Help me understand compressor theory

[david3921]

First I'll tell about my problem, the solution, and questions that came up.

So last week my compressor (HF 29 gallon) started to slow when cycling and popped the breaker. I recall seeing that issue here so I checked out the different posts with the same issue. Most at this site pointed to the unloader valve as the cause but mine was ok. I took off the belt and started it up and the motor seemed to be ok, also. Spinning the pump pulley didn't lead anywhere either as it turned with what seemed to be the correct resistance.

I did an on-line search for more clues and found the website https://fix-my-compressor.com/. It has a pretty good index to track down possible solutions. After checking the obvious things with no love, I settled on either the pressure switch or a capacitor(s). The site shows how to check a capacitor with a meter. I have 2 of them but neither had a setting to check capacitance. You can use the ohm setting but I guess that just measures whether it can hold a change. Anyway, the parts are cheap so I bought a pressure switch (with built in unloader) and 2 capacitors; start and run. I installed each, one at a time to see what was the cause. It was the start capacitor. The one I bought was a little bit larger than the original so I split the sides of the cover, welded it back up, and powder coated it. All is good now.

Now for the questions; at one point i was thinking that I would have to buy another pump. The motor is 110v , 2 HP.

Can I install a larger pump, say intended for a 3 HP motor?

Can I put a larger pulley on my current 2 HP for faster recovery?

Can I put a larger motor on my current setup? Like a 3 HP?

If so on any of these, what would be the benefits? Just faster recovery?

I really don't have a need for a larger compressor...yet. I use my mine to inflate stuff, blow debris out of the garage, and when I use my HVLP gun. Just curious as to how motors and pumps are matched. Thanks.

 

[The Cobbler]

.....

Can I install a larger pump, say intended for a 3 HP motor?

Can I put a larger pulley on my current 2 HP for faster recovery?

Can I put a larger motor on my current setup? Like a 3 HP?

If so on any of these, what would be the benefits? Just faster recovery?

......

No, you will overload the motor
No, you will probably overspeed the pump & overload the motor
No benefit

that setup you have is very likely optimized for the best results , leave it as is

 

[TuxThePenguin]

You can't reliably check capacitors with a meter, despite so many other people specifically suggest to use meters. The thing is that electrolytic, oil filled and other similar types of capacitors can leak electrolyte and still measure the correct capacitance when not heavily loaded. One better test than a capacitance test is an ESR tester, which significantly more often catches bad capacitors, but still not always. One step better than that is a leakage tester - check out the "Mr Carlson's Lab" tester. He's got a great Youtube channel. Also, ESR testers are available for about $15 on Ebay and work better than any capacitance tester for this. I would highly recommend one. They aren't the most accurate capacitor tester you can buy, but they're the best value for the money.

Having a capacitance tester on a multimeter is nice, though, and if that's the tester that someone has, it's certainly better than nothing. And it will still catch bad capacitors sometimes. I do think it's worth having a multimeter with a capacitance tester as well, though I wouldn't rush out and buy one if this would be the only time you'd expect to use it.

So anyway, onto your questions

-You can typically install a larger pump, but it has to be rated for a larger CFM at higher RPM. If you're installing a 3HP pump onto a 2HP motor, you want to spin it at around 1/2 to 2/3 its rated RPM. Trying to spin it at its rated RPM with a lower horsepower motor will either not work at all (motor stall) or it will put significant extra stress on the motor and may cause it to fail in short order.

-You probably don't have a lot of ability to adjust pulley sizes because you want a specific amount of torque at a specific amount of RPM. If you change the gear ratio to spin the pump faster, that also is going to take more power from the motor, and if you go too high you'll stall the motor out. You might be able to get a little bit more power than stock, but like a car, the more power you tune it for, the more reliability you will remove from the machine.

-You can use a larger motor with the same RPM as the stock one for the approximate same output, and a lower efficiency. If a pump is rated to take 2hp at a given RPM, it won't really change much if you use a stronger motor because the pump will only present a specific amount of resistance. But a 3hp will draw more power. It's not going to draw 50% more power for the same load, but it's still going to draw somewhat more power. Now, as far as spinning the pump faster, that's tough to say. It will affect reliability if it's outside of the manufacturer's specs of the pump. How much really varies depending on the specific pump in question.

Increasing the power increases the CFM which does increase recovery speed. But don't go crazy with trying to do this and wind up wasting your money by burning up your good components. Better to sell an underpowered compressor and buy a more suitable one.

 

[theoldwizard1]

The HF 21 gallon compressor is an "odd" compressor ! The motor is directly coupled to the pump so changing either the motor or the pump is not practical.

Matching the motor to the pump is done by looking at the maximum speed of the pump, the rated speed of the motor. Running the pump faster (by change the pulley diameters) will get it to pump up faster, but you will likely be sacrificing the life of the pump. Increasing the cut off pressure will over work the motor and likely trip a breaker.

Nothing wrong with oversizing a starting compressor.

BTW, even though that is advertised as a 2.5HP motor, the reality is, it is probably close to 1HP. A "true" 2.5HP motor would draw over 30A at 120V.

Bang for the buck, it is pretty good !

 

[sberry]

I agree, about a hp or a pinch over is about all that can get out of the circuit. If a guy is going to change a circuit should be willing to change the unit if he needs more air. I a, also gonna agree that these are pretty well optimized in this class and as proof when it went a fuzz wrong it tripped the breaker, not much headroom like there might be with a dedicated circuit.
There are a couple guys here with modest setups and 400$ comps that do what they need to do. They plan a little and provide a circuit and these type put out about 2x what a 120V will. Sears made those small 240V jobs too, they were 21 gallon but had a 15 run motor on them, little over 2 hp. They would run a spray gun for a while. We saw a couple 3 few blown up over the years mostly when guys tried to use them for real work and ran the snot right out of them.
I saw a guy grinding rivets on a truck frame with a little comp a while back, had a little die grinder and ran that little fugger about a week. I was tempted to walk over with a 4 1/2 and hit them as I was tired of hearing it run.

 

[brianh]

The HF 21 gallon compressor is an "odd" compressor ! The motor is directly coupled to the pump so changing either the motor or the pump is not practical.

He specifically said he took off the belt.

 

[david3921]

The HF 21 gallon compressor is an "odd" compressor ! The motor is directly coupled to the pump so changing either the motor or the pump is not practical.

Matching the motor to the pump is done by looking at the maximum speed of the pump, the rated speed of the motor. Running the pump faster (by change the pulley diameters) will get it to pump up faster, but you will likely be sacrificing the life of the pump. Increasing the cut off pressure will over work the motor and likely trip a breaker.

Nothing wrong with oversizing a starting compressor.

BTW, even though that is advertised as a 2.5HP motor, the reality is, it is probably close to 1HP. A "true" 2.5HP motor would draw over 30A at 120V.

Bang for the buck, it is pretty good !

I actually have the 29 gallon so it has the belt driven pump. My mistake. I corrected my initial post.

 

[david3921]

Thanks guys for the insights. I'm happy with what I got. Just wondering about the motor/pump relationship. I did install a couple of improvements while I had it apart. It came with a cheap main pressure guage. I replaced it with a fluid filled one. Also, the pressure switch nut that connects it to the tank had 3 plastic washers that looked pretty beat up. I replaced those with copper washers with the same hole diameter. I got those in a set that I bought from, yep, you guessed it, Harbor Freight.