[MechanoMan]

I've been watching other build threads on DIY brushless attempts.
Instead of tacking onto others' threads, I'll do my own here.

I would like to achieve a wide-range, high speed spindle, up to 30K RPM sounds good.

I'm playing with ScorpionCalc: Scorpion Calculator (SC) - RC Groups
Which is supposed to be pretty good. We don't care about prop selection, so I have to tweak the prop diameter and pitch manually to achieve the desired watt loading to read the estimates of motor performance.

I think any reduction gear IS a huge mistake, and defeats the purpose. And it's unnecessary- these motors run just fine over a wide speed range!

The DIY brushless motor could probably compete well with the $300 2.2KW Chinese spindle + $250 VFD. That's a lot of cash. And the Chinese spindle doesn't have consistently good bearings- in fact, for the size they use, they're just not rated for high RPM. Even if you buy your own replacement precision bearings, I don't think they'll be capable of all that much more speed. WE can build a spindle with a cheap, blank ER8/ER16/ER20 precision shank where the shank diameter is lower. The smaller the bearing's diameter, the higher RPM it handles. I don't think a wood router, or even aluminum-cutting, spindle will have problems with a 12mm spindle- just my guess. We can get high rpm steel, even ceramic, bearings at a decent price in this diameter. Plus, the 2.2KW spindle is a 220V motor, which generally requires a 220VAC VFD, but AFAIK a router won't even draw more than a 110VAC outlet can do anyways. Not everyone has 220VAC available.

There are a number of design issues here of major note:
System voltage:
Voltage drop in the supply wires. If it's a router, this is pretty major because a 15 ft run is quite possible. If it's a low voltage system, like 12v, then the drops are problematic. Most RC systems use silicone wire because it won't burn up- but a 15 ft run of 14ga wire is 0.076 ohms. At 1/2hp, the current is 31 amps @ 12v, and 2.35v is lost in the wiring, which is FAR too much. If we go with a 48V system, 1/2hp is only 10.3amps, 0.783v dropped, which is only 1.63% of the source voltage.

There's several switching power supplies on eBay for 400W-500W in the 24V-60V range. I'm just not sure if that's enough. A switching supply won't tolerate overcurrent for even a second, that'll cause a foldback which could stall the motor entirely. The CNC software has no capacity to slow down the cutting if it's drawing too much power. But Taig's motor was only rated for 1/4hp- 186.5W.

Most of the RC controllers have a Vmax of 30V at most (~24v systems in practice). It'd be hard to find a 32v/48v/60v sort of controller, but they're actually not wildly complicated to build. In fact, due to their open-loop nature and other reasons, I'm pretty much "off" the idea of using an RC controller, period. I'd DIY a 24v controller if I had to.

Motor speed KV rating:
The KV rating of the motor is the RPM per volt when unloaded at 100% duty cycle. Yes, the units are totally nonsense ("K" means 1000, and it should be "/V", it should be "RPM/V"). 30K rpm on a 48V system would be 625KV, which is a purchasable item. You'd want to go a bit higher to ensure it can reach that speed under load, supply wire drops, controller drops, and winding resistance reduce the effective voltage by a bit under full load.

No-load current:
This looks like the dominant cause of motor heating, which has nothing to do with cutter loading.
This one is throwing me for a loop as far as understanding and getting data on. As far as I can tell, this is the current is supposed to be mostly constant over the voltage and RPM range- that is, if it's got a 1A no-load current on a 700KV motor at 14.3v=10K rpm, it requires 1A from the supply, which is 14.3W wasted as heat in the motor itself (pretty significant). At 21.4v=15K rpm, it's still wasting 1A, but that's 21.4W now.

Except that approximation is flawed- At higher RPM, the no-load current is higher. Unfortunately, almost all eBay suppliers who are so kind as to list a no-load current don't list the voltage it's supposed to be from, and the voltage the motor's designed for is a wide range.

Now here's something I'm looking at. On eBay, the powerful EMP3548 comes in multiple KV ratings. Same case design, more or less "the same motor" with different KV for comparison.
The 790KV is an Io of 2.1A. That's huge, and problematic for motor heating. That's 26.6W of heat to maintain 10K rpm unloaded.
There's a 1100KV, that would require less voltage to the wattage lost should be lower. But the Io is HIGHER, 3.5A. So at 10K rpm there's 31.8W of loss. again I don't know where these no-load Io numbers were measured so it's not conclusive.

