Need 220v motor for air compressor

[Tracs]

I came across a 60G Sanborn air compressor at princess auto in the clearance section that was complete minus the electrical motor for less than 1/3 of the regular price so I snatched it up.

The tank say it is 3.7 RHP. The motor is also labeled 3.7. I have been reading about inflated HP ratings on compressors. The compressor instructions state the unit draws 15A. Would 15A make 3.7HP? If it did would that be running or peak?

My friend gave me a Emerson 230V 3450RPM 2.5 HP running, 4.2 peak HP pump motor. It looks like it will bolt up and will only need a 1/2" bore pulley instead of 5/8" to complete it.

Do you think that the compressor will perform satisfactory with the above mentioned pump motor? What will the disadvantages of a slightly less powerful motor? Low PSI? Low CFM?

Here is the compressor in full form.
http://www.princessauto.com/en/deta...ricated-belt-drive-air-compressor/A-p8348955e

 

[md21722]

Your standard 3.2-3.7 HP motors for these types of compressors actually do use 3.2-7 HP, 15A 230V. In this case 2.5HP is too small for your the compressor pump. If a motor shieve (pulley) that is too small is used, to pump may run too slow and not have sufficient lubrication. Even thought most are splash lubricated they do have a minimum RPM for adequate lubrication. Whether or not it bolts up is another issue, common frame sizes are 56 or 145 in this size. You can use either a 1725/3450 with the right belt tension and shieve size. Drill the base plate as necessary.

 

[Packard V8]

FWIW, If one had an equivalent duty cycle and RPM motor, just size the drive pulley down by the equivalent horsepower ratio. My math says 2/3 or .67; so if the OEM pulley was 6", you'd use a 4".

However, that 2.5 hp motor with only a 1/2" shaft is indicative of a light duty pump motor. It most likely is not a compressor duty motor and it's doubtful if it would live long and prosper. Since it was free, it costs only a pulley and belt to confirm.

jack vines

 

[Tracs]

FWIW, If one had an equivalent duty cycle and RPM motor, just size the drive pulley down by the equivalent horsepower ratio. My math says 2/3 or .67; so if the OEM pulley was 6", you'd use a 4".

However, that 2.5 hp motor with only a 1/2" shaft is indicative of a light duty pump motor. It most likely is not a compressor duty motor and it's doubtful if it would live long and prosper. Since it was free, it costs only a pulley and belt to confirm.

jack vines

What you said is what i was thinking. Free motor and it is rated continuous duty and same speed. I have a pulley that is 25% smaller than stock and will try that. If it wont work ill just return it all and keep looking for a good deal on a new one. Or if it works use it till i find a 3.5 hp used motor.

 

[redmondjp]

What you said is what i was thinking. Free motor and it is rated continuous duty and same speed. I have a pulley that is 25% smaller than stock and will try that. If it wont work ill just return it all and keep looking for a good deal on a new one. Or if it works use it till i find a 3.5 hp used motor.

Gor for it! The pump is probably already spinning at 800rpm (if not faster), so slowing it down a bit will not hurt the lubrication any. In many cases, minimum pump speed is in the 400rpm neighborhood.

Make sure your unloader is working so it can start easily.

 

[Tracs]

Well i tried it. Started and ran till the tank filled and cut out. I drained air till the cut in level. It tried cutting back in, turned a few rotations and stalled. Just not enough power. The 2.5hp motor didn't get hot or anything while running. It seemed that it would start spinning until the unloader valve shut and it tried making compression and it stalled.

So either keep looking for a used motor, spend $2-300 on a new 5hp motor and be into it for $200 less than a brand new unit with warranty or try and return it all even though it says no return.

 

[Packard V8]

While you're looking, go down to a 33% smaller pulley. That may be just enough more mechanical advantage to allow your motor to pull against compression.

jack vines

 

[Tracs]

While you're looking, go down to a 33% smaller pulley. That may be just enough more mechanical advantage to allow your motor to pull against compression.

jack vines

I guess I can give that a try. Wouldn't that make the compressor spin too fast?

