[sillythings]

Hi all,

I want to upgrade the dc motor on my old x2 mini mill. I want a brushless dc motor with direct belt drive. However, the brushless dc motor with at least 1 hp rating are quite expensive; even on ebay. Also there is the cost of the control circuit and power supply.


However, there are some really cheap and powerful brushless motors for rc cars.

For example this one is rated at 1000 Watt (1.3hp) and its only $25.67!

XK3665-B-2500KV Brushless Inrunner

Specs.
Max Volt: 11.1V
Max Watts: 1000W
Rpm/V: 2500kv
Dimension: 36.0mm x 65.0mm

The electronic speed controller (ESC) on hobbyking are similarly powerful and cheap. Notice the motor is rated for 11.1v which means I can use a cheap PC power supply.

The max rpm is 2500 Rpm/V * 11.1 V = 27750. Therefore I will need a 10:1 pulley to bring the rpm down. I need to wire up a simple circuit to interface with the ESC to adjust the speed. I don't even need to worry about power protection since both the esc and the power supply already have them and temperature protection too!

I think this could be a really sweet and compact setup. Am I missing something??

[Stigoe]

The electronic speed controller (ESC) on hobbyking are similarly powerful and cheap. Notice the motor is rated for 11.1v which means I can use a cheap PC power supply.

I think this could be a really sweet and compact setup. Am I missing something??

Do you happen to have a cheap PC power supply able to provide 100A on the 12V line...? 1000W / 11.1V = 90A

[sillythings]

Thank you, very good point.

I didn't realize each 12 volt line in a 1000 watt ps only provide around 32amp.
1000W ATX Power Supply Quiet Gaming 14cm Fan Dual SLI Ready for VGA ATI NVIDIA | eBay

The above on ebay has two 12V 32A lines. Maybe I can wire the 2 12v line in parallel to get 64A (not sure)? If so, I'll get 11.1v*64a = 710 watt or 0.95hp.

Do you, or anyone, know if it is safe to simply wire 2 12v lines in parallel?

On hobbyking, there are other motors with 24v and higher voltage that dramatically cut down the max current requirement. We can also go with those and wire up 2 pc power supplies in series to get 24 volt.

I just realize the 10X pulley reduction is problematic for single step reduction. Probably need to step down in 2 steps.

Thanks again for your feedback.

[sillythings]

One more idea to get some feedback.

Most of the time, machining on my mill will not require 1.3hp. It is really the occasional high load (maybe lasting 10+ minutes) that needs to be dealt with. I wonder if there is a easy way to add a 12 volt lipo battery into the system to supplement the power supply. Perhaps we can connect the battery in parallel to the power supply output? rc car lipo batteries have no problem providing 100A current draws for 10 minutes. We can use high current diodes to make sure currents from the power supply does not go into the battery and vise versa.

Unfortunately, power supply design is not my area of expertise. Any suggestions are greatly appreciated.

[The Blight]

I believe I once saw one guy using a brushless outrunner for his mill, and he just had two car batteries hooked up to a charger for power.

[jfong]

You can buy 750watt 12volt Dell server power supplies for around $25 on eBay. We use them to fast charge lipo batteries. They put out 60amps at 12volts

This should be more power than the original brushed motor.

[sillythings]

Thanks jfong! Good to know that dell server ps put out 60amp at 12v! I don't have to mess with their internals!

[sillythings]

Just found this: A simple high quality 12Volt 100Amp Power Supply- Part1 - RC Groups

Apparently HP server power supplies can put out 100 amp at 12 volt! Muhahah, more power!

Let's tackle the next issue. Any suggestion on stepping down 27750 rpm to 2000 rpm? I like belt & pulleys because its quieter and stronger. However, is it reasonable to do a 14X single step reduction with belt and pulleys? Using a 2 step reduction is the obvious solution. Any better idea?

[jfong]

You don't have to limit the x2 spindle to only 2000rpm. I think hoss ran his a little over 4000rpm stock. Spindle does get warmer but new bearings are easy to replace. Higher rpm is great for cutting aluminum anyway. I would use a 5:1 or 6:1 pulley. That will also increase your torque for cutting.

[LongRat]

There are a couple of big issues here.
That RC car motor is not rated at 1000W for anything like continuous operation. These small motors are simply not able to shed the heat that would entail, and they will get hotter and hotter and quickly expire. The same thing is true of the smaller speed controls. There is a reason 'real' industrial motors with that rating are so much bigger, and more expensive. Without any forced air cooling I would be hesitant to draw more than 200W continuously from that motor. In the RC car world, the term 'continuous' never actually means that - usually it refers to 20 or 30 seconds if you are lucky. Be very careful specifying LiPos as they too suffer from this same rating ethos. A 100C discharge LiPo will absolutely not discharge at that rate from full to flat under any conditions at all.
There are plenty of examples on the forum of people who have successfully used RC brushless motors as spindle drives, but this is normally for high speed auxiliary spindles for small diameter cutting and engraving. If this is what you would require, I would look into a larger, higher kv outrunner style motor and a heavily over-specified ESC. But frankly, you would be better off with a small frame AC motor and VFD. They aren't too expensive in the size you would need and will run continuously all day and all night without any issue at all.

[sillythings]

Thanks LongRat. I thought the efficiency of brushless motors are in the 90% and higher. If so a 1000 watt going into the motor should only generate 100 watt in waste heat. That's similar to a 100 watt incandescent light bulb right? Wouldn't a good heat sink be sufficient? Also the mill typically go through long period of easy cuts interspersed with a few heavy cut. Maybe a rc race is actually more stressful?

Anyway thank you for the feedback. I'll do more digging.

