Air compressor build (motor sizing?)

Ike Carlson · Jun 16, 2019

I'm finally making some progress on my shop air compressor. Here is what I have so far...

-120 gallon vertical lp tank (super heavy) I will hydro test to 300+ psi

-Airbase EMAX APP210524T 5 HP 2 stage compressor

-3hp 3 phase motor running on a vfd

I tore the compressor down and am making sure everything is good before putting it into service.

I believe the pump is rated at 800 rpm and the minimum is ~500 rpm. I'm planning on running it at ~600 rpm so I can use my 3hp motor.

Do you guys think the 3hp motor will do ok?

john.k · Jun 16, 2019

A 3hp 3ph motor will easily provide 12cfm at around 120 psi max.I dont know what the VFD will do.............with compressors a lot depends on whether they can start unloaded......if they can get up to near full revs unloaded ,there will be no problem......the switchgear overloads usually go when the motor stalls on start against a full head of pressure in the cylinder.......A 2 stage compressor uses a little less power,but is a at a disadvantage to a twin cyl single stage if the cylinders are pressurized at start.........this is why most of the cheapies have 2 or 3 cylinders.

muckalee · Jun 16, 2019

Install an unloader pressure switch so the motor can start without a load.

rons · Jun 16, 2019

Not as good as a 5 or 7.5 Hp.

My 120 gallon and pump like yours came with a 5 Hp single phase motor. I replaced it with a 5 Hp three phase motor and VFD. If I was just blowing dust out through the door then it would be fine. For other stuff it's right on the line.

rons · Jun 16, 2019

The Airbase EMAX spec: Built-in head unloaders designed for continuous run applications

Gordon Heaton · Jun 16, 2019

You're going to have pretty long run-times with that combination. Slowing a 5hp pump, running it on 3hp and connecting it to that big tank will take a long time on first-fill, and longer than you think when the switch kicks in. Even though the pressure switch restarts at, say 100psi, you've still got to fill twice the volume that you'd have with a 60gal tank. If you have an option, I'd go for a bigger motor and run the pump near max rpm.

If its a matter of using what you've got, then just do it and work with the limitations.

Lakeside53 · Jun 16, 2019

To use your 3hp motor at 3hp you need to change the pulley ratio such that it runs at 60hz, not using the VFD to slow it down. Slow your motor with a vfd and now you are 600/800 *3 or roughly 2 1/4 hp.

Why not just put a 3 or 5 horse single phase motor on?

I don't know what your compressor is rated at, but if 5hp was to give you say 175psi, then 90-120 will need a lot less. Don't assume you have to slow it down until you try it. The good thing about a vfd is it will tell you the FLA when running, so you know what HP you need.

Tony Quiring · Jun 16, 2019

Better would be 5 hp motor with VFD but can work in low but change pulley size to maths motor HP to compressor need.

RPM higher rather than lower.

2 or more pressure switches to control motor rpm.

At low pressure motor HP not needed as less work done so increasing motor RPM increases output, increase rpm until motor draws rated amps so motor is doing full work.

At some pressure the amps will rise fast as we one stage now working so switch tells VFD to slow down to keep amps correct.

Sent from my SAMSUNG-SM-G930A using Tapatalk

Gordon Heaton · Jun 16, 2019

All of this seems to be gross overthinking. Just get an adequate motor to run the pump. Less cost and headache in the long run.

DDoug · Jun 16, 2019

How much does the 3hp motor & VFD cost you ?

Ike Carlson · Jun 16, 2019

I already have a motor/vfd. I think I paid $850 for the pair.
I only have $30 into the tank and pump. :cool:

I am currently limited on amps to the shop until the new line gets plowed in. I'm running on 30-40 amps right now with the temporary line.

Ike Carlson · Jun 16, 2019

Lakeside53 said:
To use your 3hp motor at 3hp you need to change the pulley ratio such that it runs at 60hz, not using the VFD to slow it down. Slow your motor with a vfd and now you are 600/800 *3 or roughly 2 1/4 hp.

Why not just put a 3 or 5 horse single phase motor on?

I don't know what your compressor is rated at, but if 5hp was to give you say 175psi, then 90-120 will need a lot less. Don't assume you have to slow it down until you try it. The good thing about a vfd is it will tell you the FLA when running, so you know what HP you need.

Lakeside53, do you also work with chainsaws?? The name sounds familiar.

I'm using the vfd to speed it up for my vibratory tumbler. I run that at 88 hz.

Lakeside53 · Jun 16, 2019

yes, In a past life I had 25K posts on Arboristsite

Ike Carlson · Jun 16, 2019

I knew it.

DDoug · Jun 16, 2019

Ike Carlson said:
I already have a motor/vfd. I think I paid $850 for the pair.
I only have $30 into the tank and pump.

I am currently limited on amps to the shop until the new line gets plowed in. I'm running on 30-40 amps right now with the temporary line.

Well it looks like you should have asked here first then eh ?

compressors don't need VFD's, and they are hard on them.

Just slam in the contactor across the line, and away they go.

CalG · Jun 16, 2019

digger doug said:
Well it looks like you should have asked here first then eh ?

compressors don't need VFD's, and they are hard on them.

Just slam in the contactor across the line, and away they go.

Innocence is bliss!

I've been running my compressor through a VFD for the past 6-7 years, and really like the combination.

Starts under load without an issue. No unloader other than the built in that is part of the switch.
No voltage drop on the mains due to starting inrush.
I can "crank up" the speed 10% for when I'm using the blast cabinet without a qualm.

I'm not sure of which part of the motor VFD combination I'm not supposed to like.

Slamming across the line was hard to live with. Voltage SAG was miserable. With the VFD, It does not exist.

Ike Carlson · Jun 16, 2019

That's exactly why I want to run the vfd. Softstart, rpm control, and I can monitor motor performance and tune it.

garyhlucas · Jun 17, 2019

A couple of things to know about compressors. Compressor manufacturers all use the service factor rating of the motor. In other words the motor will be overloaded for a brief time right before it shuts off on reaching high pressure. Motor load is proportional to psi it is pumping. So when you use lots of air and the compressor can’t supply the volume the pressure and load on the motor drops.

For the 3 hp motor motor you may simply set the maximum pressure shutoff down lower until the motor is just slightly overloaded. That will give you maximum delivery volume from that motor. Changing the motor pulley smaller will give you a higher pressure at lower volume.

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