What that's telling me is whether you go for a low-voltage, high-KV rating or high-voltage, low-KV system, the motor heating is similar.

Running the motor at lower speeds and actually using a STEP-UP pulley to get a higher spindle speed would actually make the spindle idle much cooler. But, this does exponentially increase resistive losses. The 790KV motor at 10K rpm direct-drive at a 1/4HP load would need 12.66V 14.7335A. The winding resistance is 0.048ohm, for 10.4W of resistive heating on top of the 26.6W Io loss. Actually much less than the Io loss. 37W total loss. So it's not actually gonna get all that much hotter when cutting as just spinning there.

If it were on a 2:1 step-up, it would halve the Io loss to 13.3W, but double the current and quadruple the resistive loss to 41.7W, 55W total.

If we used a 1:2 step-down, Io loss alone is 53.2W, resistive heating is only 2.6W, but the total's still 55.8W, quite high.

There's this glorious huge, low-speed beast for contrast:
http://cgi.ebay.com/C80100-KV130-EMP...#ht_3555wt_927
130KV, but a 6.5KW motor. That'd require 77v to get even 10K rpm, and I'd be afraid to put 230V on it needed to reach 30K- I doubt the winding insulation could even handle that. It's got an Io of 2A, which doesn't sound that different, but that's deceptive- 2@77v no-load would be 154W to idle at 10K rpm. It's a big motor and can dissipate more heat to stay cool, but that's a LOT.

Seems to me like the no-load current loss is easily a dominant form of heat generation. Changing the motor's KV doesn't seem to change the situation, but rather, the idle heating seems to be directly related to the maximum capacity of the motor. Bigger isn't always better. The 6.5KW monster above takes a buttload of power to keep turning, even if not used, and its low resistance windings don't even matter there. More weight in iron core, more iron core losses.

Heating at idle is not entirely a problem, as long as the motor's temp is within range. It seems like the bottom line is they're gonna take a lot of power to idle in a powerful motor, it's gonna get warm, and no gearing can fix that. But again, this just isn't a deal-killer as long as the losses are calculated and within spec. I don't give a rat's ass how much wall electricity is wasted as long as it works well and works reliably.

Well I'm a bit confused here. More technical notes to follow as I work it out.

I'd REALLY like to know how many watts a router motor actually cuts with on a router hogging out oak or whatever, and at what RPM. This would give me a great deal of insight to calculate from. From what I can tell, the 2.2KW Chinese spindle never actually uses anywhere near that, and I may be wasting my time trying to calculate and figure out the power supply for even a 1KW spindle. Like I say, Taig is only a 186.5W motor- a 500W drive motor sounds like quite a hogging beast, relative to that.

[MechanoMan]

Taking into account the KV1100 780W motor here:
Outrunner C3548 KV1100 brushless motor EMP for airplane | eBay

And plugging the size into ScorpionCalc, it estimates 0.3C/W (0.54F/W) temp increase. The max temp is around 200F, past that, the magnets' performance drops off (note that in an outrunner, the magnets are on the outside where they're not directly exposed to winding heat).

In a 100F hot garage, that gives us 185W of wasted heat (not power output) capability.

At 24v, we get 26400 rpm, and a 1.8A Io, 43.2W of Io loss. That plus 71.2A of current through the 0.028ohm winding is 185W, which would yield 1567W of work output. CLEARLY too high, doesn't pass a sanity check for a 780W motor. Unless they underrated for a lower max temp, doubtful.

I think my approximations are flawed here. ScorpionCalc evidently has some other math because they're not wholly consistent with my I^2*R+V*Io estimates. I suspect that's just due to them using a figure for copper resistance rising with high temps. Then again, for that motor, ScorpionCalc says I won't hit 200F until I load it with 1346.5W of output power (1708.1W in), so it's ballpark. In which case I'm confused, this is ballpark-ish with my estimate, but the eBay seller remains much LOWER. I'd expect the eBay seller to be much HIGHER than reality, but, for the figures the seller provided, both my hand calcs and ScorpionCalc say continuous value is far higher. I think maybe the prob here is ScorpionCalc's C/W heat dissipation is wildly better than reality, that directly affects power handling.

