# In Search of A Better G0704 Spindle



## MontanaAardvark (Sep 26, 2017)

If there's one area of my mill that seems to make sense to upgrade, it's the spindle.  At least, if I keep working mostly in aluminum.   I use G Wizard Speeds and Feeds calculator, and I find that it virtually always tells me to max out my spindle at its 2200 RPM.  On steel (and I don't have a ton of experience here) the RPM is usually much lower and the spindle seems adequate.    

I'm really not that determined to increase the horsepower.  From what I see in G Wizard, most of the time I'm limited by cutter deflection and not the spindle HP.  Plus, I think these are _systems_, and if the motor is too powerful for the rest of the mill, it's more likely to cause the mill or the drive components (ballscrews, etc.) to deform too much.  I'm not going to throw out a good deal on a 1-1/2 HP motor, but I'm not looking for a 2 or 3 HP option, either.   

I have the option of a conventional 220 outlet near my mill.  If I stick with 120V, the computer, lights, control box, and everything are running on one 15A circuit.  I don't think adding more power on 120 is a good idea.  

What I'm looking for is something in the 1 to 2 HP range, that will do 5000 RPM (as a guess).  220 AC is preferred.  I'd like to control it from inside Mach3, so I think that means an encoder and I suppose that means replacing the control box on the side of the head stock.  I expect I'll have to rebuild the headstock, at least partially.  Probably improve the bearings.  It would be nice if got quieter.  

I really don't know what I'm getting into or how involved it's going to be.  Can the whole thing be done for $500?


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## JimDawson (Sep 26, 2017)

MontanaAardvark said:


> Can the whole thing be done for $500?



Probably not using new hardware, using used hardware....maybe.

220 (240) volt is the only way to go.  Not sure that Mach3 requires an encoder for spindle control, I don't think so.  You will need a BoB that has a provision for spindle control if you don't already have one.  You will want a 3 phase, 4 pole (1725 RPM) motor and a single phase input VFD rated for the motor.  Most of the small (<5hp) Baldor motors are rated at 6000 RPM max, I wouldn't try that with an import motor.  Carefully check the motor specs before running one up that high.  I would recommend going direct drive and use a sensorless vector VFD.  A 2 HP would be my choice for your machine, you don't have to use all of it, but it will give you better low speed performance.  I can run mine from about 10 to 5000 RPM.  My mill is quite a bit bigger than yours and I'm running a 3 HP Baldor, has plenty of power.

For the bearings, you're on your own there.  Can't offer any advice.


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## dave2176 (Sep 26, 2017)

For spindle bearings look to what Hoss did (Daniel last name fell out of my brain at the moment). He has G0704 dot com. I would also go with a 2 HP 3 phase motor plus VFD. Running a 3 phase motor on single phase 220 only produces 2/3 of the rated horsepower. Starting with 1-1/2 HP would only yield 1 HP output and the ponies seem smaller than they used to be years ago.
Dave


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## spumco (Sep 26, 2017)

Yes, you can - but just barely and depending on components selected.

Basic components:
220v 3-phase Ebay motor, look for "vector duty" (I'm partial to Marathon BlackMax motors)
VFD with 220 1p input (as Jim mentioned) and 3p output.  Automation direct's GS-3 series is quite nice and has optional high-speed encoder inputs.  I went with a Hitachi VFD, and it's worked well.
BOB (or sub-board) with 0-10v output to control the VFD.  Mach3 outputs a PWM signal that the BOB converts to the 0-10v

_Optional_ upgrades:
- Belt drive conversion -* highly* recommended.  You won't need the gears if you get the right motor and pulley ratio.  Determine lowest reasonable speed you'll be cutting anything (for me it's tapping at 500RPM), and the max spindle speed you want.  When you're shopping for a motor, check the name plate to see what the max safe RPM of the motor is and multiply that by your pulley ratio.  There's your top end.

Belt drive also lowers the head weight, and is much quieter.  No more oil in the head - no more dribbling.  No gear backlash so rigid tapping is possible.

Your spindle bearings will eventually get wrecked, but just run the stockers for a while with some good grease and when they give up the ghost replace them with as nice a set as you can afford.  Don't worry about them right now.

