I'm about ready to purchase a new compressor pump for a replacement for my old sears compressor. I was thinking of going with that 3 hp one from harbor freight, the rpm is 1050 max (scfm 10@90psi and 11@40psi). What type (size) of motor would I need to keep it 120? I just don't want to have it 220, and I was told that most compressors 3hp and up were best to be ran on 220. If thats the case then I'll probably go with the 2hp Mcgraw that HB sells, just the scfm are way lower, (5.3@90psi and 6.2@40psi)
The right Air Compressor motor
- Thread starter St3v3
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pretty much anything over a 1-1/2 hp motor will be 240 volts , a 1-1/2 is marginal on a 15 amp 120 volt circuit
seber
Well-known member
You won't start anything over one hp on a standard 20 amp circuit. Even 1 1/2 will blow the breaker at startup.
This isn't right. A 1.5 hp motor is fine on an unloaded 15 amp circuit; I have three or four in my shop at the moment. They pull around 15 amps at startup and less than that under load. (1.5 hp is 1119W or ~9.3 amps at 120V. The efficiency of the motor won't be 100% so actual draw is a bit higher.)
seber
Well-known member
They must be exceptional motors. Even my one horse motors will occasionally trip the breaker.
I wouldn’t say so. Most ~1.5 hp motors are in the 80% range for efficiency so they should stay well under 15 amps under load. The math backs up that my motors have no problems on a 15 amp circuit. Stick an ammeter on your motor and see what it’s actually pulling. My guess is you have other stuff on the circuit so the motor pushes you over the edge.
edit: typo
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barrysuperhawk
Well-known member
...Or you have marginal breakers that you should replace?
I've been running a 1.5 hp CH air compressor for nearly 40 years on 15 amp circuits (at more than half a dozen garages). Never, ever, have I tripped a breaker doing so. Not ever.
bsaint
Well-known member
Multiply by 4 rule usually you can only get 6. 5cfm because of inefficiency
My feeling is if a 240V 15A motor is sufficient for the rated output then half the motor power should deliver half the output and half the pump rpm. So can you size a motor pulley small enough to get the correct pump RPM? Also, is that RPM below the minimum operating speed of the pump?
My feeling is if a 240V 15A motor is sufficient for the rated output then half the motor power should deliver half the output and half the pump rpm. So can you size a motor pulley small enough to get the correct pump RPM? Also, is that RPM below the minimum operating speed of the pump?
seber
Well-known member
I did some searching. The CH 1 1/2 hp compressor is 8 1/2 amps. That is actually one hp. There has been a lot of hyper power ratings over the years since Sears started inflating numbers. You can even get a five hp compressor that runs on 115 VAC. Sure you can.
Walkers
Well-known member
Just fyi, the tanks on those small compressors expire. I think they have a 10 year date on them. Usually by the time the pump is shot everything else should be scrapped as well. At the very least you should thoroughly wash out the tank and inspect the bottom of the tank for rust pits and cracks on the inside.