4 pole Vs 6 pole

[Stephen Beal]

I'm looking to convert a PTO driven roller mill to electric. The rolls need to run around 540rpm, so the motor will be geared down via pulleys.

Is there any advantage in running a 6 pole motor Vs a 4 pole motor?
I know a 6 pole will have more torque however it will have to be geared higher to achieve the same rpm, so presumably this advantage is mitigated?
6 pole motors seem slightly heavier build, would they be more durable?

Many thanks for your advice.

 

[Gr8blu]

First - a correction to your post. 4-pole (on 60 Hz) operates at nominal 1800 rpm. 6-pole at 1200 rpm. Since operating speed for the process is only 540, the "gear" ratio for the 4-pole will be higher (3.33 to 1) compared to the 6-pole (2.22 to 1).

Reasons to use slower speed motor (at same power output):
1) higher gearbox ratio results in higher gearbox cost (and typically maintenance)
2) higher speed machine is under more mechanical stress - which means increased likelihood of mechanical failure (bearings, etc.).
3) slower speed machine means more torque output - which typically translates as "larger / heavier"
4) If using pulleys instead of gearbox, the slower speed machine (6 pole) can typically withstand more radial loading on the shaft, which means less overall alignment issues

The higher speed machine may be cheaper, but by the time all the above is factored in, the net result could be a wash from an initial cost perspective. Over the equipment lifetime, the slower speed machine will generally work better / last longer / require less maintenance than the faster version.

Converting energy to motion for more than half a century

 

[LittleInch]

Could you not just run your 6 pole motor via a VFD and throw away the gearbox?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LionelHutz]

LittleInch - You'd need a motor with about 2x the HP requirement of the mill to do that. A 10-pole or 12-pole would be more suitable if doing that.

 

[LittleInch]

Might still be cheaper and simpler to maintain than a gearbox...

Just throwing it into the mix.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cranky108]

Consider the application and location. If a gearbox or VFD would be easer to work on/maintain. Also think about who will be able to work on it.
A VFD can be quite complex.

 

[LittleInch]

Well a PTO at 540rom is a tractor driven thing so I guess the most robust would be good, but a fixed speed VFD should be fairly simple to set up.

Or just strip a PTO drive off a tractor and fit an electric motor? what is the engine RPM for 540? 1500 RPM?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Stephen Beal]

Thanks for the replies.

I'll be using belts and pulleys to achieve the correct speed.

A VFD would be a neat solution if I was attaching the motor directly, although as mentioned I'd need something with a much higher rating to get enough output (22kw).

Thanks

 

[LittleInch]

Belts and pulleys eh? the definition of "agricultural" !

Just make sure they are guarded properly....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[edison123]

If budget and space is available, why not 10 pole 600 RPM machine eliminating all the belts and pulleys? Or you need precisely 540 RPM at all time?

Muthu

http://www.edison.co.in

 

[waross]

VFD to replace a 3:1 ratio? You would need a 3:1 HP ratio as well to develop the torque.
So motor 3 Times the needed HP plus the cost of a VFD?
At least double the cost, possibly triple the cost.
1800 RPM based motors are by far the most common and readily available motors.
1200 RPM based, More torque for the same HP, but more cost. This will probably cost more even if the HP is less.

Or just strip a PTO drive off a tractor and fit an electric motor? what is the engine RPM for 540? 1500 RPM?

At first look, wildly impractical.
At second thought, it may work well.
"what is the engine RPM for 540? 1500 RPM? About 1800 to 2100 RPM.";
1800 input would be usable.
"just strip a PTO drive off a tractor" Easier said than done.
The PTO is not a bolt on accessory like a fuel pump, alternator or govrnor (on some tractors).
The PTO is integral with the transmission housing, and the transmission case is probably the largest and heaviest component of the tractor.
Then I thought, I farming country I can't drive many miles without seeing several old. abandoned tractors sitting in fields.
If you are able to source a smaller older, no longer loved, tractor with possibly a seized engine, take out the engine and fit an 1800 RPM motor to the transmission input shaft.
Leave the wheels on to make it easier to move.
Maybe not for everyone but it is doable.
(The tractor may even be able to move itself on the end of a long extension cord. The world's largest drag-line walked around on the end of a 13,000 Volt extension cord.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Um, er, ten poles would be 720 rpm @ 60 Hz, no?

Shouldn't that be twelve pole [ two sets of six ] [ per phase ] for which synchronous speed would be 600 rpm? And since an induction motor at full load will run somewhere less than that at full load, would the slight overspeed above 540 be acceptable? This would completely eliminate the need for a gearbox, VFD or other speed reducer, as well as periodic belt replacement.

Keep it simple.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wayne440]

At one time Winpower made a PTO driven generator that could also be used as a 540 RPM power source if fed with 230V 3 phase.
Perhaps the gearbox from a PTO driven generator could be adapted to a motor.

 

[waross]

Compared to all the other suggestions, a 3/1 pulley drive will be the cheapest and simplest.
A standard 1760 RPM motor will give 587 RPM. Close enough for the farm.
If you want it closer to 540, buy a 3" or 4" adjustable pitch drive pulley and set the speed you want (At or below 587 RPM.)
If you buy a variable pitch pulley, do comparison shopping.
On a quick search, I see prices ranging from $14 to $140.
I expect you should get a suitable pulley for $35 to $50.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Something else to consider: it’s fairly easy to get a 4 pole replacement motor off the shelf in a hurry, 6 pole not so much. So consider your down-time costs.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

CR - Yes, for 60 Hz, it should be 12 pole to get 600 RPM.

Muthu

http://www.edison.co.in

 

[Stephen Beal]

Thanks again for the replies.

I'm actually on 50Hz here in the UK, so a 1/6 less rpm.

Jraef, good point about 4 pole motors being easy to source, I hadn't really thought about that!

As mentioned a belt and pulley system is the most economical solution. I won't have room to mount inline with the input shaft and also thought belts would dampen the vibrations from rolls the best.

Cheers