- Encoder for spindle and/or motor.  This will permit rigid tapping and much more accurate speed control if the VFD gets information back from the actual motor RPM.  No more 'bogging' down on heavier cuts unless you overload the VFD completely.  It also permits significantly increased torque at low RPM (like 200% over name plate in some cases), so your 'loss' of low-end torque with a 1.5:1 belt drive is no longer an issue.

Check your control software (Mach3?) and see if it is capable of dealing with a pulley ratio on the motor while still permitting rigid tapping.  If not, you may be stuck deciding if you want rigid tapping or feedback to the VFD - you can't have both in this case.

- External braking resistor for VFD.  Permits stopping the spindle RIGHT NOW.

*Here's a really slick idea...*

Buy a big 1.8kw (2.5hp) servo and DYN4 drive from DMM Technology and install it as the spindle motor.  No separate VFD needed as you'll be using their servo drive to power it.  The drive takes 220v 1phase.  Already comes with an encoder, and Mach3 is capable of driving a spindle with step & direction pulses.  You get:

1. Smaller, lighter motor than the equivalent power induction motor
2. Motor is new, not used.
3. Motor and drive are a set from the same company - no tearing your hair out with incompatible signal voltages or encoder signal types.  Trust me, this is huge.
4. Built-in extremely nice encoder, with  - and this is important - encoder signal pass-through features.  The encoder signal can be sent to Mach3 or other controller so you can have a real-time RPM display.
5. Factory made shielded cables so you aren't fiddling with sizing big 8ga wires from the VFD to the induction motor and chasing down RFI/EMI.  VFD's are noisy.
6. Spindle positioning capability.  Make a single point tool and have the spindle index it - instant broach for square corners or internal splines!

Has a 3000RPM max, so a 2:1 pulley ratio would be the ticket for your aluminum milling.  And you'll have gobs more low-end torque, even with the pulleys, than your current set-up.

$378 for the motor
$267 for the drive
$60 for two cables (signal and power)

*OR...
*
Go with the .75kw (1hp) 5000RPM servo (half the price) and do a 1.25 or 1.5:1 pulley ratio.  Less money, still plenty of torque down low for steel cutting and bigger drills, and potentially a higher top end.


-S


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## Boswell (Sep 27, 2017)

spumco said:


> Here's a really slick idea...



I have not had a big desire to modify my gearhead PM-45M CNC mill, but this is a very compelling plan.


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## MontanaAardvark (Sep 27, 2017)

spumco said:


> *Here's a really slick idea...*
> 
> Buy a big 1.8kw (2.5hp) servo and DYN4 drive from DMM Technology and install it as the spindle motor. No separate VFD needed as you'll be using their servo drive to power it. The drive takes 220v 1phase. Already comes with an encoder, and Mach3 is capable of driving a spindle with step & direction pulses. You get:
> 
> ...



This _is_ a really cool sounding idea.  Seems like it hits all the features I'm looking for.  

My whole thing here was "what are other people doing, or wish they should have done?"

I need to look around a bit.


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## spumco (Sep 27, 2017)

When my motor, VFD, or anything else spindle-related breaks, this is what I'm going to do.  I may not go with the DMM servos, but I'm absolutely going to have a spindle servo.  All the 'grown-up' VMC's I've seen or investigated use servos because you need the indexing/homing capability to align the drive dogs on CAT30/40/50 taper tool holders.

I researched used servo motors and drives for a while before punting when I stumbled across a really nice, really cheap Marathon motor.  I was pretty overwhelmed & discouraged because new servo/drive packages from brand-name sources are unbelievably expensive, and trying to find just the right motor-drive match in a sea of used ebay stuff takes a degree in servo tech.

If I ever start making things for profit and not just fun, the first $1k my mill makes me is going towards servos for the x/y/z axis.  The second is going to a higher-speed servo spindle setup I outlined above.


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## MontanaAardvark (Sep 27, 2017)

spumco said:


> ... and trying to find just the right motor-drive match in a sea of used ebay stuff takes a degree in servo tech.



Ain't that the truth.  I could use a primer on spindles, motors and controlling them.  Applications notes from the manufacturers are usually a good place to learn.  I need to learn what techniques are common, who makes what parts, the trades between different approaches and all that practical knowledge that comes from working in a specialty.

