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# Compressor pullies - size/speed

#### fredhh47

##### Aluminum
I have a 60 gallon upright compressor. The original 3.7hp Chinese motor died a horrible death and I replaced it with a 5 hp Baldor motor I happened to have. The original motor was 3450 rpm and the Baldor is 1850. I have the original 12" puiley on the pump and an approximately 4" pulley on the motor. Obviously it runs slow, and takes a long time to get up to pressure. I need to know how large of a pulley to put on the 1850 motor to bring the driven pulley's speed up to approximately what I was getting with the original 3450 motor. The original motor pully was about 3" diameter. I'm very sure that the Baldor will take the heavier load, it used to run on a 100 gallon horizontal 2-cylinder compressor, which was obviously set up for the slower rpm. Any math wizards have a suggestion?
Any ideas would be appreciated.
Thanks.

#### rawen2

##### Aluminum
If I calculated right a 5" pulley on the motor would run the compressor at about 89% of original speed.

A 6" pulley on motor would run it about 7.25% faster than original. Probably not good for long term pump life.

A 5.5" pulley would be 98.3% of original.

3/12 = .25 so .25*3450=862.5 original RPM

5/12=.416 so .416*1850=770.83 RPM
5.5/12=.458 so .458*1850=847.9 RPM
6/12=.5 so .5*1850=925 RPM

Last edited:

#### Bill D

##### Diamond
3450/1850=1.86

3 x 1.86 = 5.58
So a 5.58" diameter motor pulley. Call it 5.5" and be good enough.
Bill D

#### fredhh47

##### Aluminum
Fantastic response. Thank you all. I wish all my problems were this easy to solve.

#### Bill D

##### Diamond
A compressor will have a minimum and maximum speed. You sound fine at these speeds being discussed. Minimum is around 750 RPM for oil splash lubrication. Top rpm I have no idea but 3600 seem very fast. to me.

#### rons

##### Diamond
Your Baldor will last longer with a motor pulley on the small side.
You didn't say about the motor being single phase or three phase.
If your current motor is single phase then wait for it to get burned on that compressor.
Your next motor should be a three phase motor and power it using a self-start with a 10 Hp VFD.

#### deltap

##### Cast Iron
I have memorized - driver diam. x rpm = driven diam. x rpm. Solve for one unknown.

#### scsmith42

##### Aluminum
Pulley diameter x 3.14 gives you belt movement In inches per revolution.

X motor speed give you belt inches travel per minute.

So, your 4” pulley moves 12-1/2” of belt per revolution. X 3450 means that your belt was moving at 43,332 inches per minute / or 3,611 feet per minute.

To utilize your current pump pulley and create the same RPM, you’d need a 7.5” pulley on the 1850 rpm motor.

7.5” x 3.14= 23.55 inches circumference. 23.55 x 1850=43,567 ipm / 3630 FPM.

#### metalmagpie

##### Titanium
Your new pulley should be twice the diameter of the old one.

#### Lakeside53

##### Stainless
Are you sure about the 1850? 4 pole motors are 1800 (less slip at load so maybe 1750?), 2 pole motors are 3600 (3450 at rated load of 3.7hp).

As above, 2x diameter of the old pully.

#### WizardOfBoz

##### Diamond
As an engineer, I know that torque factors in here. This seems to be a non-issue practically (from the posts). Is this because a higher HP motor at lower speed will have to have higher torque? Does starting torque matter?

#### scsmith42

##### Aluminum
As an engineer, I know that torque factors in here. This seems to be a non-issue practically (from the posts). Is this because a higher HP motor at lower speed will have to have higher torque? Does starting torque matter?
For an air compressor, usually yes because the head will start developing pressure before the motor is up to full speed. Years ago electric motors for compressor use tended to be longer than non compressor motors - supposedly to provide more starting torque.

#### Lakeside53

##### Stainless
most quality compressors have an unloader - the motor is allowed to get up to speed before pumping starts.

An 1800rpm motor of the same rated hp has 2x the torque of a 3600 rpm, but when you double the pully diameter the effective torque is halved. 3.7hp verses 5 - yep the 5 has more torque even after the larger pully... Real question is do you believe the name plate of the original 3.7?

#### Scruffy887

##### Titanium
I have a 60 gallon upright compressor. The original 3.7hp Chinese motor died a horrible death and I replaced it with a 5 hp Baldor motor I happened to have. The original motor was 3450 rpm and the Baldor is 1850. I have the original 12" puiley on the pump and an approximately 4" pulley on the motor. Obviously it runs slow, and takes a long time to get up to pressure. I need to know how large of a pulley to put on the 1850 motor to bring the driven pulley's speed up to approximately what I was getting with the original 3450 motor. The original motor pully was about 3" diameter. I'm very sure that the Baldor will take the heavier load, it used to run on a 100 gallon horizontal 2-cylinder compressor, which was obviously set up for the slower rpm. Any math wizards have a suggestion?
Any ideas would be appreciated.
Thanks.
Something tells me that any larger pulley that is close to 2x same circumference will do the job until the compressor part dies a horrible death as well. Note that the compressor will be very reliable until it craps out on a Sunday evening and you are a few hours short of getting an order out for Monday morning.

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