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High Speed Spindles - ATC

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Robert LaLonde

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#1
I've about decided that when the current high speed spindles on my small mills start acting wonky I'll replace them with some ATC spindles with the ISO20 taper. I probably won't build a proper ATC carousel or even a fixed stand. I just want the fast tool changes and repeatable tool heights using the tool table built into the control software.

In my searching I noticed there are 400Hz and 800Hz spindles rated at the same net power and peak RPM. A little digging shows the 800hz spindles are 4 pole and the 400Hz spindles are 2 pole. That's exactly as I figured. Not accounting for motor slip 60RPM per Hz is normal for 2pole motors and 30 for 4 pole.

Now some folks may say it takes a special VFD for the 4 pole motor, but some research shows many reasonably priced VFDs have the capability to produce higher frequency output. Its just that 400Hz equipment is so common.

Now I think the 4 pole motor will have more torque at lower RPM, but I'm not sure. Its seems like it should even if it has a similar linear horsepower curve to the 2 pole motors. I know my 1.5KW spindles still have some ability to cut down to about 8000RPM. Its fractional horsepower, but its consistent and reliable if I plan my cuts accordingly. Most of the time it makes no difference to me really. I tend to small cutters at or near maximum RPM. By extension do you think I would have similar net power at 4000 RPM with a 4 pole high speed spindle?

There are times when it would be nice to get the surface speed down a bit more. I recently found myself having to finish some steel parts on the little high speed mills. (My big mill lost an axis motor) I was able to do it, but sometimes there was a light show and boy howdy was it hard on tooling. If I could shift the torque curve up with a 4 pole 800hz spindle for those rare occasions when something goes sideways on another machine and I have to get a job done I will go that way. They are expensive spindles, but the difference in cost is negligible.
 

magicniner

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#2
High speed integrated spindles don't produce good torque at lower rpm, that's why commercial VMCs use big 3 phase motors with step-up gearing or belts, they don't use the power at high revs but the big motor ensures adequate torque at low revs, 7kW isn't uncommon in a modest VMC.
I'm considering a BT30 Skyfire spindle and 3 phase motor for a small VMC build this year.
 
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Robert LaLonde

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High speed integrated spindles don't produce good torque at lower rpm, that's why commercial VMCs use big 3 phase motors with step-up gearing or belts, they don't use the power at high revs but the big motor ensures adequate torque at low revs, 7kW isn't uncommon in a modest VMC.
I'm considering a BT30 Skyfire spindle and 3 phase motor for a small VMC build this year.
I am aware of that, but it does not answer the question I asked. It boils down to this.

What is the difference between a 1.5KW 4 pole spindle turning 24,000rpm at 800Hz and a 1.5KW 2 pole spindle turning 24,000rpm at 400Hz other than those differences inherent and stated as part of this question?

More specifically is there any difference in the torque curves over the same ranges of RPM?

A difference in a couple hundredths of a horse power can be significant at lower RPMs however. It can mean the difference between there being a way to do a job and not being a way to do the job. Or at the very least it can mean there is an opportunity for significantly different approach to a job.

I realize you can't compare a 1.5kw motor to a 7.5Kw motor and no part of my question was intended to in any way shape or form. Big motor. Little Motor. 20 pound motor. 200 pound motor. I get that. I can see that. I can feel that. I'm not asking about that. LOL.

Of course to muddy the waters the voltage levels can be tweaked at lower RPMs on some better quality water cooled spindles to actually change the torque curve and give a little more power at lower RPMs, but that can quickly overheat and burn up a cheap motor or a poorly cooled motor. For the sake of this query lets just assume a standard linear or near linear setup without advanced programming.
 
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Robert LaLonde

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I knew as soon as I posted before that I should have worded my original query differently.
 

magicniner

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Looking at the torque spec for 3 phase motors of the same power and frame size and comparing torque figures for the 2, 4, 6 and 8 pole models will give a definitive answer.
A friend of mine who uses a high-speed spindle on his mill is about to swap to a conventional milling head because he needs lower speeds than the high-speed job can work at.
 
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JimDawson

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Normally a 4 pole motor has twice the torque of a 2 pole motor for a given HP, but they normally turn at 1/2 the RPM. Now, would that allow you to slow down a 24K RPM, 4 pole motor to 4K rpm and still have be it useful? I don't know. If you use a sensorless vector VFD then it could develop full torque at nearly 0 RPM, but the motor might melt down into a puddle of molten iron and copper. :)
 

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I knew as soon as I posted before that I should have worded my original query differently.
Bob, part of what I was getting at with my first reply is that if there was a way to make a high speed motor produce usable torque at low speed without gearing of some sort then there is a good application for that in commercial VMCs, and yet everyone goes for the big motor to supply adequate low speed torque despite significant reductions at low speed.
 
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Robert LaLonde

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Bob, part of what I was getting at with my first reply is that if there was a way to make a high speed motor produce usable torque at low speed without gearing of some sort then there is a good application for that in commercial VMCs, and yet everyone goes for the big motor to supply adequate low speed torque despite significant reductions at low speed.
Oh, I get it, but I'm not talking about 5HP at 3600 RPM and 5HP at 96 RPM.

I had hoped my second post would get the point across that even little as 0.02 more reliable horsepower at "lower" rpm could be significant when problem solving at the edge of possible in order to get a job done. I am aware that "More power!!! Grunt! Grunt! Grunt!" is the easiest way to solve a problem, but its not always the fastest or the most practical in every possible set of circumstances.

Normally a 4 pole motor has twice the torque of a 2 pole motor for a given HP, but they normally turn at 1/2 the RPM.
Yep got that and even detailed it above. ~ "Typically, except for motor slip 30 rpm per Hz and 60 rpm per Hz respectively."

Now, would that allow you to slow down a 24K RPM, 4 pole motor to 4K rpm and still have be it useful? I don't know. If you use a sensorless vector VFD then it could develop full torque at nearly 0 RPM, but the motor might melt down into a puddle of molten iron and copper. :)
 
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Robert LaLonde

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What is the difference between a 1.5KW 4 pole spindle turning 24,000rpm at 800Hz and a 1.5KW 2 pole spindle turning 24,000rpm at 400Hz other than those differences inherent and stated as part of this question?
 

magicniner

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4/5 of 5/8 of Nak all, but 4 poles give more torque at lower speeds, but still not enough for low speed machining with HSS tools on materials that require it ;-)
 
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Robert LaLonde

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#11
I think you are missing the fine margins I am talking about. I've machine medium carbon steel like 4140HT on spindles in this class at high feeds and speeds. I'm not guessing about that. Just trying to quantify the differences.
 

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#12
Bob, from someone who works in the motor drive industry, be aware that there is a US government (and EU) restriction on the manufacture of variable frequency drives which operate beyond 590Hz. This is due to limit the potential export of motor controlling electronics which could be used in nuclear centrifuges. This was spawned by EU Regulation No. 428/2009. You may have difficulty finding a suitable VFD to run these motors at 800Hz as all devices after 2009/2010 should be in compliance.
 
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Robert LaLonde

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That is good to know, but there are 1000hz VFDs upto 4.5kw available on Ebay from domestic vendors. They look like the Chinese made Sunfar VFDs. I had one rated at 0.8Kw some time back for use with a 60,000 rpm spindle. I didn't look any further than that for right now.
 

magicniner

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#14
I think you are missing the fine margins I am talking about. I've machine medium carbon steel like 4140HT on spindles in this class at high feeds and speeds. I'm not guessing about that. Just trying to quantify the differences.
Bob, it looks like your answers will come from manufacturers.
 
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