 

[Packard V8]

You mentioned you're using a 25% smaller drive pulley than OEM. Going down to 33% smaller will slow the compressor by that percentage. It's like shifting down a gear in a car; more power, but less speed at the same RPM.

jack vines

 

[redmondjp]

I guess I can give that a try. Wouldn't that make the compressor spin too fast?

A smaller pulley on the motor will make the pump spin slower. I agree with this idea. You won't get the same flow, but it should be able to pump up to shutoff pressure without an issue.

 

[CharlieADelaney]

I noticed a huge difference going from 25% to 33%.

 

[Tracs]

I made a rather large calculation error in selecting my first pulley. It turns out it is only 12.5% smaller than stock. Stock is 4" and I got a 3.5". So a 33% smaller pulley would be 2.68" Not sure how close I can get to that, 2.5" or 2.75" would probably be common sizes.

Stock pump RPM is around 1100 RPM. A 2.5" pulley would lower it to about 725 RPM which I think is within the pump RPM range. I can't find the exact specs though. It is a 755H model pump from what I can find.

http://www.mastertoolrepair.com/complete-pump-040-0357-p-26170.html

 

[redmondjp]

I made a rather large calculation error in selecting my first pulley. It turns out it is only 12.5% smaller than stock. Stock is 4" and I got a 3.5". So a 33% smaller pulley would be 2.68" Not sure how close I can get to that, 2.5" or 2.75" would probably be common sizes.

Stock pump RPM is around 1100 RPM. A 2.5" pulley would lower it to about 725 RPM which I think is within the pump RPM range. I can't find the exact specs though. It is a 755H model pump from what I can find.

http://www.mastertoolrepair.com/complete-pump-040-0357-p-26170.html

Either pulley should be fine, but the 2.5" would be the safer choice. Any pump speed in the 700s will be plenty fast enough for lubrication.

 

[Tracs]

Either pulley should be fine, but the 2.5" would be the safer choice. Any pump speed in the 700s will be plenty fast enough for lubrication.

I'll get a 2.5" pulley and belt in the morning and give it a try

 

[Tracs]

Got a 2.5" pulley and belt to fit, put it all on and fired it up. Tank ran up to 135psi and cut out. I drained it down to 105psi and it cut back in and ran up to 135psi. No obvious signs of the motor struggling.

Took about 10 minutes to fill the 60 gallon tank from empty. It took 1:50 to fill it from 105psi back up to 135psi.

What will spinning the pump 33% slower do to the CFM delivery? Can i assume 33% less CFM? Is CFM proportional to pump RPM?

 

[redmondjp]

Got a 2.5" pulley and belt to fit, put it all on and fired it up. Tank ran up to 135psi and cut out. I drained it down to 105psi and it cut back in and ran up to 135psi. No obvious signs of the motor struggling.

Took about 10 minutes to fill the 60 gallon tank from empty. It took 1:50 to fill it from 105psi back up to 135psi.

What will spinning the pump 33% slower do to the CFM delivery? Can i assume 33% less CFM? Is CFM proportional to pump RPM?

In general, yes, flow is proportional to RPM. Your elapsed time numbers sound correct for that size of tank.

Use Compressor And Prosper, my friend!

 

[Tracs]

In general, yes, flow is proportional to RPM. Your elapsed time numbers sound correct for that size of tank.

Use Compressor And Prosper, my friend!

The recovery time from 0psi is stated at 11-12 minutes and thats about what mine will do turning the pump at 725RPM instead of 1100RPM. Princess auto sells a Pro Point brand 20 gallon compressor with the exact same compressor pump and 1.9HP 120/240V motor. It makes 155PSI and 5.7 CFM at 90PSI. The pro point has the same size flywheel and uses a 2-5/8" pulley on the motor. The Pro Point turns the compressor pump at 780RPM.

Would it be safe to assume that my 60 gallon compressor now operates in the same CFM range as the 20 gallon Pro Point? What roles does tank size play in CFM delivery when the pump is not running, eg. When the tank is full or near full?

