Which is best single phase motor – 2 pole or 4 pole

[AjohnwParticipant @ajohnw51620]

Not sure why I started nosing around on this subject but it probably relates to buying a compressor and looking at what is sold. In some ways use on compressors is most interesting as the motor starts on a high load not far off it's max. Other things are also regarded as having a high start up torque requirement but these must be one of the worst.

Most of them seem to be fitted with 2,800rpm cap start, cap run. Leaves me wondering if this is just due to cost difference over 1400 rpm. As they are speed reduced by a belt drive the motor could be either type even with the rather large pulley that goes on the pump. That may be there on some to serve as a flywheel and also a fan in some cases.

However there may be other reasons. 3HP seems to be a popular motor size. I would have thought this would stand a fair chance of blowing a 13 amp fuse in a 3 pin plug on start up but maybe the 2 pole motors help but I would have thought 4 pole would be better in all respects.

Most info on the web gets wrapped up in torque differences when what actually matters is motor power and that can be exactly the same. Also size but as far as I am aware there is little difference.

It's left me wondering if other than cost one type has other advantages over the other.

Anyone know ?

Normally I would avoid 2 pole motors like the plague if given a choice but maybe that is the wrong attitude to them.

John

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[AjohnwParticipant @ajohnw51620]

And why ?

[Nick_GParticipant @nick_g]

Posted by Ajohnw on 03/01/2017 18:00:28:

It's left me wondering if other than cost one type has other advantages over the other.

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One runs at about 1400 rpm the other at about 2800 rpm

Different uses require different requirements for end application.

Nick

[JasonBModerator @jasonb]

Posted by Ajohnw on 03/01/2017 18:00:28:

. In some ways use on compressors is most interesting as the motor starts on a high load not far off it's max.

Most half decent compressors have an "unloader valve" even the machine mart ones so they don't start under load. Ever heard that puff of air as the motor cuts ou when the tank is full? that is your unloader valve.

J

[Neil WyattModerator @neilwyatt]

All things being equal the 2800 will run much cooler as the effectiveness of a fan varies with the square of speed, so they are probably more reliable in the long run.

Neil

[AjohnwParticipant @ajohnw51620]

Posted by Nick_G on 03/01/2017 18:09:39:

Posted by Ajohnw on 03/01/2017 18:00:28:

It's left me wondering if other than cost one type has other advantages over the other.

–

.

One runs at about 1400 rpm the other at about 2800 rpm

Different uses require different requirements for end application.

Nick

That's what crops up on the web a lot. It doesn't really matter on the face of it when the final speed is much lower than both and the power at that speed the same.

John

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[JasonBModerator @jasonb]

It does matter if you are tight for space as you won't need such a big reduction ratio to get a 1400 motor down to a low spoeed as you would a with a 2800

[Nick_GParticipant @nick_g]

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You trying to double the output of that new compressor of yours John. ?

Nick

[JasonBModerator @jasonb]

Posted by Nick_G on 03/01/2017 18:41:02:

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You trying to double the output of that new compressor of yours John. ?

Nick

He'll just get a lot of hot air come out of it Nick

[AjohnwParticipant @ajohnw51620]

I'd forgot the unloader Jason. Even the Einhell has one. Doesn't have much to do with 2 or 4 pole though. The unloader means that there must be some rotation before it sees any load.

Don't think Neil's reason is on either. They rate motors for a given temperature range which really means temperature rise when in use so probably less copper in the 2,800 rpm ones. Cost again.

No Nick if needed I'll change the pump head – also depending on replacement cost when and if the pump head wears out. A look round youtube shows just how disgusting the innards of some compressors are.

If it's not done for cost reasons then it needs to be electrical.

John

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[Nick_GParticipant @nick_g]

Posted by Ajohnw on 03/01/2017 18:52:03:

A look round youtube shows just how disgusting the innards of some compressors are.

John

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That is mostly due to many people putting them under the bench and forgetting about them until they start rattling or chuck their conrod through the casing.

I have a cheap (presumably far east) JCB branded one. I have seen the same model with lots of different badges on. – After a guesstimated 1 hour running I dropped the oil and replace it with a known quality product. Then did the same after a few additional hours.

I was quite surprised how little oil the sump holds. i.e. a little has to do a lot, so it's little wonder they complain if they are forgotten.

Nick

[bodgeParticipant @bodge]

Hi John

I may well have this wrong but if one used 1428 rpm motor one would only get half the free air per minute compared to 2800 rpm unless one plays around with pulley sizes , a 1428 motor is slowing itself down in effect, so am thinking 2800 more efficient ?