I understand servo controls in general.  I'm a EE and I designed feedback loops for a living for 30 or 40 years.  In very different context.  No motors were involved, everything was at frequencies motors can't work at.  I should be able to understand this stuff, but it's a whole different field from anything I've had to study.


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## cjtoombs (Sep 28, 2017)

dave2176 said:


> For spindle bearings look to what Hoss did (Daniel last name fell out of my brain at the moment). He has G0704 dot com. I would also go with a 2 HP 3 phase motor plus VFD. Running a 3 phase motor on single phase 220 only produces 2/3 of the rated horsepower. Starting with 1-1/2 HP would only yield 1 HP output and the ponies seem smaller than they used to be years ago.
> Dave



The loss in horsepower is a feature of solid state phase converters, not VFDs.  VFDs will provide the horsepower they are rated for at the machines design RPM.  Lower RPM than the design RPM decreases horsepower linearly (half the design RPM, half the rated horsepower).  This is why folks above are recommending 2 to 3 hp motors, because of the HP loss at lower RPM.  Speeds higher than the design RPM are at the rated horsepower.


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## cs900 (Sep 28, 2017)

cjtoombs said:


> The loss in horsepower is a feature of solid state phase converters, not VFDs.  VFDs will provide the horsepower they are rated for at the machines design RPM.  Lower RPM than the design RPM decreases horsepower linearly (half the design RPM, half the rated horsepower).  This is why folks above are recommending 2 to 3 hp motors, because of the HP loss at lower RPM.  Speeds higher than the design RPM are at the rated horsepower.


I'm no VFD expert, but I do believe TORQUE is constant up to the rated RPM of the motor (usually 60Hz on the VFD).  So while while I agree with what you say bout HP, torque is what you actually care about as it is the force you can deliver to the cutting edge. 

That in mind a 1.5 HP motor rated for 3600 RPM (2.19ft-lbs) will need to be geared up 39% to get up to 5000RPM. You'll get an equal loss in torque, so at full speed you're only at 1.33ft-lbs. Compared to a 3 Hp you're at 2.66 ft-lbs. I believe this to be the reason people choose larger motors.

Also interesting, and to compliment what cjtoombs pointed out, after 60hz HP stays constant and torque drops off non-linearly. So running your motor right at 60Hz is the peak of HP and torque! That's good to know if you plan on doing lots of a particular material, you can plan your gearing around what HP/torque your application will require.


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## cjtoombs (Sep 28, 2017)

That torque is more important than horsepower is a common misconception.  If you gear a 1/4 hp motor down to 1 RPM, you will have about 1300 lb*ft of torque, but you won't be able to do any more work with that than that same motor turning 5000 rpm with about 1/4 lb*ft of torque (ie.  neglecting gearing inefficiencies, your metal removal rate would be the same if the full 1/4 hp were being used).  That's because horsepower is the measure of the amount of work that can be done.  Torque kind of equates to voltage in electricity, it is the amount of force with which you can rotate a shaft like voltage is the amount of pressure exerted to move current.  Without the RPMs for shaft rotation or the amperage for electricity, neither torque or voltage can produce work.  The linear torque relationship of the VFD is what translates to the linear Hp curve, since the relationship is linear.  The loss in torque does become a problem for VFDs, as large cutters run at low motor speeds tend to stall in the work at anything approaching reasonable cut depths and feed rates (I have way more experience with this and I would like).  The stalling is a function of torque, since the movement stops.  That's generally why it's best to err on the side of a large motor, or better yet size the motor for the minimum horsepower you want to cut at.  If you need to slow a nominally 1800 RPM motor down to 900 RPM and you want 1 hp at the shaft at all times, you need a 2hp motor.


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## spumco (Sep 28, 2017)

Which is why if someone is shopping for a motor, they'd be well served to look for the highest CT ratio (constant torque) possible.

3P, 1.5hp motors:
IronHorse general purpose - 4:1 CT, ($175) nameplate torque available from 1750 to 437rpm without overheating
Marathon BlackMax- 1000:1 CT, ($454) 1750 to *17.5rpm* without overheating....


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## cjtoombs (Sep 28, 2017)

Yea, that's the other problem with running motors at low speed, the fans don't work and they overheat.  I think some of the motors specifically designed for this have fans driven by a separate motor.  Not a problem for fanless motors that rely on sheer mass for cooling, but most of those are antiques.


