2 vs 4 pole electric compressor motor?

[99graveyard]

Hi All,
I have a Quincy 216 that I would like to find a motor for. As I understand it this pump is rated at 500-900rpm with a 2-3hp motor. I would like to be in the upper range of that for the cfm increase. I could buy a newer 2 pole 3hp motor (3,450rpm) that would put the pump in that rpm range. I’ve also read that newer motors are more efficient. However I’ve read a lot of post where folks prefer 4 pole (1,750ish rpm) motors. If I go that router than I would have to change pulleys to get around 800 pump rpm, the pulley reduction would be close to 2:1 then. Wouldn’t it be harder on the electric motor with less pulley reduction? Thank you all in advance!

 

[bsaint]

The torque on the pump is the same. The more poles, the slower the motor but the increase in torque. As HP is a function of force and speed. As long as you are running the pump at the correct speed,

 

[American Locomotive]

4 pole motors tend to be more efficient, and the larger pulley required with the 4 pole motor gives it more grip on the belt.

 

[vanapplebomb]

Four pole also typical last longer since they run at half the speed. Often, they also tend to start better and generate less heat when coming up to full running speed. Not so important for constant or intermittent cycling. This becomes a big deal when the compressor frequently cycles on and off. Quickly kicking on and off is really hard on motors, and builds a lot of heat. The less heat is generated at startup the better.

 

[BukitCase]

"4 pole motors tend to be more efficient, and the larger pulley required with the 4 pole motor gives it more grip on the belt"

Another "side" benefit is that the larger pulley will increase belt life due to less stress reversal effects... Steve

 

[pancho400cid]

As touched on above, the size of the motor is driven by horsepower-per-rpm (aka torque).

A 2 HP 4-pole is a bigger and more expensive motor that a 2 HP 2-pole. The 4 pole nominally has half the RPM and will make nominally twice the torque of a 2-pole with the same rated HP.

You need to check the mounting holes and pulley alignment before pulling the trigger on a motor.

A common mistake is buying a two-pole motor because they are cheaper for the HP rating, but then finding out the driving pulley ends up being too small to drive the load so to make it work you end up needing to switch out BOTH driving and driven pulleys which can devolve into a big mess.

...

 

[seber]

"4 pole motors tend to be more efficient, and the larger pulley required with the 4 pole motor gives it more grip on the belt"

Another "side" benefit is that the larger pulley will increase belt life due to less stress reversal effects... Steve

No, the tension on the pulley is the same at running speed but faster starts mean additional tension.

 

[99graveyard]

Wow! All very good information! I knew this was the place to ask! Thank you all much! Should i keep an eye out for a used on or look for a good deal on a new?

 

[BukitCase]

"tension on the pulley"

I wasn't talking about tension, altho that DOES contribute - "stress reversal" as in the constant flexing of the belt from straight to curved around the pulley. The smaller the pulley diameter, the more flex; and the more flex, the more HEAT will be produced. same as tire ratings for big equipement - a lot of those have a "ton/mile per hour" rating, because the more weight the more the tire flexes... Steve

 

[vanapplebomb]

Yup, a smaller pulley will have less wrap angle and a tighter bend radius on the belt...both of which will decrease the belts useful working life.

 

[Citation]

Do keep in mind that compressors in the 2-3hp range are typically driven by 2 pole motors. Why do something different?

 

[marinusdees]

Do keep in mind that compressors in the 2-3hp range are typically driven by 2 pole motors. Why do something different?

Because, as per the comments, it's arguably "better".

 

[Citation]

Because, as per the comments, it's arguably "better".

Arguably but in any practical way? A pressure lubricated oil system is arguably better than the splash lube system used by my lawn mower. My mower is 20 years old and has had no lubrication related issues.

In this case, is the difference one that will matter in practical terms? If we were talking about a 5+ hp pump I can see why one would pick a 4 pole vs 2. However, when we are talking under 3hp why bother? How does that practically better translate into something that will matter in practice?

 

[American Locomotive]

Arguably but in any practical way? A pressure lubricated oil system is arguably better than the splash lube system used by my lawn mower. My mower is 20 years old and has had no lubrication related issues.

In this case, is the difference one that will matter in practical terms? If we were talking about a 5+ hp pump I can see why one would pick a 4 pole vs 2. However, when we are talking under 3hp why bother? How does that practically better translate into something that will matter in practice?

If a 4 pole and 2 pole motor can be acquired for roughly the same price, in the same frame size, then why NOT go for the 4-pole motor when it offers ~10% higher efficiency and the larger pulley will reduce belt wear?

 

[Citation]

If a 4 pole and 2 pole motor can be acquired for roughly the same price, in the same frame size, then why NOT go for the 4-pole motor when it offers ~10% higher efficiency and the larger pulley will reduce belt wear?

So if all else is equal, yes go for the 4 pole. However, most 2-3hp compressors would have come with a 2 pole motor and the correct pulleys for a 2 pole motor's higher speed. Also consider things like thermal resets. A 2 pole motor meant for compressors is likely to have a thermal overload protection vs needed to have a starter to provide the same protection.

