AC motor hp and torque relationship 1

[cabledude]

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.

 

[jraef]

HP is just a convenient way of describing a relationship betwween torque and speed. Lower HP and higher speed can still means less torque than a larger HP at a lower speed. The basic formula for this is HP = Tq (in ft. lbs.) x Speed (in RPMs) / 5250, so to get Tq alone, it is HP x 5250/speed.

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

 

[itsmoked]

Ah you are ignoring the horsepower!!! If you need 5HP to run the comp at 1000RPM, YOU NEED FIVE HORSEPOWER!

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.

 

[aolalde]

A basic principle of physics:

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.

 

[electricpete]

All good answers. As itsmoked said, you may be able to decrease the horsepower requirement of the compressor to within your motor's capability by running it at a lower speed (smaller motor sheave or larger compressor sheave).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[itsmoked]

cabledude; I am running exactly what you have. I bought a local facility air conditioning control pneumatics compressor at auction. It's a large twin cylinder single stage. It had a 5hp 3-phase. I brought it home and tossed on a 3hp 1-phase, reduced the motor pulley size accordingly as explained above. It still puts out tons of air.

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.

 

[cabledude]

Thanks to all who responded to this problem. I have smaller pulleys for the motor and understand that I can turn the compressor slower and still get a good bit of air from it. I was really looking for some formulas to help me understand better. The basic formula above was a good tool for this problem. That said, suppose I was to turn the compressor at half of its rated speed. If I apply the above formula would I not require more torque?
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.

 

[jraef]

This formula is for determining the torque OUTPUT of a motor, not for determining the REQUIRED torque of the load. That is something that the compressor manufacturer must do, it is far more complex, and in this case they already determined that at the rated SCFM and pressure, they needed 5HP. But suffice it to say that if you turned the speed down to 1/2 of what the compressor was designed for, you will get less output, AND your HP requirement will be less. That is essentially what itsmoked was telling you.

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

 

[electricpete]

jraef is definitely correct. I will try to say it in a sequential way:

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[itsmoked]

continuing;

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.

 

[jraef]

Final continuation...

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

 

[itsmoked]

9- dang I can't think of anything!!

cabledude; What do you want the compressor to provide air for? We can advise if 3HP will do the trick.