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## mksj (Sep 28, 2017)

TENV and TEBC motors (typical inverter rated motors) are designed to be used down to ~0 RPM without overheating (they run cooler at low speeds), the primary problem with the more traditional TEFC motor is that they tend to overheat at low or very high RPM. Most conventional rated 3 phase 1750 RPM motors when driven by a variable speed drive are usually limited to an operating range of around 15-90 Hz, but one still has the limitations that the Hp will be 1/4 of the rated nameplate at 15 Hz.  The constant torque range is ~4:1 for standard 1750 RPM motors, inverter motors are for all intensive purposes maintain full torque down to 0 RPM (CT is usually > 1000:1 ratio). To some degree VFDs can compensate for short periods to increase the rated current/output, and inverter motor can be pushed a bit further, around 150-180% for 1 minute. Conventional motors would be something in the 120% range.  I use an inverter 2 Hp motor on my lathe set to 150% overload setting, and it will still trip the VFD running at 60Hz in a deep continuous cut with large stock. The other factor that is often overlooked is the gearing/belt ratio and the applied power to the spindle. If you run an inverter motor at 120 Hz on a 2:1 ratio to the spindle speed vs. a conventional motor run at 60 Hz with a 1:1 ratio, the inverter motor will be delivering twice the spindle Hp and the spindle torque will be about the same. On a mill of this size, one would most likely not be able to hang a 2Hp 3 phase inverter motor on the head of this mill, nor do I think it is practical. If you look at small CNC mills, they all overdrive their motor usually to something like 6000 RPM. So my recommendation in this setting for this size mill would be a 1 or 1.5 Hp inverter motor with a sensorless vector VFD, or a BLDC motor with controller which would offer more Hp in a more compact package.  A number of individuals have done CNC conversion of larger mill like the PM932 and PM940 and the 2 Hp Marathon Black Max motors have been more than adequate for their purposes.  They have used a 1:1 drive ratio if I recall. I use a Baldor IDNM vector motor on my lathe, it is absolutely quiet and smooth at speed.

The discussion of using an encoder for spindle feedback is appropriate for full blown CNC machines, but would be an expensive and unnecessary adventure in this level of CNC conversion. The main reason for encoder feedback, is for precise spindle positioning,  maintaining a 0 Hz position or very low speed control, and absolute speed accuracy which exceeds 0.01%. The accuracy of a sensorless vector VFD/Vector or BLDC motor is 0.1%, usually withing +/- 1 RPM even under varying load. Both my current lathe and mill which use sensorless vector control maintain this level of speed control. There are also other variants of feedback control, w/wo an encoder.  Many VFDs do not have the ability to use encoder speed control feedback, those that do are usually 2-3X more expensive.   The VFD frequency range on my lathe is 20-125Hz using a TENV inverter motor, the mill is 20-200 Hz and uses a factory TEBC inverter motor. The other possibility as other have mentioned is to use an integrated motor/controller system, some possibilities are below. The major limitation I see is the Kw motor rating and the limited top speed ability of anything over 1 hp (0.75 kW), as mentioned previously you could go with a DMM 2.5 Hp drive and use a 1:2 drive ratio  which is a very nice combination. 
https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/
http://www.dmm-tech.com/Dyn4_main.html

Baldor 1 Hp inverter motor $200-300. A decent VFD will be about $250. 
http://www.ebay.com/itm/Baldor-Inverter-Drive-Motor-Inventory-Clearance-/182739811235
http://www.ebay.com/itm/BALDOR-1-HP-3-PH-INVERTER-DRIVE-MOTOR-NEW-/191731639588

If one looks at the application in practice, one needs to look at the purpose of the machine. In general, a CNC mill conversion is looking at higher speeds, and the need for low RPM is not a priority, nor would you expect to use it to be used to bore large holes with a large drill bit. If you need precise control to do say tapping, then you are looking at a completely different scenario/cost factor. I do not see the practicality of trying to do a CNC conversion and also expecting to drill holes at 100 RPM. The flip side is that you need speed, so you are looking as something like a usable speed range of 600-6000 RPM, maybe a bit lower with a 2 speed head. Standard gear head nor the bearings will  live very long at these speeds, so you are looking at a belt drive.


