 10 amps per horse power
230 Volts------- 1 phase-------- 5 amps per horse power
208 Volts------- 3 phase-------- 3 amps per horse power
230 Volts------- 3 phase-------- 2.5 amps per horse power
460 Volts------- 3 phase-------- 1.25 amps per horse power
574 Volts------- 3 phase-------- 1 amp per horse power
NOTE: THE MORE ENERGY EFFICIENT THE MOTOR - THE LESS ENERGY IS USED. 


4. Air Piping Size by CFM and Pressure Drop:
Compressor Room Header---0.25 PSIG pressure drop per 100 feet of piping
Main Line-----------------0.5 PSIG pressure drop per 100 feet of piping
Loop Line-----------------1 PSIG pressure drop per 100 feet of piping
Branch Line---------------2 PSIG pressure drop per 100 feet of piping
NOTE: THE LESS AIR PRESSURE DROP - THE LESS ENERGY USED

5. Size Compressed Air Line Filters to be twice (2x) your compressor CFM flow rate.
This will lower your pressure drop 2-3 PSIG and save an additional 1% on electrical energy costs.
Elements will last twice (2x) as long and this can save on your maintenance costs.

6. Lowering Compressor Pressure settings 2 PSIG will result in a 1% energy savings.

7. Lowering Compressor Inlet Air Temperature 10 F will result in a 2% energy savings.

8. The average energy cost to operate an air compressor is approximately $0.10 per horse power per hour.

9. Compressed Air system leaks totaling the size of a 1/4" orifice, at 100 PSIG, running 24 hours a day will waste approximately $15,000 worth of electrical energy a year.

10. Using Synthetic Compressor Lubricants can save you up to 9% of the energy cost of operating your compressor as compared to using a non-synthetic lubricant.

DEFINITIONS:
CFM= Cubic Feet per Minute - Volumetric air flow rate.
Inlet Pressure = The actual pressure at the inlet flange of the compressor.
PSIG= Pounds Per Square Inch Gauge.
AMPS=Abbreviation of the plural for Ampere, a unit of electrical current

Disclaimer: The information contained in this Article is for general information purposes only. The information given is with reference from various books & articles and while we endeavor to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the Article. Any reliance you place on such information is therefore strictly at your own risk.


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## FOMOGO (Jul 10, 2019)

Do you have the specs for the pump? I generally tend to go the buy once, cry once route. With a real 5hp motor you will have better performance, and if you ever find a better pump you will have the power to run it. Noticed the Baldor motors have gone up substantially since I bought mine last year. Trying to run blast cabinet with an under powered unit will be frustrating. Here is what looks to be a good deal from a good seller. Best of luck getting it all together. Cheers, Mike









						184 5 230 18 DP  | eBay
					

Find many great new & used options and get the best deals for 184 5 230 18 DP at the best online prices at eBay! Free shipping for many products!



					www.ebay.com


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## Investigator (Jul 10, 2019)

This is one of the motors I'm considering:









						5hp Air Compressor Duty Electric Motor 22amp 3450 RPM 56 Frame 7/8" Shaft 230v for sale online | eBay
					

Find many great new & used options and get the best deals for 5hp Air Compressor Duty Electric Motor 22amp 3450 RPM 56 Frame 7/8" Shaft 230v at the best online prices at eBay! Free shipping for many products!



					www.ebay.com


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## markba633csi (Jul 10, 2019)

Here's a WEG made in Brazil :  not a bad price- shipping included








						5 HP Air Compressor Electric Motor 56HZ Frame 3440 RPM Single Phase WEG 7/8  | eBay
					

Find many great new & used options and get the best deals for 5 HP Air Compressor Electric Motor 56HZ Frame 3440 RPM Single Phase WEG 7/8 at the best online prices at eBay! Free shipping for many products!



					www.ebay.com
				




I don't know what "higher rpm" they refer to- 3440 is the higher of the two common speeds, 1725 being the other


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## Janderso (Jul 10, 2019)

You gonna put a condenser on it? Dryer?
Makes a huge difference.


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## Investigator (Jul 10, 2019)

Janderso said:


> You gonna put a condenser on it? Dryer?
> Makes a huge difference.



For the immediate future, no.  Or should I say not yet.  I'm in the planning stage of a new shop, likely 35'x35' or so.  That should happen in the next year.  When that happens, the compressor will be central located and air piped around and throughout the shop.  At that point I will definitely add stuff.  Right now, it is going to replace my little homeowner wheeled unit.


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## Investigator (Jul 10, 2019)

I suppose using a 3ph motor is an option, but not one I've really considered.  I have a 7.5hp idler motor on my RPC.  I'm not sure that is enough for a compressor.


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## pontiac428 (Jul 10, 2019)

I retrofit that same HF compressor head onto my compressor about 12 years ago.  It's a good unit.  I did the math to get the speed close to it's rated max with my pulleys, so that part is good.  The motor is a high mileage imported "5 HP" 230V/1PH, and it is definitely the weak link.  Startup is fine, but as the pressure climbs the motor overheats and trips the thermal breaker on the motor after running for  a while (30 mins continuous, depending on weather).  I would recommend buying a little more power.  It seems the 5 HP rating for the head means a real 5 HP on the shaft, because my motor simply falls short of doing what it should.


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## pacifica (Jul 10, 2019)

I built a compressor: used a 5 hp ,1725 WEG 220 volt motor single phase. It was vg quality at a decent price and has worked well for 5 years so far. If you want the best that is baldor, leeson or marathon with various levels of duty depending on how much you spend.
I sized the pulleys so rpm at pump is 900. The 2 cylinder piston Pump is _much quieter(about 70 db)_, will last longer and runs cooler. My oil-less compressor drove me crazy with the noise. Total cost was under $500 and output is about 15 cfm at 90 psi. Always try to get the most cfm since you never know when you'll need it.


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