What i can tell you though is not all 13 amp fuses are the same , as some one who runs a 3 hp compressor on a standard 13 amp square pin plug / socket , one particular brand of fuse will blow as quick as you can change them, and another brand will last ages

It took awhile for me to figure that one out……i cant remember where i bought the good fuses other than to say at one of the local supermarkets but you can tell the difference by looking at them once you have both types………b

[John Stevenson 1Participant @johnstevenson1]

The 'good' ones are probably made by China Nail and Silk Flower Factory

[bodgeParticipant @bodge]

Posted by John Stevenson on 03/01/2017 19:57:17:

The 'good' ones are probably made by China Nail and Silk Flower Factory

You may well be right at that, but not joshing or any thing, not all fuses are the same , tried it on a couple things that are a bit thirsty on the sparks an sure enough same thing happens one brand will blow other brand dont !……..b

[AjohnwParticipant @ajohnw51620]

That;s what struck me about 3hp bodge, A lot used to reckon that a 3hp lathe motor couldn't be run on a 3 pin plug. I did have one. Some one had fitted a 3hp to it so that he could turn certain tool steels. It was a Viceroy. I'd guess that the belt drive just wouldn't transmit that much really. It didn't blow fuses though even though the full load current draw is usually about 13 amps so start up could be a lot higher.

There used to be a 110v plug used in factories. The earth prong on the usual 3 pin was turned through 90 degrees. I understand some people used the 15amp fuse from those.

It was mostly many of the oil free that looked disgusting. They probably work because the air pressure tends to keep the piston square in the bore. The ones with oil vary.

I do wonder if one of 2 pole or 4 pole is more efficient or a different starting torque characteristic etc. I suspect it's just min cost. Many have a large pulley on the pump. Why not make it smaller as it would be cheaper. Leaving it the same size and increasing the other would cost more. The size of the big pulley might be needed to give the power transmission that's needed.

John

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[bodgeParticipant @bodge]

Posted by Nick_G on 03/01/2017 19:21:14:

Posted by Ajohnw on 03/01/2017 18:52:03:

A look round youtube shows just how disgusting the innards of some compressors are.

John

–

.

That is mostly due to many people putting them under the bench and forgetting about them until they start rattling or chuck their conrod through the casing.

I have a cheap (presumably far east) JCB branded one. I have seen the same model with lots of different badges on. – After a guesstimated 1 hour running I dropped the oil and replace it with a known quality product. Then did the same after a few additional hours.

I was quite surprised how little oil the sump holds. i.e. a little has to do a lot, so it's little wonder they complain if they are forgotten.

Nick

I guess you must be talking about mine then !……..lives outside under the bench

Though it does get a pretty through inspection before use ,drain cocks left open when not in use only had it twenty years !

Change the oil often works just fine.

Not sure where it was made, Italy for the pump , UK for motor & receiver …………..b

Having said that i have seen some real rough ones receivers more full of muddy water than air,…..

[SillyOldDufferModerator @sillyoldduffer]

Umm,

Guessing again:

Compared with a 2-pole of equal power, 4 pole gives ½ rpm and 2 × Torque

So a 4 pole motor would be better whenever torque matters, often at start up. An air compressor doesn't need a lot of torque to get going and then the higher rpm of a 2 pole motor would get the pressure up by driving a small (ie cheaper) piston faster.

Dave

Edited By SillyOldDuffer on 03/01/2017 21:23:22

[bodgeParticipant @bodge]

Posted by Ajohnw on 03/01/2017 20:18:15:

That;s what struck me about 3hp bodge, A lot used to reckon that a 3hp lathe motor couldn't be run on a 3 pin plug. I did have one. Some one had fitted a 3hp to it so that he could turn certain tool steels. It was a Viceroy. I'd guess that the belt drive just wouldn't transmit that much really. It didn't blow fuses though even though the full load current draw is usually about 13 amps so start up could be a lot higher.

I do wonder if one of 2 pole or 4 pole is more efficient or a different starting torque characteristic etc. I suspect it's just min cost. Many have a large pulley on the pump. Why not make it smaller as it would be cheaper. Leaving it the same size and increasing the other would cost more. The size of the big pulley might be needed to give the power transmission that's needed.

John

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Going from memory on m/c i have the motor pulleys is about 4 inch dia, & the driven pulley a good 12 inch dia , the spokes on the big pulley are fan blade shaped to get a bit of air blast over the cylinders so 2800 rpm is going to be better than 1400 rpm in that respect.

though of course the big pulley is not doing 2800 rpm

It looks like just your average compressor as sold by Clarkes / machine mart , though i bought it from a company that went by the name SIP years ago, ..3 hp = about 2.2 KW there are quite a few house hold appliances that draw that these days so i dont really see any problem with using a 3HP motor . Clarks and co must of sold hundreds of them over the years!