For an opposite case, slow milling- 2000RPM.
1.818V. Io losses are only 3.2W, ScorpionCalc says that they're lower, but I don't care too much, it's negligible. ScorpionCalc seems to be "breaking" for this low-voltage stuff so I'm going with my hand calcs.
It'll overheat at 81.325A, which is basically 147W of work output. Less than 50% efficient, but I really don't care about efficiency, just "what can it do?"

The 2K RPM case is pretty slow anyways. I mean, looking up, a 1/4" cutter through aluminum should be over 9000RPM. If you go with bronze though, it's way different, that could be as low as 1220RPM. I suspect the cutter will do just fine at this slow speed with 147W of work output though.

BUT, going on the assumption that SC's heat dissipation estimate was like 2x better than reality (which brings power handling back in line with the motor's stated power) then we're down to 70W of cutting power at 2000RPM.

[MechanoMan]

OH WAIT... I'm looking at the SURGE capacity of the motor in SC.

Change "motor ON time" to 60 min.
Motor reaches 200F in a 100F ambient with 1334W input power, 1135W output power.

Ah, now if you let it autocalc for "peak eff", it drops motor to 156.8F and input power is 865.8W in/752.7W out. THAT is consistent with seller ratings.

Novac here says 175F is absolute max, 160F is "good performance". A 155F limit sounds reasonable for milling use.

Interesting, if we bump that motor up to 36V, adjusting loading for 156.8F to match the other result, we get 37.8kRPM at....583.7Win/476.4W out.

[MechanoMan]

Well, on the "low" end, it's REALLY hard to get ScorpionCalc to yield a stable reading, the RPM varies wildly with load as you to to hit a 156.8F target temp. I got 156.5F, 2357RPM, 221.9W in, 111.2W out. At 4.2Vin, 52.823 amps- that's serious.

A 500W 24V supply is rated for 21A out. You get from 24V to a 4.2V "average" through pulsing (chopper), but the 53A average current is being pulsed too. That's approx a 1:5.7 "on" time to deliver current, so the pulses would be >300A peaks. The despite the supply's average current being under 10A, these pulses are brutally high. Capacitors can't readily smooth this out. Electrolytic caps themselves have an internal resistance high enough to render them useless at this level, and the transistors of a modestly designed supply can't switch this high of a current. And the pulse frequency is irrelevant.

For the matter, the wiring from the controller to the motor is a problem if it's got length to it. Even when the average current is the same, when you run that current 10x higher at 1/10th the duty, the average heating in the wire is 10x higher, and the voltage drop on the wire is 10x higher. This makes the motor RPM much more variable when loaded down.

Now if you say "ok, it draws 35A @ 24v, I have a 35A @ 24V power supply, if we do a 50% duty for half the speed, what happens if we stay within the 35A supply rating on the pulse peaks?" Then average current and torque is halved, and since RPM is half, the total power output is 1/4. An RC plane won't notice this because the a fixed-pitch prop's loading drops to 1/4 power when you halve the RPM. But a mill can load the motor independent of RPM, and may ask for higher torque on a low RPM job with a large endmill.

[JRoque]

Hi. Wow, that was some write up.

My 2.4kW Porter Cable router is a behemoth compared to those motors you mention. It spins at 21K RPM max and at full speed/power it gets bogged down if I dive it in too hard. They all do. And if that Porter Cable gets loaded like that, the motors you mention above will too.

The key ingredient missing from those hobby motors is torque. I seriously doubt they get much useful torque out of that 6.5kW input. Also, it's not easy to get 6.5kW out of a standard outlet. You'll need 220V @ 30A for that. You will also need a spindle with bearings that can take the cutting loads.

If you want to cut oak, your best bet is a wood router like the Porter Cable. It's got lots of torque, an integrated fan and the bearings to do the work. It will be loud and your oak chips will fly everywhere but it'll do it.

JR

[Pplug]

Hi. Wow, that was some write up.

My 2.4kW Porter Cable router is a behemoth compared to those motors you mention. It spins at 21K RPM max and at full speed/power it gets bogged down if I dive it in too hard. They all do. And if that Porter Cable gets loaded like that, the motors you mention above will too.

