AC motor hp and torque relationship
AC motor hp and torque relationship
(OP)
I have just bought a new air compressor pump. The specs are as follows: twin cylinder,5 hp, 1000 RPM (max). Would like to know if a 3 hp motor with run speed of 3450 RPM would have the hp and torque to run this compressor if the V-belt pully was roughly sized 3.5 to 1. This ratio would be needed to keep the compressor RPM close to its maximum specified RPM. This sounds logical but it does not work in reality. I do not understand why a smaller motor with a faster RPM could not handle this compressor.





RE: AC motor hp and torque relationship
So if you need 5HP at 1000RPM, the torque requirement is 5 x 5250/1000 or 26.25 ft. lbs.
Your 3HP 3450RPM motor develops 3 x 5250 / 3450 or only 4.56 ft. lbs. If you reduce the speed through a pulley to 1000RPM, you increase the shaft torque by an equal ratio, so your 4.56 ft. lbs becomes 15.96 ft. lbs, but still way short of the required 26.25.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: AC motor hp and torque relationship
So if you want to use a smaller HP motor you must spin the compressor s l o w e r... Period. The problem is figuring the correct new compressor speed since it isn't really linear.
Generally I grab a smaller motor pulley and try it. Once the compressor actually runs you absolutely want to measure the current draw of the motor using a clamp on ammeter. Make sure you aren't pulling more current the name plate allows or the motor will absolutely toast! You must check over the whole pressure range too. It will draw the most current at only a certain point.
If the motor is drawing too much current you must reduce the motor pulley more.
BTW you can have a 1/4HP motor turning 10,000RPM. It won't do anymore work than a 1/4HP motor turning at 1,725RPM it just spins faster.
RE: AC motor hp and torque relationship
Energy = work
Power= work/time
If you want to make some work in a specified time the energy required per unit of time has to be provided. There is no way to avoid that.
RE: AC motor hp and torque relationship
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RE: AC motor hp and torque relationship
Remember that if you put a true 3hp (not the lying Sears(et al) 6hp peak jibberish) on a compressor it will actually move quite a lot of air.
RE: AC motor hp and torque relationship
Tq=HP*(5250/RPM) Tq=5(5250/500)=52.5 ft-lbs
I am sure that is not true, probable because I no longer require 5 HP. It sounds like the pulley change is just to increase the torque untill I have the originally required torque of 26.25 ft-lbs. I would be grateful for any other formulas or conversation that might apply.
RE: AC motor hp and torque relationship
But be carefull, compressors often have mechanical lubrication systems that need a minimum speed in order to function, and 50% might be too low. Whatever you plan to do, check it out with the compressor mfrg first.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: AC motor hp and torque relationship
1 - Motor always operates very close to it's sync speed (neglect a few percent slip).
2 - Sheave ratio together with motor speed determines compresssor speed.
3 - Compressor speed (for a given compressor) determines hp requirement. Higher speed almost always means higher horsepower.
4 - Motor will always match the horsepower required by the compressor (neglecting belt losses). If motor has to provide power above it's rating and protective devices, it may trip or be damaged.
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RE: AC motor hp and torque relationship
5- You lower the required HP by slowing the compressor.
6- You slow the compressor by reducing the motor pulley diameter. (you don't make the compressor pulley bigger because it always carries the integral fan)
7- Check your results with an ammmeter.
RE: AC motor hp and torque relationship
8- You get less SCFM from the compressor. Pressure is not necessaryilly affected, but if you have a high pressure and SCFM requirement, you slower compressor may not be able to keep up, just so you know.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: AC motor hp and torque relationship
cabledude; What do you want the compressor to provide air for? We can advise if 3HP will do the trick.