[109jb]

I am a big fan of the RC brushless motors. Having been a modeler for my entire life they have created another avenue for enjoyment of the hobby. I actually flew one of my brushless powered RC airplanes yesterday. Having said that, I have my doubts as to their suitability for a milling machine.

The brusless motors that I use on my airplanes are generally in the 100-500W range. The propeller selection, which determines power output of the motor is done using a wattmeter to measure the input power. I set mine up to achieve 90% of the motor's maximum rated power input when full throttle is selected. Even when installed behind a propeller, and on an airplane moving in free air at 50 mph or more, the motors can get very hot. Too hot to hold your hand on for more than a second or two. For this reason, I would definitely recommend that if used on a mill, a fan to cool the motor be used, and a motor used that has a much higher advertised power rating than what you want. If I wer to do this and wanted a reliable 1kW out of the motor I would probably use a motor advertised as a 3kW or so.

The use on an RC car or airplane is a pretty short duration affair. I would say airplanes are harder on the motor than a car since a car has to be decelerated for every turn whereas I generally fly my airplanes at full throttle for the duration of the flight. Those flights are never longer than about 15 minutes. I don't track it, but I figure I get about 100-200 flights before I need to replace the motor or the bearings in the motor. That is only about 25-50 hours of intermittent use. For my smaller airplanes, which by the way are great fun due to lower cost and less care if crashed, I just throw the motors away and buy new ones. For my larger airplanes with the better motors, I have replaced the bearings. For a mill, I would not want to be replacing bearings every 50 hours.

As for the cheap motors, you get what you pay for. They are hit or miss. I have had some chepos that were fine and others that weren't. For a mill, I would not suggest a cheapo motor. The higher quality motors aren't that much more money.

RC cars generally use inrunner brushless motors and they have a high kV (high rpm/low torque) which means a large reduction drive for a mill. Airplanes generally use outrunner brushless motors which have lower kV (lower rpm/higher torque). If I were going to use a brushless RC motor for a mill it would definitely be an outrunner.

Machine spindles powered by brushless RC motors isn't new. There are tons of examples on the internet of people building spindles for their machines powered by RC brushless motors. The thing is though that you really only see the concept, design, and no-load testing. There is little information on actual cutting with them. I have only seen a couple videos showing these cutting anything, and believe me I have searched. The other thing that there is virtually no information about is the longevity of these adaptations. I have yet to find any information from those that have adapted RC motors that says anything about hours of use, failures, etc.

[sillythings]

Hi Blight,


Thank you for your suggestion of using brushless outrunner motor! Your suggestion is a much better solution than the rc car motor I had in mind! The belt gearing problem is solved!

For folks reading this thread, here is a link to outrunner motor https://en.wikipedia.org/wiki/Outrunner. These are the brushless motors often used in rc planes to turn large props without gear boxes. They turn slower but has much higher torque.

Here is a $41 motor from hobbyking NTM Prop Drive 50-60 Series 380KV / 2665W

Kv: 380rpm/v
Max current: 90A
Max Power: 2665W @ 30v (8S) / 2000W @ 22v (6S)
Shaft: 8mm
Weight: 468g

Prop Tests:
17x8 - 25.9v - 1900W - 73.5A - 7.03kg thrust
17x10 - 25.9v - 2124W - 82A - 7.26kg thrust

A maximum 2665 watt (3.5hp) in a puny 1 pound package! Eat that AC motor! Muhahaha!!

I'll connect up 2 server power supplies in series to get the 24 volt. I only need about 1000 watt output which means 42 amp current is sufficient (24V * 42A = 1008W). Even though the power supply can do all 100 amp!

The max rpm = 24v * 380rpm/v = 9120rpm

A easy single step 3:1 timing belt and pulley reduction will achieve ~3000 rpm. Or 4:1 reduction to run the motor cooler!

If heat is a problem (even though the motor is only operating at 40% capacity during the occasional heavy cutting), I'll point a cpu fan at it!

This thing has gobs and gobs of speed and torque! All crazy good!

[LongRat]

As I suggested, an outrunner is a much better idea. But you must be careful, with the exposed rotating outer magnets. If you are cutting anything magnetic, seal it all up in a housing and then your cooling issues will get more of a problem.
But I must reiterate, don't think these motors outperform AC motors of the same size - they don't. It's just that the ratings are calculated differently. Brushless motors for RC are given efficiency ratings yes, but that's at their max efficiency RPM. Manufacturers tend not to mention that. And only the very best would be at 90% even then. Chances are that under variable load cutting conditions you'd be operating at an RPM somewhat away from the max efficiency RPM (either higher or lower), which could pull the efficiency right down to 50% or lower. Then you are talking about dumping 500W+ of heat into something the size of a jar of baby food. That gets hot, fast.
My opinion is that these are good motors for this application, only if you need the high RPM they can deliver for engraving type applications. If you actually need speeds close to the stock set up, an AC motor and VFD will deliver a better result with great reliability for a similar cost.

[handlewanker]

Hi, have you considered a 3/4 HP 3 phase 3,000 rpm motor with a VFD?

They are quite small......approx. 120mm diam and 217mm long overall with a 11mm diam keyed spindle.

A small 3/4hp 3 phase motor running off of a VFD will run from the single phase 240 volt main supply, and it's speed variable via the VFD too, so no problem with reduction pulleys etc unless you want more speed or torque...... whatever.

Oh Oh.....you're in UK....hmmmmm..........I'm refering to some 3 phase motors on EBAY that are selling for A$59, but as they come from China they may be available on EBAY in UK too, or similar.......more details if you want specs of the motor.
Ian.