120 gallon compressor pully questions

[Kbcoatings]

I just bought a 120 gallon compressor and it came with a 10 hp three phase motor. I am replacing it with a 7.5 hp single phase weg motor. I was wondering what size pully I'd need. Thanks for your help

 

[A_Pmech]

Welcome to GJ.

To answer that question we need to know:

Speed of the 3-phase motor
Speed of the 1-phase motor
Datum diameter of the sheave on the 3-phase motor

 

[Kbcoatings]

The 3 phase motor is 1750 rpm and the sinhle phase motor is 1750 rpm. The pully that's on the 10 hp 3 phase motor is 6 inch I do believe

 

[larry_g]

I just bought a 120 gallon compressor and it came with a 10 hp three phase motor. I am replacing it with a 7.5 hp single phase weg motor. I was wondering what size pully I'd need. Thanks for your help

I'd start with a motor pulley that was 70-75% of what the 10hp motor had. That said, check the specs on the compressor head to make sure that it will operate in this slower speed range. The above assumes that the motors have the same speed spec. There may be a few other items that come into play, but you did not ask about them.

lg
no neat sig line

 

[Leoruiz]

I just bought a 120 gallon compressor and it came with a 10 hp three phase motor. I am replacing it with a 7.5 hp single phase weg motor. I was wondering what size pully I'd need. Thanks for your help

Call the tech dept of the COMPRESSOR head manu.
Not the tank or electric motor or Crapsman or HD or HarbFart..
Find the maker of the actual compressor and talk to them.The rest are nonsense. Get it right or you'll likely burn it up. Those guys will get you straight.

 

[Kbcoatings]

Ok thanks for the quick response guys ! I'll try a 4 inch pully and see how that goes

 

[thebeekeeper1]

Ok thanks for the quick response guys ! I'll try a 4 inch pully and see how that goes

That's what you got out of the posts? With all due respect, that sounds like "famous last words."

 

[A_Pmech]

I'd start with a motor pulley that was 70-75% of what the 10hp motor had. That said, check the specs on the compressor head to make sure that it will operate in this slower speed range. The above assumes that the motors have the same speed spec. There may be a few other items that come into play, but you did not ask about them.

lg
no neat sig line

Agreed.

Then, test for peak motor current at pressure switch cut-out. The motor current should be equal to or less than the motor nameplate current.

 

[Kbcoatings]

Thanks man!

 

[Movover]

maybe a amperage draw test while its running to make sure your not over-loading the 7.5hp

 

[Movover]

A_Pmech beat me to it..

 

[Kbcoatings]

Ok I will make sure I do an amperage test. Thanks for the info guys

 

[Brad J.]

My 7.5 HP Quincy with a 340 pump has...

19" pulley on the pump
8 7/8" pulley on the motor (probably 9" if you have to buy one)

Pump runs around 800rpm with handheld tachometer.

4" pulley would be extremely slow.

 

[Kbcoatings]

Really? I have a 6 inch pully bought now, I'll give that a try first and if it's too slow I'll get a bigger one. I'll be setting the air compressor and refrigerated air dryer up within the next few days. Thanks man!

 

[Brad J.]

Whats the large pulley dia?

Search a pulley/speed calculator and they are usually very close to what I find with my tachometer.

Input 2 pulley diameters and input speed (1750) and that's your info.

If it's a pressure lube system you can operate at a minimum 400 rpm's up to around 1000 rpm's. Splash lubes usually need a minimum of 600 to keep oil splashed on everything.

 

[CompressorPros.com]

You would want to check with the air compressor manufacturer to see if they will recommend a target RPM for the pump turning at 7.5HP. Unfortunately, not all 10HP pumps can be turned by a 7.5HP single phase motor and you could have high amp problems. The basic formula to figure out what pulley you need is:

(pump RPM x flywheel diameter(pump pulley)) divided by motor RPM