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## MontanaAardvark (Sep 28, 2017)

mksj said:


> TENV and TEBC motors (typical inverter rated motors) are designed to be used down to ~0 RPM without overheating (they run cooler at low speeds), the primary problem with the more traditional TEFC motor is that they tend to overheat at low or very high RPM. Most conventional rated 3 phase 1750 RPM motors when driven by a variable speed drive are usually limited to an operating range of around 15-90 Hz, but one still has the limitations that the Hp will be 1/4 of the rated nameplate at 15 Hz.  The constant torque range is ~4:1 for standard 1750 RPM motors, inverter motors are for all intensive purposes maintain full torque down to 0 RPM (CT is usually > 1000:1 ratio). To some degree VFDs can compensate for short periods to increase the rated current/output, and inverter motor can be pushed a bit further, around 150-180% for 1 minute. Conventional motors would be something in the 120% range.  I use an inverter 2 Hp motor on my lathe set to 150% overload setting, and it will still trip the VFD running at 60Hz in a deep continuous cut with large stock. The other factor that is often overlooked is the gearing/belt ratio and the applied power to the spindle. If you run an inverter motor at 120 Hz on a 2:1 ratio to the spindle speed vs. a conventional motor run at 60 Hz with a 1:1 ratio, the inverter motor will be delivering twice the spindle Hp and the spindle torque will be about the same. On a mill of this size, one would most likely not be able to hang a 2Hp 3 phase inverter motor on the head of this mill, nor do I think it is practical. If you look at small CNC mills, they all overdrive their motor usually to something like 6000 RPM. So my recommendation in this setting for this size mill would be a 1 or 1.5 Hp inverter motor with a sensorless vector VFD, or a BLDC motor with controller which would offer more Hp in a more compact package.  A number of individuals have done CNC conversion of larger mill like the PM932 and PM940 and the 2 Hp Marathon Black Max motors have been more than adequate for their purposes.  They have used a 1:1 drive ratio if I recall. I use a Baldor IDNM vector motor on my lathe, it is absolutely quiet and smooth at speed.
> 
> The discussion of using an encoder for spindle feedback is appropriate for full blown CNC machines, but would be an expensive and unnecessary adventure in this level of CNC conversion. The main reason for encoder feedback, is for precise spindle positioning,  maintaining a 0 Hz position or very low speed control, and absolute speed accuracy which exceeds 0.01%. The accuracy of a sensorless vector VFD/Vector or BLDC motor is 0.1%, usually withing +/- 1 RPM even under varying load. Both my current lathe and mill which use sensorless vector control maintain this level of speed control. There are also other variants of feedback control, w/wo an encoder.  Many VFDs do not have the ability to use encoder speed control feedback, those that do are usually 2-3X more expensive.   The VFD frequency range on my lathe is 20-125Hz using a TENV inverter motor, the mill is 20-200 Hz and uses a factory TEBC inverter motor. The other possibility as other have mentioned is to use an integrated motor/controller system, some possibilities are below. The major limitation I see is the Kw motor rating and the limited top speed ability of anything over 1 hp (0.75 kW), as mentioned previously you could go with a DMM 2.5 Hp drive and use a 1:2 drive ratio  which is a very nice combination.
> https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/
> ...



Thanks for this.  Lots to think about.  

You talk a lot about a machine this size and that's welcome.  I know that G0704 is a relatively small machine, while bigger than the more common SIEG X2-size mills.  It's not something like a Haas or Tormach, and one of the things I think about is that just because I have CNC control of four axes doesn't fundamentally change the machine.  I don't think adding ballscrews turns a $1400 G0704 into a $10,000 Tormach or $40,000 Haas.    

Ultimately, it all comes down to "one needs to look at the purpose of the machine" as you say.  I'm not likely to be doing large pieces, exotic geometries or exotic materials.  If I get one to do, I'll muddle through as best as I can.  I'm not thinking I need to do tapping under CNC in the spindle, although I could see where that could be a really cool capability.  To be honest, the machine is pretty good as it is, and it's a good compromise.  I'm not in business, I'm a hobbyist.  I think that means I'm sort of like the small job shops that might work on custom pieces for different companies, a few of several different parts every week.   