The key ingredient missing from those hobby motors is torque. I seriously doubt they get much useful torque out of that 6.5kW input. Also, it's not easy to get 6.5kW out of a standard outlet. You'll need 220V @ 30A for that. You will also need a spindle with bearings that can take the cutting loads.

If you want to cut oak, your best bet is a wood router like the Porter Cable. It's got lots of torque, an integrated fan and the bearings to do the work. It will be loud and your oak chips will fly everywhere but it'll do it.

JR

Hey if you really like that router then you should try a 3 or 2.2kw Chinese spindle & vfd. They are designed to retain torque even when cutting oak. I switched from a pc router to the spindle and it's no comparison. Plus the chips don't fly everywhere! And even the cheap Chinese bearings hold up better than the two in the pc.

[studysession]

Serious power and torque comes at a serious price -

Motors » Lehner Motoren Technik

Lehner has some great options but are very expensive

[MechanoMan]

Hi. Wow, that was some write up.

My 2.4kW Porter Cable router is a behemoth compared to those motors you mention. It spins at 21K RPM max and at full speed/power it gets bogged down if I dive it in too hard. They all do. And if that Porter Cable gets loaded like that, the motors you mention above will too.

The key ingredient missing from those hobby motors is torque. I seriously doubt they get much useful torque out of that 6.5kW input. Also, it's not easy to get 6.5kW out of a standard outlet. You'll need 220V @ 30A for that. You will also need a spindle with bearings that can take the cutting loads.

If you want to cut oak, your best bet is a wood router like the Porter Cable. It's got lots of torque, an integrated fan and the bearings to do the work. It will be loud and your oak chips will fly everywhere but it'll do it.

JR

Really? How many amps (or watts) is your PC rated for? Do you have it on an extension cord or anything?

You're misusing the term "torque". A hobby motor can most definitely have more torque than a PCR. The system will have peak power limits.

Yeah, you would not get 6.5KW out of a 110v socket. That's only for "reference". In fact, it seems like a DC power supply would itself be limited to around 500W.

A PCR, if I was gonna go that router, I'd go for a Chinese spindle. The noise of a router is just not practical for me.

[studysession]

The noise of a router is just not practical for me.

I want to reduce the level of noise from the router. I understand that cutter passing through the wood can still be loud on big jobs sometimes, but the noise level of the router is just way to much for me.

[MechanoMan]

Yeah most routers I know are brushed beasts. They make so much noise you can't be in the room for long without hearing protection, with or without load.

Those brushes are also prone to wearing out. And the armature itself wears out, esp if used with worn brushes. Brushless of course has no brushes. They have nothing that wears except the bearings.

[MechanoMan]

Serious power and torque comes at a serious price -

Motors » Lehner Motoren Technik

Lehner has some great options but are very expensive

Aside from being very expensive, I see no indication that these are any different than similar motors on eBay or wherever.
A brushless motor delivers torque (or power) at different RPM depending on a fixed set of formulas. Usually the power supply presents a limit before the motor does, or the motor presents a thermal limit for sustained use.

AFAIK the tech's limits are pretty much met in even the better cheap Chinese motors. You can't add more copper to reduce the resistance- there's no room. You can make the motor larger to increase the power, which is itself a problem for RC use, but it also increases the no-load losses.

[Fish4Fun]

MechanoMan,

You might have seen the thread I started on this same topic:

http://www.cnczone.com/forums/diy-cn...otor_bldc.html

I have been doing a lot of research on the topic and would be happy to share some reading materials I have found, but for now I will attempt to respond to some of your observations.

First, you need to understand that BLDC motors as a family are "constant torque" over a fairly large RPM range; however, this does NOT imply they are "constant power", quite the contrary. Rotational power is defined as Power = Torque * Angular Velocity. What this means is that for a given torque value, if you double the Angular Velocity you double the power. The second thing you need to understand is that, like many consumer goods, these motors garner their "power rating" from power consumption rather than shaft output power. This means that they are rated more on their ability to sink power than convert it efficiently into mechanical energy (here I am speaking specifically of the RC family of BLDC motors).

Please do NOT think I am casting shadows on these motors, I am fascinated by them; I just feel it is important to understand some of the marketing hype.