 

[md21722]

Yes it would operate in the same range based on the comparable pulley sizes. Yours is a bit lower because you are using a smaller pulley on the motor (2-1/2" vs 2-5/8").

Tanks do a few things. They provide an air reserve, they serve as a condensation point, and they serve as a buffer against the natural pulses of an reciprocating air compressor so you get smooth air out the line.

Tanks don't have a specific CFM rating. However, consider that many of the bigger tanks have 3/4" outlets that you plumb into 3/4" or 1" line. These large lines can support high CFM, much higher than the pump CFM. So for example, an impact wrench that uses 37 CFM in short bursts, gets the CFM it needs. Once tank pressure drops, the pump comes on to refill the tank.

 

[Tracs]

It seems most of the motor choices are 3 or 5 hp and nothing in between. If I get a 5hp motor in place of the original 3.7hp and put the original sheave on, will it draw the full amps the 5hp motor runs at, or will it be proportional to the work it does? If the 3.7hp motor draws 17.2 amps at full load is that what a 5hp motor will draw?

 

[md21722]

You can get 3.5 or 3.7 HP motors at NAPA... look at their performance piston line for part numbers.

 

[redmondjp]

It seems most of the motor choices are 3 or 5 hp and nothing in between. If I get a 5hp motor in place of the original 3.7hp and put the original sheave on, will it draw the full amps the 5hp motor runs at, or will it be proportional to the work it does? If the 3.7hp motor draws 17.2 amps at full load is that what a 5hp motor will draw?

The 5HP will not draw its full-load amps if not fully loaded. What it draws will depend upon the motor, but I'd guess it will be under 20A @ 240VAC.

Why not just use the motor you have on there for now? I'd run it as-is for now.

 

[Tracs]

The 5HP will not draw its full-load amps if not fully loaded. What it draws will depend upon the motor, but I'd guess it will be under 20A @ 240VAC.

Why not just use the motor you have on there for now? I'd run it as-is for now.

I will use it for now, but the hot tub motor with its 48 frame size and 2.5hp spins the pump at 33% less RPM than original. I don't have a use for the additional CFM right now but in the future I might. I envision maybe using paint guns and spot blasters in the future. Also if I want to sell the compressor I don't think it would be a very fast sell with a funny looking hot tub motor.

 

[md21722]

To meet electrical codes you'd still need to wire it as a 5 HP and would need a mag starter or a 5 HP motor that doesn't require it.

 

[redmondjp]

I will use it for now, but the hot tub motor with its 48 frame size and 2.5hp spins the pump at 33% less RPM than original. I don't have a use for the additional CFM right now but in the future I might. I envision maybe using paint guns and spot blasters in the future. Also if I want to sell the compressor I don't think it would be a very fast sell with a funny looking hot tub motor.

Be really careful there. Unless you can get a larger motor for a really good deal, you're not going to get your money back out of buying another motor for the compressor just to sell it. Used compressors SELL (actually sell for, not asking prices) for pennies on the dollar of their new prices.

 

[Tracs]

Be really careful there. Unless you can get a larger motor for a really good deal, you're not going to get your money back out of buying another motor for the compressor just to sell it. Used compressors SELL (actually sell for, not asking prices) for pennies on the dollar of their new prices.

You are right, If i bought a new motor it would not be worth it. But a used motor would make it more even. I wish Canada had good deals on used equipment like America does. There are no deals to be had on anything, doesn't matter what it is. This compressor was new minus electric motor and they list for $729 CAD. As it sits now with the motor i put on it im only into it for $250 CAD. The last compressor i seen for sale in the area was a CH 20 gallon and they guy was asking $200 and it sold in 15 minutes.

 

[Tracs]

I tried out a beefy 3hp 56 frame motor today. Turns the compressor pump at full speed using the original pulley without breaking a sweat. Got it used for $40.

New pump up times. 0-135psi - 5:23. 105-135psi - 1:06.

Now it should put out the full CFM rating.

Into the final product for less than $300.

 

[md21722]

Good deal, that's excellent performance. By rough calculation that's 13.6-14.8 SCFM.

 

[redmondjp]

Awesome! You'll be very happy with that compressor now.