Compressor "rule of thumb"

[Rod Upfold]

If you have a 3hp compressor...what size of motor is needed to drive the compessor?

Compressor 3hp needs a 3hp motor
Compessor 5hp needs a 5hp motor

and so on


Thank you


Rod

[Steve Leverich]

Rod, generally compressors are labeled by the horsepower of the motor, so a 3 horse compressor would automatically have a 3 horse motor - but there's a catch, caused by "market-speak" - I've seen "6 horsepower" compressors in non-pro stores that run on 120 volts - if they were REALLY 6 horsepower, it'd take about 50 amps to run them on 120 volt. My 5 horse DevAir 80 gallon has a 5 horse Baldor motor and it draws around 27 amps on 240 volts, so if it were strappable to 120 volts it'd draw twice that.

I think all this "exageration" is done to sell unsuspecting consumers stuff that's nowhere near what it's billed as - similar to the period in the 70's where stereo gear was rated in things like "music power" and "MAP", which stood for Maximum Available Power - in the shop I ran at the time, we tested one such unit that was rated at 100 watts - it started to clip (serious distortion) at around 5 watts.

Maybe if you tell us what all you hope to run with a compressor, we can come closer to recommending a realistic unit... Steve

[Rod Upfold]

Thanks Steve...I just seen compressors in stores around here and see their hp rating and wondered what hp motor was needed to run the compressor.


I was in electronics for 30 years and like most things manufactures like to stretch things for sales. I can remember when there where pushing "12 transistor" radio's and only 6 or 8 where actually used in the circuit... the rest of them were on the circuit board but weren't attached to the radio's circuit.

Thanks again

Rod

[Kyle Kraft]

As many will echo, my favorite rule of thumb is motor nameplate horsepower rating. Many compressor manufacturers who play the "alleged horsepower" game like to put the initials 'SPL' in the horsepower line on the nameplate. They are afraid you'll compare the nameplate rating with the advertised rating and call their bluff.

They assume that most weekend warriors don't know Ohm's law and how to apply it. Volts x amps = watts and there are 746 watts per horsepower.

Have some fun with this on your next trip to Sears or the BORG.

[Steve Leverich]

"there are 746 watts per horsepower."

Right, Kyle - but this is "consumed" horsepower, IOW how much energy was used - to get "developed" horsepower, you'd need to rig up a lifting system, connect the motor to it, and find out how many pounds the device could lift in a second - the original James Watt definition of a horse power was the abililty to lift 32,580 pounds in one minute, which has been rounded off to 33000 pounds - also stated as 550 pounds lifted one foot in one second.

I guarantee you will NOT get any motor that uses 746 watts of power to lift 550 pounds one foot per second, due to efficiency.

But I agree also, that calling any motor 6 horsepower that runs on 120 volts without blowing a normal breaker is simply a LIE... Steve

[harry strasil]

I would be more interested in CFM air output and whether the motor was continuous duty or not. I shake my head at some shops of all kinds that use air tools. ie; running a 10 to 50 hp motor to make air instead of using elect powered tools that only have a 1/4 to 3 hp rating. seems to be a most inefficient way to do things.

[Mike Seals]

I would be more interested in CFM air output and whether the motor was continuous duty or not. I shake my head at some shops of all kinds that use air tools. ie; running a 10 to 50 hp motor to make air instead of using elect powered tools that only have a 1/4 to 3 hp rating. seems to be a most inefficient way to do things.

Has to do with torque and the amount of torque a tool can develope. Which is also a confusing matter for the HP of a compressor. It takes torque to compress air. In my old car days we used a formul for developing a HP rating(hp) = (Torque (ft-lb) * RPM) / 5252

[Jim Davenport]

The Volts X amps formula in AC applications will only give you apparent power, or “volt amps” (VA). You have to plug in the “power factor” (PF) or efficiency of the motor. This is loses due to capacitance, reactance, and other loses in the motor. A high quality single phase AC motor will be about 70% efficient or have a PF of .7 cheaper motors will have a PF of .5, or .6
The formula is: Watts (real power) Single-Phase = Volts x Amperes x Power Factor.
You can plug in the PF of .7, .6, or .5 depending on how you feel the quality of your motor is.
To get electrical Horse power divide the watts by 746. This is actually just a ball park figure, but it’s close enough for our purposes.