As for your observations about current/voltage and supply wires: For any real world spindle that is expected to run more than a fraction of an hour, supply wires are going to need to be sized appropriately. No getting around it. The torque developed in any particular commutation is a function of the current through the stator windings, voltage is primarily important in overcoming the inductance of the stator. In a nutshell, higher voltage forces current to flow faster which leads to a higher potential RPM (there are mitigating factors that impose limits to the "upper speed limit").

In a brushed DC motor we have to worry about armature and brush deterioration; with BLDCs, we have to choose a drive that is sufficiently rugged. The RC hobby does not require tight speed control (and has the luxury of a high current/low voltage supply), so commutation, by design, occurs based strictly on rotor position and power is varied to approach the required RPM. For a spindle we would want to "force" compliance to a preset RPM, so it makes sense to use a "synchronous" approach where commutation timing is "fixed", and power is adjusted based on "slip". The difference is both subtle in effect and distinct in approach. To be completely honest I am not sure what to expect from these motors. These motors are designed specifically to operate from high current, low voltage sources and have nebulous specifications. I plan to "play" with a couple of motors I have ordered. I am starting with a fairly standard driver and a low voltage supply, but really hope to move to a line driven supply early in the prototyping phase. 30kRPM in a 12 pole motor yields a 333uS (3kHz) Electrical commutation rate which implies a PWM rate of < 33uS (>30kHz). The switching losses are potentially significant, but efficiency is not the primary goal. In a real world design, step pulleys will almost certainly be requisite to achieve useable power over the 500RPM to 50kRPM range.

One of the observations you made was that the same motor frame was available in several different "KV" ratings. This is achieved by different stator winding configurations. The magnetic flux density in the stator is a function of both the current through the windings and the number of turns in the stator windings. Assuming the stator core permeability and winding resistance can be ignored (they absolutely cannot), you can adjust RPM per AMP by altering the number of stator turns. In a real world design, winding resistance and core permeability impose limits on both RPM and Torque, but still allow several viable configurations. In a typical 12 coil RC BLDC motor (four coils per phase), with each coil wound with the same number of turns of the same size wire there are 3 potentially viable wiring schemes: 1) All four coils of each phase wired in parallel, 2) All four coils of each phase wired in series and 3) two coils wired in parallel connected in series to another two coils wired in parallel. In both the motors I have, the connection appears to be all four coils wired in parallel. This configuration draws the highest current and presents the lowest inductive and resistive load.

BLDC motors reach maximum power at the highest RPM they are rated for. It is possible to drive them to a higher torque (at any given RPM) than they are designed for; however, this must be intermittent.

.......I started this reply last night, but work has been crazy and I simply have not had time to finish, so I thought I would post what I had so far....... This is a topic of interest to me, and I am actively experimenting with these motors and working on a driver for them.

Fish

[JRoque]

Hi. Fish is right in his assessment.

FYI, I don't run a wood router on my CNC machine. I built my own spindle and drive it with a DC motor. DC power supplies are not limited to 500 watts, I've seen 640kW ones. If I'm allowed another misuse of the term, brush motors of equal size have better 'torque' than BLDC.

An RC motor needs a spindle to do the cutting. There's just not enough radial and thrust load strength in those bearings and housing.

I like those Chinese spindles they're selling now. I wish they had them when I was building mine. A friend on this forum just installed a water-cooled one that seems great.

JR

[chcraft]

Just stumbled on this thread and thought I would add my two cents. For the past three years I have been using a 13.8VDC brushless outrunner motor paired with a mill head from Taig Tools as the spindle on my 24" x 24" wood router. The motor is rated at 500KV which gives me 5900 RPM at 13.8 VDC this is measuured with an optical tachometer. Using the pulleys supplied with the Taig Mill head I can get either 12,300 or just over 18,000 rpm at the cutter depending on the pulleys used. I am using a DC power supply rated at 46A at 13.8VDC. I have an ESC (speed controller) rated at 40 amps and use an Astroflight servo tester for variable speed although I normally run at maximun speed.

With no load the motor draws 3-5A and during most of my cutting I see a 10-15A draw from the motor using the meters on the powersupply. Using a .25" ball nose bit taking a .125" cut in white oak at 100" per minute I see about a 20A draw. While I am not running 8 hours per day 5 days a week I probably average 15-20 hours per week usage with most of that in 4-6 hour chunks. My typical project is a 30 minute to 2 hour cut time. I do a lot of 3D work in hard woods.