So 4 pole gives you a 10% boost in power efficiency, and less stress on belts. In exchange the motor is likely to cost more and require some type of external overload protection. A typical 2 pole, 3hp replacement compressor motor will probably cost less, not require an external overload protection, have the correct size pulley (assume the OP has the original) and the difference in belt wear is likely to be a non-issue for most people.

Sure, 4 is "better" but is it better in a way that will matter?

 

[vanapplebomb]

At home, maybe not, but in a commercial setting, or even a heavy DIY shop, the four pole motors do live noticeable longer. There is a reason that the higher end models have the four pole motors, and not the two pole style. Sometimes the same compressor is offered with both motors. Same pump, but the option of a better four pole motor with a mag starter vs a two pole TOPS motor. In heavy duty applications it is worth the extra cost because of the extended life you will get out of it.

 

[Citation]

At home, maybe not, but in a commercial setting, or even a heavy DIY shop, the four pole motors do live noticeable longer. There is a reason that the higher end models have the four pole motors, and not the two pole style. Sometimes the same compressor is offered with both motors. Same pump, but the option of a better four pole motor with a mag starter vs a two pole TOPS motor. In heavy duty applications it is worth the extra cost because of the extended life you will get out of it.

That is pretty much what I'm getting at. If this were one of those HVAC compressors (the ones that provide pneumatic power for large building hvac system) I would agree to go for the heavy duty option. But it sounds like this isn't a heavy duty case thus the extra cost and effort isn't delivering any benefit. Kind of like having a watch that's water resistant to 2000 meters when you only swim in a pool.

 

[csp]

Most likely this 216 was originally driven by a 4 pole motor.

 

[redmondjp]

That is pretty much what I'm getting at. If this were one of those HVAC compressors (the ones that provide pneumatic power for large building hvac system) I would agree to go for the heavy duty option. But it sounds like this isn't a heavy duty case thus the extra cost and effort isn't delivering any benefit. Kind of like having a watch that's water resistant to 2000 meters when you only swim in a pool.

But those HVAC compressors are made to run low and slow for 1000s of hours w/o much attention. Most of those units turn on at 80psi and off at 100psi, and are fairly low-flow. Just a heads-up to people as these units show up on Craigslist now and then, and people need to know that they are not going to run their blast cabinet with 100psi and 4cfm.

My big, true 5HP compressors all have 4 pole motors (1725-1750 rpm), and my consumer-grade 3.3HP 60 gallon has a 3450rpm 2 pole motor. I agree that it really doesn't matter for this application.

 

[Citation]

But those HVAC compressors are made to run low and slow for 1000s of hours w/o much attention. Most of those units turn on at 80psi and off at 100psi, and are fairly low-flow. Just a heads-up to people as these units show up on Craigslist now and then, and people need to know that they are not going to run their blast cabinet with 100psi and 4cfm.

My big, true 5HP compressors all have 4 pole motors (1725-1750 rpm), and my consumer-grade 3.3HP 60 gallon has a 3450rpm 2 pole motor. I agree that it really doesn't matter for this application.

Yeah, I think we are in total agreement here. The HVAC units I've seen are clearly designed to be trouble free for decades but they aren't a good choice for power tools. I think most could be adjusted to have higher pressure but the pumps are typically 1-2hp pumps so they won't flow much if any more than a higher end 120V pro-sumer compressor.

 

[Citation]

....

 

[marinusdees]

Arguably but in any practical way? A pressure lubricated oil system is arguably better than the splash lube system used by my lawn mower. My mower is 20 years old and has had no lubrication related issues.

In this case, is the difference one that will matter in practical terms? If we were talking about a 5+ hp pump I can see why one would pick a 4 pole vs 2. However, when we are talking under 3hp why bother? How does that practically better translate into something that will matter in practice?

If you don't get it, don't sweat it. You'll never know the difference.

 

[Citation]

If you don't get it, don't sweat it. You'll never know the difference.

Umm... I get the difference. I'm just saying for most people, and likely the OP, the difference isn't meaningful. I know the difference both in terms of accuracy and durability between my Fluke multimeter and my Uni-T meter. I also know that most of the time the difference isn't important.

The point I'm trying to get across is if the OP doesn't have a specific need for a more expensive 4 pole motor (that will likely need a starter to provide overload protection) vs a typical 2 pole, compressor motor with built in overload protection, then perhaps he should just stick with what the compressor likely came with.

However, if you can't articulate why it is better for the OP then perhaps you don't know?

 

[redmondjp]

The four pole motor will have a higher starting torque and will have an easier time starting a compressor with cold 30W oil in it. So there's one difference.

 

[99graveyard]

All very good perspectives, and thank you all for your input. I don’t have the original motor. I found the pump on a trailer mated to a Wisconsin gas engine. I bought just the pump. So I will most likely be buying a puller either way. This will be used in a residential setting.