# Compressor pulley size question, converting a 2HP 1440 RPM to a 2850 RPM motor.



## JoeBloggs (Mar 27, 2020)

This old thing was given to me because that massive electric motor (works) shorts out the power in the house.
It's a 2 HP 1440 RPM, motor pulley size - 120 mm, pump pulley size - 230mm, and i'm wondering what pulley sizes (combination) I'd need to convert it to another motor that I have laying around which is a 2 HP 2850 RPM?
Any advice would be much appreciated.


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## stevon (Mar 5, 2019)

Joe,

what rpm at what HP is required for the pump say pump RPM for a 5 HP motor is 1000 rpm, 3HP is 800rpm, etc. Pump manufacturer list this information. then you can use an online tool to get the motor pulley size

Stephen


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## JoeBloggs (Mar 27, 2020)

Hi Steve
I was hoping the original confirguration would give a clue
I'm guessing i'd need a pulley twice the size on the motor to bring the revs down, or to match the RPMs (not sure)

Have you got a link to the online tool please?


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## JoeBloggs (Mar 27, 2020)

Right, i googled & discovered a calculator
Thanks for the steer!
Seems i need a 60mm pulley for the motor.


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## stevon (Mar 5, 2019)

Joe,

The pump rpm must stay the same. This means that faster electric motor would need a smaller drive pulley to achieve the same pump speed. Link to this neat pulley speed and size calculator:





Pulley Calculator. RPM, Belt Length, Speed, Animated Diagrams - Inch







www.blocklayer.com




Keep in mind there is a limit to how small a pulley you can use 21/4" I _believe_ is the smallest you can go for a particular HP, 2 HP I think.

Stephen



JoeBloggs said:


> Hi Steve
> I was hoping the original confirguration would give a clue
> I'm guessing i'd need a pulley twice the size on the motor to bring the revs down, or to match the RPMs (not sure)
> 
> Have you got a link to the online tool please?





JoeBloggs said:


> Hi Steve
> I was hoping the original confirguration would give a clue
> I'm guessing i'd need a pulley twice the size on the motor to bring the revs down, or to match the RPMs (not sure)
> 
> Have you got a link to the online tool please?


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## JoeBloggs (Mar 27, 2020)

OK, what's the smallest limit? LOL is there an online tool for this too?


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## JoeBloggs (Mar 27, 2020)

21/4" 

Just figured out you meant 2 1/4", cheers


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## iowagold (Jan 22, 2020)

it is all math. ratios...
you need 1/2 the dia on the motor side for the new motor. as the new motor speed is close to 2x faster. watch the pitch on the belts. there is sae and metric on them.
the older compressors needed more power at times.. and if you get in to a late model of motor some of the start current issues are better. Dayton, baldor , and a few other companies have some good choices of motors. just watch the start current. and if the compressor does not have a modern unloader valve, get one on the fast!! that will help on the dead head current on the pump!


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## stevon (Mar 5, 2019)

Iowagold,

Joe is in Australia, Closer to China and the power over there is to the European Spec, EG 240 volt/50 HZ. If he bought an American spec motor first it would have to be marked as 50/60 Hz and the speed changes with the Hz. A 2880 rpm motor @50hz will run 3400 rpm on 60 Hz. His extra motor is fine if current drive pulley is 120mm then a 60mm will do the trick with the new motor.

Stephen


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## stevon (Mar 5, 2019)

Joe,

No tool but a listing. With the small diameter pulley there is less surface area to transfer power. There is a listing of how much power and the smallest practical size pulley you can use for that power.
NEMA list (American) you will have to convert SAE sizes to metric sizes:


https://www.bestorq.com/info/NEMA.htm



Stephen



JoeBloggs said:


> OK, what's the smallest limit? LOL is there an online tool for this too?


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## stevon (Mar 5, 2019)

Joe,
I just thought of something else. The NEMA listing is for single pulley power transfer, a 60mm double pulley should give you no problems. 2.4 inches = 60.96mm
PS. SAE= Society of Automotive Engineers

Stephen


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## JoeBloggs (Mar 27, 2020)

Steve, Coz of the information provided, I decided to figure out what I need to do with this motor for a tablesaw (bit weak with it's current 1 HP motor ripping firewood) & I've got two working compressors anyway.
I got the right single belt pulley to suit, the shop suggested I buy a V-ribbed belt for better grip, & coz centre to centre is only 180mm

But i'll keep your tip in mind if i ever decide to get that compressor going in future, it has a double belt pulley on the pump.

Cheers.


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## stevon (Mar 5, 2019)

Joe,

That's good, I forgot motor shafts are in MM too in Australia.

Cheers,
Stephen


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