About six years ago I aquired a 60 gal vertical Ingersol “ T-10” with the motor burned up for free. I bought a Harbor Freight “5HP” compressor duty motor with one of their 15% off coupons. To my surprise it’s still running good today.
Lets look at that “5 HP” motor. Using a conservative PF of .7. The amps are 15@ 230 volts. 15 X 230 X .7= 2415 watts. Divide by 746, gives you about 3.23 HP.

[Larry Marley]

I worked for a few years in the woodworking machinery industry, and my experience was that what Jim Davenport is saying is true.

My company wanted to upgrade our "3hp" tablesaw to 5hp to compete with other well known manufacture's 5hp offerings. We ordered a 5hp motor from a major US manufacturer and when it arrived it was physically twice the size as the motor on the Well known brand, "5hp" tablesaw we had in our shop.
When we contacted the motor manufacturer to say there must be a mistake they replied, "most of your customers sell this as a 7 hp motor. Do you want us to change the plate to read 7hp?"

As it turns out, if you hang a 3hp motor on a Dyno and load it up enough, it will generate 5hp. If you do this for more than a few minutes, you will also burn up the motor. So, the argument is that in a low duty cycle like "compressor duty" or "saw duty" where the motor cycles on and off or from load to no load you can get away with overworking it.

Our experience was, that it is a common practice to add up to 30% to the mathematical " watts to Horsepower" or " continuous duty" rating on a motor.
While in the truest since, it may not be a lye, it is definitely optimistic.

Looking at how many Watts are consumed by the motor is going to be a better measure of horse power than the stated horse power on the motor label.
This is the world according to Larry and where it varies from reality is for you to decide. Member FDIC. Discontinue if rash occurs.

[Rod Upfold]

I have seen this before...in Jim's photo of a motor plate, they are saying a compressor motor. What is the difference between a standard electric motor and the so called compressor motor - torque?

Thanks


Rod

[Rob Will]

We got into this in a thread called "Electric Motor Performance" not long ago.

IMHO, air compressors are one of the few shop tools that are still made in the USA. I would buy a USA made compressor with a USA made brand-name motor.:D

Curtis
Quincy
Ingersol Rand
etc.

After looking at a few "real" compressors vs the wannabes, you can judge the quality just by looking at the motor (well almost).

Rob

[Jim Davenport]

I have seen this before...in Jim's photo of a motor plate, they are saying a compressor motor. What is the difference between a standard electric motor and the so called compressor motor - torque?

Thanks


Rod
compressor duty motors have a higher "locked rotor value" which means more starting torque available. My "HF" motor has a class "L" value which equals to 9.0-9.9 KVA per horse power. one KVA 1,00 volt amps my motor is rated 15 x 230= 3450 VA, or 3.45 KVA. the locked rotor value for my motor would be 3.23(HP) X 9.0=29.07 KVA.
The motor on my Jet contractors saw has a E code
which equals a 4.5-5 KVA per HP rating.

[Jim Davenport]

We got into this in a thread called "Electric Motor Performance" not long ago.

IMHO, air compressors are one of the few shop tools that are still made in the USA. I would buy a USA made compressor with a USA made brand-name motor.:D

Curtis
Quincy
Ingersol Rand
etc.

After looking at a few "real" compressors vs the wannabes, you can judge the quality just by looking at the motor (well almost).

Rob
Ingersol has gone off shore on some of their products. A lot of their quality has decreased most noteably the current "T-30" series compressors.

[Bill White]

ever tried figuring out lady's dress sizes? Let's see, my big compressor is a 16 missus, my pancake is a 3 petite, and my 1 gal. model is a 1 jr. petite. Then there are the "plus" sizes....
Why do I give myself headaches?
Bill