I noticed that the closest equivalent-sized Tormach models use 5000 and 10,000 RPM spindles, of about the same 1 HP size.  That's what made me think I should upgrade the motor to run 5000 RPMs.  10k would be better for engraving, but I don't do much of that.   

All I'm looking for is:
About 1HP.  I'm not going to throw away a clearly better 1-1/4 or 1-1/2 HP motor, but the 1 HP range is around what this was designed for and it makes sense to not get too far from there.  As a bonus, CNCCookbook's G Wizard speeds and feeds program can run free for life if you limit it to 1 HP.    
I'd like to get to 5000 RPM, or close.  
I'd like to turn the motor on and off with the SW (Mach3) and set the spindle RPMs from inside the G-Code, too.  
It would be very, very convenient to run the motor on 240V.  I have high current service into that corner of my shop, more than a motor this size would want.  (I think it's 220 with a 50A breaker).  Right now, I'm running my computer, my CNC Control box (motor drivers with a 50V, 10A supply), the spindle, and some lights (LED)  all on a 120V 15A circuit.  I have this deep fear that if everything should hit its max current at the worst possible moment, that the breaker may go.  

That's it.  Positional accuracy enough to turn a tap and back it out is interesting, but not a strong requirement.  

You said:  "So my recommendation in this setting for this size mill would be a 1 or 1.5 Hp inverter motor with a sensorless vector VFD, or a BLDC motor with controller which would offer more Hp in a more compact package."  I think my emphasis will be the BLDC motor.


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## spumco (Sep 29, 2017)

For the price of a BLDC and driver _from a known source_, you can get the DMM servo package in 1-1.5hp.



mksj said:


> On a mill of this size, one would most likely not be able to hang a 2Hp 3 phase inverter motor on the head of this mill, nor do I think it is practical.



Very, very true.  Mine's in the almost-PM932 size, and my 2hp BlackMax is - frankly - too big.  Mine is the cast iron frame motor, and the sheetmetal version of the same motor would probably be a bit more reasonable.

There are a couple 1hp sheet metal BlackMax motors on ebay for about $200 shipped.  Sheet metal frame would be smaller/lighter than the Baldor above (or other cast-iron motor).  Size 56 frame would probably work OK for the G0704 head.

Automation Direct GS3 sensorless vector VFD for $242.  And an encoder input card is only another $50.  Very nice drive with instructions in English.


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## MontanaAardvark (Sep 29, 2017)

spumco said:


> Automation Direct GS3 sensorless vector VFD for $242. And an encoder input card is only another $50. Very nice drive with instructions in English.



That's this one, right?  The VFD, without a motor?

I was just looking at Automation Technology because that's where I got my Steppers and Stepper controllers for the CNC conversion.  A stepper is a BLDC motor as well, and my only issue is converting stepper motor specs into HP.   These NEMA42 steppers are spec'ed at 4230 in-oz.  In a stepper, the torque is ordinarily maximum down to zero RPM (you'll see it called holding torque) and and stays constant as RPMs go up.  After some RPM threshold torque goes down as the RPMs go up.   

I found a site that tells me 4230 in-oz = 29.9 Watt-seconds, and I think to convert to total watts, you divide by the number of seconds in a revolution, then divide watts by 750 to find the number of HP.  That says the HP depends on the RPM that motor is running, but like I said, steppers tend to have constant torque vs RPM over much of their range.  I don't know if it's done at the RPM value where the torque starts dropping or if everyone agrees to solve it at some number of RPMs.   I can turn  29.9 W*s into just about any number for HP depending on how I set up the problem.   

I'll send an email to AT to see if they have helpful numbers.


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## spumco (Sep 30, 2017)

MontanaAardvark said:


> That's this one, right? The VFD, without a motor?


Yep, that's the bunny.



MontanaAardvark said:


> A stepper is a BLDC motor as well,



I'm not a motor expert, but for your and my (layman's) purposes - no. _[I know there are some really bright electrical people on this forum.  If you can explain this more simply, or correct anything - please do so.]_

A BLDC motor has a feedback signal to the drive/amplifier.  It's usually a 3-position (120 degrees apart) Hall effect trigger.  Without brushes and slip ring, the only way to commutate the rotor poles is with a feedback devise, such as an encoder (expensive) or a Hall-effect trigger (cheaper).  The drive receives the position/rpm signal back from the motor and can adjust the commutation timing and amperage to achieve & maintain commanded RPM.