Have seen some real old compressors that where really slow running , but with compressors its all about how much free air cubic feet per minute , which with a 3 HP motor is usually around 14 CFM

i agree that a 2 pole motor is going to be cheaper to make than a 4 or 6 pole motor but i still got a nagging idea it may be a bit more efficient but i dont know enough about electric motors to be sure about that ,…not much help really , well got to go put the kettle on that draws 2 Kw, thirsty work typing…………b

Edited By bodge on 03/01/2017 22:01:37

[MWParticipant @mw27036]

Given the amount of manufacturers who sell lathes and mills with motors today are 4 pole motors, I would jest that they seem to think this is the best compromise between speed and torque, I have on rare occasion seen 6 pole motors on lathes(around 900rpm tops?) but I haven't seen a realistic proposition for a decent lathe (of say more than 3/12" centre height) running on a 2 pole motor.

This is a bit of a conundrum because, if you wanted a motor that gave less torque but more speed, then you'd probably opt for a DC motor rather than an AC squirrel caged one, because it has a much smaller footprint. . Usually this scenario crops up where you need a fast motor for very small drills and milling, so therefore you aren't likely to consider a big motor because of the strain it would add to the weight of a mill head. So it renders that proposition practically obsolete.

If you wanted to run a general range from low to high speeds, then you'd go for a 4 pole motor, if you knew from day 1 that you only wanted a machine to turn tough grades of steel and nothing else, then you could look into a 6 pole motor.

I would go out on a limb here and say that these kind of 2 pole motors are intended for uses other than "machining" with cutting tools and more intended for grinding. 

Michael W

Edited By Michael-w on 03/01/2017 22:15:52

[bodgeParticipant @bodge]

Posted by Nick_G on 03/01/2017 18:09:39:

Posted by Ajohnw on 03/01/2017 18:00:28:

It's left me wondering if other than cost one type has other advantages over the other.

–

.

One runs at about 1400 rpm the other at about 2800 rpm

Different uses require different requirements for end application.

Nick

Yes i should think that just about sums it up .

As John says "would avoid 2 pole motors like the plague" me too, but they have there uses……b

[AjohnwParticipant @ajohnw51620]

I've never looked at single phase motor specs before. Turns out to be interesting. Have to compare 2hp motors as there are footnotes on the 4 pole 3hp one. These are for capacitor start, capacitor run

Full load running current 9 amps and 8.7 amps —- that's 2 pole then 4 pole rest will be too

Full load torque 5.01nm and 10nm —– so about what would be expected

Starting torque ratio 2.5 and 2 —— that's times the full load torque

Starting current ratio 5.75 and 5.1 —– that's times the full load running current.

Efficiency 78 to 79.5% and 76 to 80% —- They quote power factor figures against these the 4 pole low figure is better.

So it looks like 2 pole win out on stall torque to me. 25% increase for bit more than a 10% increase in current. Both 2hp ones seem to use size 90 frames. At 3HP the 4 pole switches to a 100 but the 2 pole is still in a 90. The running current of the 4 pole is 15 amps though rather than 13.2 for the 3HP 2 pole one. The 2 pole is also more efficient in 3HP.

Wow. I bet your all wanted to know this.

John

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[bodgeParticipant @bodge]

Hi John

Well it makes for an interesting foot note.

Also clears up for me on how i come to be thinking 3 HP 2 pole faired a little better, ( which i forgot to state is cap start & cap run on the compressor i have here ) its along time ago since i bought it about 20 years ago, I did look into it a bit at the time, and it goes some way to explain why the vendors go for 2 pole 3 HP rather than 4 pole it would probably blows most 13 amp fuses , as would think the ones that dont blow must be very close to the limit……so thanks for the info………………….b

[AjohnwParticipant @ajohnw51620]

I looked around at compressors before buying the Einhell one and asked why 2,800rpm motors. Not sure he said but think it's due to the stop start running. I wonder if it was just cost. Seems not.

If I do finish up putting another pump on the Einhell I would have been inclined to fit 4 pole.

John

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[Michael BriggsParticipant @michaelbriggs82422]

I am sure there will be other members on the forum that can throw more (accurate) light on this than I can. I suspected that a two pole motor would be noisier than a four pole, looking in an ABB publication they are quite a lot noisier though given the racket my Clarke compressor makes, you probably would not be able to tell the difference. Regards, Michael.

[bodgeParticipant @bodge]

Posted by Ajohnw on 04/01/2017 09:34:53:

I looked around at compressors before buying the Einhell one and asked why 2,800rpm motors. Not sure he said but think it's due to the stop start running. I wonder if it was just cost. Seems not.

If I do finish up putting another pump on the Einhell I would have been inclined to fit 4 pole.

John

One other reason i can think of is with over frame cooling on the motor one gets more air flow to cool the motor with 2800 rpm, just a thought.! , though it tie in with the stop / start running nature of compressors

I had a look on line for Einhell compressors but could only find small ones, any chance of a pointer where to look and what model to look for similar to the one you have.? …………………..b

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