As I said I have been using this combination for three years and have finally worn the bearings in the Mill head out (they are getting very noisy) but the motor is still running fine. Sent the head to Taig for bearing replacement and just received it back. Working in my shop with the air temperature around 90-95 degrees the motor is not too warm to touch. I have some photos of the configutation that I can post if anyone is interested.

-------------------------------------------------------

Craft Enterprises, Inc.

[PaulRowntree]

I have some photos of the configutation that I can post if anyone is interested.

Please do! Did you do the design/integration yourself?
Cheers!

[chcraft]

Here are a couple of photos of my configuration. I assembled the components my self. In an old thread (search CNC9000) another user suggested the brushless motor and Nick Carter of CarterTools (The Taig Lathe and Milling Machine) helped me with the configuration of the Taig Mill components. I had to machine an adapter for the 5mm motor shaft to fit the .375 diameter drive pully for the taig mill head. I origonally used a Porter cable 690 router but it was very noisy and the fan in the bottom of the router blew chips everywhere. It also did not work well with small bits. The Taig has an ER-16 collet system which will take up to a .375" shank. Almost all of the bits I use are either .25" or .125" shanks.

The first image is a closeup of the spindle and the second shows the power supply in the background.

[MechanoMan]

Fish4Fun: Yes, the torque is constant- but you are neglecting the Io loss- which is easily much more than the I^2*R losses. And there's a problem in the driver providing the rated current in pulses.

JRoque: Yes there are certainly bigger supplies- but once you get into the hundreds of $ range, it is not cost-competitive with the 2.2KW Chinese spindle. For a router, the length of wiring run can become a major problem for high current/low voltage. In theory you could put the power supply on the router carriage and run 110v and control signals to it, but the weight would be awkward.

chcraft: Did you run the pulley on the BLDC's bearings, or drive a shaft on secondary bearings that held the motor pulley?

I did notice the Xbox 360 supply is a cheap, compact, readily available 12V power beast. The older style is almost 20A rating!

The Taig pulleys have been a persistent problem for me on the fastest and slowest settings- it slips too easily. I put as much tension as I can manage. This probably increases bearing heat on both sides.

Well, I'm not certain, but it appears the bearings in the Taig head- which I haven't used that much- just aren't good enough for fine PCB milling. I say that because when I put it at the top pulley (~10K rpm) with a fine carbide engraver (6 mil tip), it cuts great at first, then breaks the tip about 15 min into the run and it seems like it breaks the tip very easily if I replace it and restart. I think it's because the expansion of the steel bearing as it warms up increased the runout past acceptable bounds.

One could blame the collet or whatever, except it seems to be linked to the head heating up. That won't have a significant effect on the collet, but the bearings, being the very source of heat, might be the problem.

[MechanoMan]

They said the Taig head uses a 17mm ID/40mm OD bearing. That would be double-angular contact which might make it a 3203-2RS, which has a 18K max RPM on grease (21K on oil).

You might be able to find it in a higher RPM ceramic-hybrid or full-ceramic version. Ceramic is faster but not crazy faster- the primary benefit is they don't loosen up with heat like steel does. For this reason, I DO kinda want to try the Taig on ceramic instead, maybe with a belt-driven BLDC.

It's almost practical to still direct-drive over a wide speed range by swapping out motors rather than changing the belt setting. Weird- but the logic is there.

[chcraft]

MechanoMan: I am running on the bearings on the brushless motor. I have only had a couple of issues with slippage and those were a result of taking a much deeper cut with a large bit than I planned to.

As for smaller bits I routinely use ,065 diameter engraving bits with a .005 tip without any problems. I am however cutting wood instead of routing a PCB. I have attached an image of some wooden grips where the checkering was done with a 60 degree engraving bit with a .005 tip. The star was cut with a .0313 ball end bit. The 60 degree bit has been used to cut over 50 sets of these grips and has never broken in use. I have broken a couple by hitting them with collet wrenches or dropping then but never under normal usage.

[MechanoMan]

I went for the sharper 20 deg (and 10 deg) bits. The tips do break with just a little runout. The traces need steep walls, unfortunately. The board height can vary and with obtuse angle tips the trace gets thinned a lot if it cuts a thou or two deeper than at another spot.