Can you run a 5hp compressor pump with a 2hp motor?

[John Donhowe]

What about cfm? I guess what I'm saying is that I need this guy to pump so I can run my HVLP sprayer without it getting out of breath

How many cfm do you need for your sprayer? With your 2hp motor turning the HF pump at 2/5 of the speed for 5 hp, you'll get 2/5 of 17 cfm, or 6.8 cfm @ 40 psi.
Also, remember that the compressor is filling the tank, which acts as a buffer, so you can get by with lower compressor output if you're not spraying continuously- i.e., you let the compressor catch up and refill the tank when you're not spraying.

+1 on checking valves on the old pump.

[Mike Dowell]

Ok - well I will go ahead and pull the head off the old pump to see the valves and report back. In the mean time, my sprayer needs 12cfm @45psi.

[Mike Dowell]

So I can't get the head to give it up. The valve cover popped right off but the head won't budge. <sigh>

Anyhow, spinning the crank with the valve cover off, I get no exhaust valve movement and a very, very small amount of movement from the intake valves. What opens and closes the valves? pushrods of some sort?


QUESTION: http://www.grainger.com/product/CHAM...mp-Stage-1Z944

Am I reading those specs correctly? That compressor puts out 2.9cfm? That can't be right.

[Peter Quinn]

I don't think either of those little compressors are going to come close to 12 scfm's at 40 psi in any event, if you want to run an HVLP gun on a small compressor, I'd be looking at a different gun. There are a number of LvLP guns today that demand much lower scfm's than previous models, worth a look.

[Myk Rian]

QUESTION: http://www.grainger.com/product/CHAM...mp-Stage-1Z944

Am I reading those specs correctly? That compressor puts out 2.9cfm? That can't be right.

That one doesn't matter anyway. Discontinued.

[Curt Harms]

So I can't get the head to give it up. The valve cover popped right off but the head won't budge. <sigh>

Anyhow, spinning the crank with the valve cover off, I get no exhaust valve movement and a very, very small amount of movement from the intake valves. What opens and closes the valves? pushrods of some sort?


QUESTION: http://www.grainger.com/product/CHAM...mp-Stage-1Z944

Am I reading those specs correctly? That compressor puts out 2.9cfm? That can't be right.

I don't know about your compressor but small units usually use reed valves. As I understand it (no expert here) they're sort of a flapper valve so no actuating mechanism. If one seems to be moving and one not, I'd be suspicious of the one not moving. Do you have some sort of penetrating oil? Something like liquid wrench or PB blaster? Maybe soak the bolts and wait a day, or soak 'em again and wait another day. Even if you can't fix the pump, it'll be a learning experience and it's worth the same as scrap whether assembled or in pieces.

[Mike Dowell]

*UPDATE*

So I ended up slapping a 3hp Harbor Freight compressor pump on there. I'm spinning it with my 2hp motor hooked into 220v. Works like a champ. I've got my switch set to cut off at 130psi and cut on at 80psi. It's an old pressure switch. I would like it to cut back on closer to 90psi but oh well. It works, it pumps and, it *almost* pumps as fast as my 12cfm HVLP spray drains it. I'd have to do a 30' dining table to run out of breath I think.

Thank you all for your input on this. I've managed to spend only $75 on this project because the compressor was free and, my father-in-law felt bad that it didn't work so HE bought the compressor pump! Not only that, but my landlord(retired electrician) wired up the 220v receptacle for FREE!(he claims he wants to keep me happy so I stay).

Not bad. Not bad at all. Now all I've got to do is come up with a way to keep the dang thing from dancing around on the floor when it's pumping. Maybe I'll anchor piece of wood to the concrete floor and bungee it to that.

[Art Mann]

The load on a compressor is a function of pressure. You might be able to get by with the smaller motor if you seriously derate the maximum pressure - say from 175 psi to 120 psi. This will require altering the compressor cycle switch rather than just the output regulator. Others have already mentioned derating the cfm capacity.

[Curt Harms]

Glad to hear it's working. Re the pressure settings, some pressure switches have two nuts compressing springs. One large nut and one small nut. The large nut is the cut-out pressure and the small nut is the cut-in pressure. You might be able to set something like 130/100 if you're so inclined.

[Sean Tracey]

Think of it this way. You are on a bicycle at the bottom of a hill. You pedal as hard as you can and it takes you 3 minutes to get to the top. The exact same bicycle is given to an olympic gold medalist. He pedals as hard as he can and gets to the top of the hill in 1 minute.

No matter what bicycle they give you, the gold medalist is going to beat you to the top.

The electric motor is the driver. The compressor must require less power than the motor is capable of producing. For a given compressor, the only way to reduce the power required is to slow it down by installing the appropriate pully and the slower you go the less cfm you will get.

[Sean Tracey]

If you hook up a 3hp compressor to a 2hp motor, you are likely to overheat the motor or circuit. This is not good practice and may result in a fire.

[Curt Harms]

If you hook up a 3hp compressor to a 2hp motor, you are likely to overheat the motor or circuit. This is not good practice and may result in a fire.

Not if you go with a smaller than recommended pulley on the motor (or larger on the compressor). You're 'running in a lower gear'. It'd be wise IMO to check the amp draw on the running motor, preferably when the pressure is close to cut-off and the compressor is working the hardest. That would tell the tale of the motor load. Mike has it working and it hasn't tripped the overload. If the motor isn't running nearly too hot to touch, he's most likely good.

[Sean Tracey]

I didn't see where an rpm and thereby load reducing pulley arrangement was actually chosen and installed.

The exact words were, "slapped a three horsepower compressor Harbor Freight compressor on there" and "Works like a champ." This was prefaced with a basic lack of understanding of power required, power provided, and numerous other things.

Nor did I see that there was any understanding of motor protection.

The circuit breaker is designed to protect the wiring in the walls. It is entirely possible that the circuit breaker could protect the wiring in the walls, but the motor would burn up or the cord connecting the motor to the wall because that breaker is designed to protect wiring that can handle a much higher amp load than the motor can and possibly the cord from the wall to the motor.

When you buy a complete system with compressor and motor, presumably all of this has been designed to work together properly. When you put mismatched components together yourself, then you would need to have the knowledge required to redesign the system so that it works properly.

If the motor does not have it's own thermal or amperage based breakers to protect it, overheating could occur unchecked by protection devices.

Frequent starts would make this more likely.

My post is meant to caution that just because a motor may actually carry the load and appear to be working like a champ, there may be other problems. It's winter now. Using the system lightly in cold ambient conditions may be different than heavy use in hot summer ambient temperatures.

[Mike Dowell]

For what it's worth, I've been running this combo for a week now with no problems. It's got enough breath for my HVLP guns and is overall a thousand times better than my little toy noise box I was using before.