How to Determine Pulley Size

[ss2]

ok you guys, i need some help.
i have to replace a 5 hp electric motor.
the original motor is 1750 rpm with a 7" drive pulley.
the new motor is 3505 rpm.
how do i determine the correct size pulley to keep the compressor pump speed close to what the original motor was producing?
ideally i would like to slow the pump a little more if possible but not any great significants.

 

[wilbilt]

Trade the new motor for one that produces the proper RPM.

Otherwise, use a 3" - 3 1/2" motor pulley. Not the best solution.

 

[ss2]

thanks Will,
what i have is a 3 phase 5 hp motor.
i don't have 3 phase in my shop.
i have considered building a rotary phase converter and still may do that but i have this single phase 5 hp on another machine that is not being used at this time. course the rpm's are different which is my problem.
what kind of problem am i getting myself into by using the 3505 rpm motor?

 

[wilbilt]

thanks Will,
what i have is a 3 phase 5 hp motor.
i don't have 3 phase in my shop.
i have considered building a rotary phase converter and still may do that but i have this single phase 5 hp on another machine that is not being used at this time. course the rpm's are different which is my problem.
what kind of problem am i getting myself into by using the 3505 rpm motor?

Obviously, there is the difference in RPM. This can be directly compensated for by reducing the drive pulley size. Trade-offs will be increased power consumption, increased vibration, more noise, searching for the correct belt, etc.

Using a small enough drive pulley may require a grooved or segmented belt. Less longevity, increased cost.

 

[Franz©]

3.5" pully on the motor will give you the same driven speed on the compressor. You may need to add a backidler setup or extend the distance between drive and driven to get more than 50% wrap on the motor pully though.

 

[nissan_crawler]

at the risk of sounding dumb, is there any reason you can't just get the right rpm motor? You're looking at $40-70 to swap the motor pulley, and it might be getting sketchy with that small of a pulley. Personally, I would go a little smaller on the motor, and a little larger on the pump.

I just bought a 5hp 1750 rpm 23 amp baldor off ebay for $180 shipped. If you have to change both pulleys, you'll be near the cost of a motor anyway.

 

[ss2]

at the risk of sounding dumb, is there any reason you can't just get the right rpm motor? You're looking at $40-70 to swap the motor pulley, and it might be getting sketchy with that small of a pulley. Personally, I would go a little smaller on the motor, and a little larger on the pump.

I just bought a 5hp 1750 rpm 23 amp baldor off ebay for $180 shipped. If you have to change both pulleys, you'll be near the cost of a motor anyway.

i would gladly spend the $180 to get the right motor.
is it possible your seller on ebay has another motor?
what i have seen on ebay is $255 plus shipping.
please let me know.

 

[nissan_crawler]

*inserts foot in mouth* It was $280 with free shipping, not $180. If you're still interested, search "weco-group", they have an ebay store. They were the best price I coudl find on Baldor's when you figured in shipping.

 

[dxdexter]

ok you guys, i need some help.
how do i determine the correct size pulley to keep the compressor pump speed close to what the original motor was producing?
.

3.14159 x Pulley Diameter x rpm = inches per minute

Just rearrange equation and solve for unknown.

I would get a 1725 RPM as was originally in place.