A stepper does not have a separate feedback device to permit the drive to adjust its output to match reality _*(ignore 'closed-loop' stepper products for the moment)*_.  There are a bunch of individual winding pairs inside the housing, and the rotor has a bunch of 'teeth' around the circumference.  Opposite pairs of poles/windings are energized sequentially, and because the rotor 'tooth' is drawn to the energized pole, the rotor turns to the newly-energized winding.  If you fiddle with the voltage/amps so that one winding has a bit of juice, and the next one has a bit more, the rotor tooth is pulled partway to the second one.  Result - microstepping.  The big difference is that the rotor teeth are, simplified for my small brain, what is commutating the stepper motor and not a feedback device.

Steppers lose torque as RPM goes up.  Gecko drive systems has an excellent series of articles on how steppers work, but essentially it takes time for the windings to energize, and it takes time for the rotor teeth to react and move to the magnetic field, and at some RPM it simply can't move fast enough to keep up with each sequence of winding pulses. RPM maxes out, and there's no torque available.  Figure about 1000RPM is the upper limit for run-of-the-mill steppers and drives before they can't do any useful work beyond just turning themselves or an unloaded axis ball-screw.

BLDC motors don't have 'teeth' like steppers, so they're more like AC servos than steppers in that they have a few rare earth magnets on the rotor pose and a series of windings.  The difference you and I need to know about BLDC and AC servos is that BLDC, while having feedback, are 'dumb' and can't position.  The 120 degree rotor sensor pulses are simply too far apart for positioning, and the drives aren't designed for anything other than RPM control.  AC servos have encoders with many more pulses per rev (thousands in some cases) so they can be positioned.

Here's the other part - grown-up servo/drive combinations can also be programmed to run in RPM mode and not just step/direction.  You command 1000rpm, and the drive spins at 1000rpm, having done the math to know how many feedback pulses it should receive per time unit.  You don't have to keep sending it step and direction signals - it just runs like a 'normal' motor, but with the ability to sense & increase current if the RPM dips due to load.  When you're ready to position the spindle (alignment, tapping, whatever) the computer puts the servo drive in position mode and then issues a series of step/direction moves.  Spin end mill, stop, rotate to index the tool holder drive dogs, hold position rigidly, tool change, back to RPM mode and making chips.  This cuts down on the number of signals needing to be sent to the motor, especially at higher RPM (high frequency of step pulses).

Pretty cool.

So Steppers can be used as spindle motors, but that would be a bad idea due to the rapidly dropping torque and comparatively low top RPM limit.  A BLDC motor is way better, but it's crippled when compared to AC servos.  If reliable BLDC motors & drives (think Tormach 440) were available at 1/10 the cost of AC servos it'd be a no-brainer, but with the cost of decent servos being so low, and much more reliable than 'dodgy import' BLDC drives, the only advantage BLDC motors have is that they generally have higher RPM limits.

You want 10kRPM?  BLDC, cause a 10kRPM servo is going to be outrageously expensive.  You can live with 5kRPM and a pulley system to get you 10k?  AC servo all the way.

-S


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## markba633csi (Sep 30, 2017)

Isn't a BLDC motor similar to a stepper without the "teeth"? Don't they both have permanent magnets? 
Mark S.


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## MontanaAardvark (Sep 30, 2017)

markba633csi said:


> Isn't a BLDC motor similar to a stepper without the "teeth"? Don't they both have permanent magnets?
> Mark S.



Yup.  If you look up stepper motor on Wikipedia (for example), it says:

A stepper motor or step motor or stepping motor is a brushless DC electric motor that divides a full rotation into a number of equal steps.​It's one of cases where this specialty has adopted a specific meaning for BLDC.


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## JimDawson (Sep 30, 2017)

MontanaAardvark said:


> These NEMA42 steppers are spec'ed at 4230 in-oz.


If you can get more than 600 RPM out of that stepper I would be surprised.




markba633csi said:


> Isn't a BLDC motor similar to a stepper without the "teeth"? Don't they both have permanent magnets?
> Mark S.



They are similar in that they both have permanent magnet rotors.  But the BLDC is a 3 phase motor, where the stepper is a 2 phase motor.  There are some other differences in construction also.

