im thinking of changing the motor on my compressor, which was changed before i brought it, and is a little down on power to what it originally had, taking it a while to fill the 200litres. the motor on it is 1450rpm, and im looking at changing it to a 2800 rpm motor. am i right in thinking this will give it almost twice the cfm, or am i being really stupid and overlooking something???
i ask because most compressors power/output is stated in hp not rpm, and i dont know if the relationship is completely different.

I suspect you will run into cylinder head overheating probs. if you double the motor speed

but it was already made for a faster motor, so it should already be made for it?

Is it belt driven? You could change the pulley to increase the speed.

you stated in your 1st post that the motor fitted was a smaller power rating , not slower speed.
You could fit a motor with 10 times the original power rating but it will not alter the speed if they are both 4 pole motors (1400rpm)

well thats sort of what im trying to understand- is power and speed the same thing? i know it had a higher cfm before the new motor was installed, but is that a change of hp or rpm?
im finding it hard to explain....
it is belt driven but changing the pulley ratio wont increase it that greatly would it? i was looking at new motors from ebay and i could get one under £20 that are 2800prm, which i thought could be a cheap, fairly easy upgrade.

power & speed are not related as such in this case.
The power rating is just more available torque.
So you could in theory double the piston area & put a more powerful motor on to power it.
I do see your point about CFM.
If you double the piston speed you should in theory double the cfm.
My only thought is that with the piston moving at twice the speed it was designed to, the friction caused will cause overheating on a cylinder that was designed to dissipate heat for a specific CFM.
If that makes sense

certainly makes sense yeah thanks, you understood what i was trying to get at, which i just couldnt put into words. the compressor was designed for a higher cfm than it has at the moment, but as its old i dont know what it was designed for- maybe ill just test it and if it gets too hot, il take the motor out and use it for a dust extraction system!!!

most compressors that I have come across use 1450 (4 pole) rpm motors.

If you run one of these at much higher speed you are likely to distribute bits of broken compressor all over your workshop and kill yourself into the bargain.

If the compressor spins ok on the motor, more power won't help.

The motor power is needed to turn the compressor. If the motor stalls before the pressure switch kicks in then it needs more power. If the pressure switch kicks in ok then the only way to get more air is a bigger pump AND motor on the compressor, so basically a new compressor as the tank is worth nothing anyway!

I'd find out exactly what the original motor rating was, and stick with those figures, when buying a new one. The output is only related to the motor speed, but you will also need one of the original HP. Contact the compressor maker for spec details.
Cheers J&J

The big killer with compressor motors is starting - it takes a powerful motor to give it the first kick, then the flywheel effect means that it's easier after that. If you want to try and make it go better, get a higher HP/wattage rated motor of the same speed - but remember that you may need to change the switch gear to compensate (they tend to have overload protection that'll trip out if the current increases).

One thing that does stop compressors starting (!) is an undersized extension lead - the startup always needs a LOT of current, and if the extension lead drops too much voltage at that current then the compressor will never start, and may burn out the motor trying.

I made a dedicated extension lead that's just a plug, a rubber socket and the biggest flexible cable I could find to go between them.

David