An AC servo is a BLDC motor with an encoder on it to feedback the position data to the controller/drive.

By definition a servo is any closed feedback loop system.  Anything else is open loop, where the controller has no idea what the controlled device is actually doing.  The heating and/or air conditioning system in your house is a servo system.  Also your car is a servo system where you are the controller.


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## MontanaAardvark (Sep 30, 2017)

JimDawson said:


> If you can get more than 600 RPM out of that stepper I would be surprised.
> 
> They are similar in that they both have permanent magnet rotors.  But the BLDC is a 3 phase motor, where the stepper is a 2 phase motor.  There are some other differences in construction also.
> 
> ...



Jim, pardon me, but this terminology confuses the heck out of me.  BLDC stands for "Brushless DC", right?  An AC servo is an AC motor, not DC.  So how are they the same?  From what I can tell, there are both single phase and 3 phase BLDC motors, so is nobody using the single phase?  

One of the things I'm stumbling over as I try to learn this stuff is interpreting the words I see, so at this point I'm easily confused.


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## JimDawson (Sep 30, 2017)

It is confusing.  A BLDC motor is a 3 phase AC motor with a permanent magnet rotor.   The drive input can be DC, single phase AC, or 3 phase AC depending on the system.  But the output from the drive to the motor is 3 phase.  For instance, the Milwaukie Fuel series of battery drills have BLDC motors in them, the motor itself is 3 phase.  All of the magic is done in the controller.

Commonly there are 2 main parts to an AC servo drive.  First the rectifier section to convert the input AC to DC, then the DC is converted to a variable frequency 3 phase output to the motor.  This is true for VFDs also.  But a VFD does not have the close control that a servo drive does.  With modern VFDs, this line is beginning to blur a bit.

There is a lot of confusion surrounding the term ''servo'', it is misused a lot.  As you know, it is not a servo system without a closed loop.  A BLDC motor could be purpose built to be installed in a servo system, and hence the inaccurate term ''servo motor''


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## spumco (Sep 30, 2017)

I find that trying to understand every single variable in motor/drive terminology has rendered me senseless.  As Jim wrote, it's confusing.  What I've done - in my mind - is categorize the motors (likely used in hobby CNC applications) to types with_ significant_ differences.

1. Stepper (permanent magnets, lots of poles, used for non-feedback positioning in most cases, low max RPM)
2. Induction (typical 1 or 3phase motor driving spindles and so forth.  Big drill press kind of thing)
3. BLDC (permanent magnet, windings energized with DC at highish voltage, commutated using hall effect sensors, not used for positioning.  Higher RPM and greater power density per weight or volume than Induction or stepper)
4. Servos
4A. DC servos (brushed DC motor with encoder for feedback.  Used for positioning.  Old-school CNC axis servos.)
4B. AC servos (brushless permanent magnet, windings energized by magic, used for positioning and/or constant RPM, has an encoder for feedback to the drive enabling commutation and precise positioning.

Yes, there are overlaps in function and terminology but I don't have the reserve brainpower to identify the nuances.  How much HP, and at what RPM?  What's the max RPM, and is the torque curve fairly flat?  Can I position with it, and do I need a magic box just to spin it up? How much does it cost?



JimDawson said:


> With modern VFDs, this line is beginning to blur a bit.



For example, my VFD can position my induction motor due to the encoder, but it'll never be as precise as a true servo simply because the encoder has a much coarser resolution than is typical on 'servo' packages.  That, and the rotor weighs a _ton_ compared to a permanent magnet servo motor of the same nominal power - it just can't start or stop as fast as a lightweight servo rotor.

On the other hand, you don't see 400hp servos (magnets are the limiting factor in high-HP applications), but you do see induction motors in the hundred (or thousands) horsepower range.  Pump motors at a municipal water plant come to mind.

-S


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## MontanaAardvark (Oct 1, 2017)

I found a technical note from a company that makes controller chips for motors, Monolithic Power Systems.  It discusses a lot of these things with diagrams, waveforms and such.  I'm finding it helpful.   It's incomplete and doesn't include servo motors. 

It's at http://www.monolithicpower.com/Port.../appnotes/Brushless DC Motor Fundamentals.pdf


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