Diy Brushless Spindle

[routercnc]

The motor linked to above is rated to 70A which should match OK with the 100A SC. In theory this would give 1920watts (24v x 70A) which sounds enough to machine wood. The actual amps pulled (for a given motor) will depend on the cutting load, but I'm not sure how easy this is to calculate. If you pull too many amps during the cut the SC will probably pop.

For this reason myself and other model aircraft users often use an ammeter to check the current draw on the ground for a given prop to make sure all is well before the first flight. If the SC gives up in the air, since it sometime also powers the reciever you loose the plane.

You could do the same thing for the first cuts to check your safety margin.

[paul.hunaban]

Yep power output would easily do the job. But, given the supply will be PWM then this shall have a resultant affect on the current drawn, dependant on speed required / length of pulse. What I hoping for is the input from those members who are now using these brushless motors to cut wood to help by letting us know from their experience, motors used – power supplies etc. I will poach with pride!! I’m happy to be told information resulting from experience as it does not always relate to theory...
Regards Paul

[Jonathan]

Power supply is tricky (unless you have a large budget) since 70 amps is a pretty large current:

http://www.ebay.co.uk/itm/HOT-Univer...ht_2890wt_1037

£40 for 20A .. not great, but there's another way. If you place a battery which can deliver the peak load in parallel with the ESC&PSU then the PSU only needs to supply the average current. This is where it's easiest to just measure how much current the motor draws when cutting and get the PSU to match. A standard multimeter wont measure 70 amps since they tend to be fused at 10-20A. Something like this is excellent as you can measure the current and it records the peak current:

http://www.hobbyking.com/hobbyking/s...Analyzer_.html

Or the cheap way is to use a multimeter to measure voltage across shunt resistor, which can just be a short length of wire of known resistance. I used one of the following batteries, with a 0-50V, 0-20A lab PSU in parallel on mine:

http://www.hobbyking.com/hobbyking/s..._Rex_500_.html

That battery is rated for 60A constant, or 90A for 10 seconds which is plenty. Since my PSU has variable voltage output and current limiting I can set it to just below the fully charged voltage of the battery (i.e. <22.2*4.2) and it will never overcharge the battery, which is very important since overcharging a lipo battery is a good way to start a fire. So whatever PSU you get if you decide to use in combination with a battery will need to have good stable voltage regulation below 25.2V. Two computer ATX PSUs in series is nice and cheap and will give very close to 24V, however it is CRITICAL to ensure they are electrically isolated before putting them in series. The battery would never fully charge, but that's a good thing if anything as it increases it's life expectancy. You could get the battery first, use it to power the motor to measure the current then buy a PSU to suit.

We can estimate what power you require; what cutter diameter and depth of cut do you generally use (at 2.5m/min presumably)?

For example:

12mm cutter, 2.5m/min and 60% stepover at 6mm depth of cut is 108cm^3/min material removal rate (12*0.6*2.5*6=108). That's about 740W required...and a pretty rigid machine to sustain that. So you'd ideally want a PSU rated for a bit more than that to be safe. Also it's a good idea to put better capacitors on the ESC, especially with such long wires, to reduce the impedance...

Hopefully it'll turn out the motor draws a lot less current and you can just use a standard PSU.

[vtcnc]

That battery is rated for 60A constant, or 90A for 10 seconds which is plenty. Since my PSU has variable voltage output and current limiting I can set it to just below the fully charged voltage of the battery (i.e. <22.2*4.2) and it will never overcharge the battery, which is very important since overcharging a lipo battery is a good way to start a fire. So whatever PSU you get if you decide to use in combination with a battery will need to have good stable voltage regulation below 25.2V. Two computer ATX PSUs in series is nice and cheap and will give very close to 24V, however it is CRITICAL to ensure they are electrically isolated before putting them in series. The battery would never fully charge, but that's a good thing if anything as it increases it's life expectancy. You could get the battery first, use it to power the motor to measure the current then buy a PSU to suit.

Thanks Jonathan for this very helpful information. I hadn't realized that it was possible to connect ATX PSUs together in series to increase voltage -- just assumed they were regulated in a way that would prevent this. Makes sense really, when I think about it. Battery cells have nominal voltages and can be series connected into a battery of higher voltage. Somehow I got hung up on the regulated part of it.

Anyway, I've gone ahead and ordered two ATX supplies with 12V 30A outputs, and looking forward to making a 24V supply with them for a spindle PS. I will isolate the negative buss from the case, and I know about setting up the switch line and adding a starting load resistor on the 5V lines to keep the supplies running.

I build and fly R/C planes, so I'm familiar with the rest of the motor side components. I have two 5055 1500W rated motors -- one 400 kV and one 540 (I believe) -- both have 8mm shaft dia. I have an 8mm ER-11 collet spindle, and collets.

ps. I also wanted to post a possible correction to the above -- I think the bolded text above should read "(i.e. <6*4.2)"

Thanks for your PSU info!

[vtcnc]

(deleted and prior post corrected)

[paul.hunaban]

Jonathon this is really good stuff, please keep it coming.

So if I’ve understood this correctly, you are running a 70A motor supplied by a 20A power supply supported by a battery for peak current demands? What voltage do you have the supply set at?

I trust you are controlling the motor via a speed controller, I’ve ordered this one today “100A Brushless Motor Speed Controller” but just realised it has an input voltage of 22.5, which may now give me an issue.
Speed controller purcahsed from ebay: http://www.ebay.co.uk/itm/1606311667...84.m1497.l2649

I don’t understand your translation from the material removal rate of 108cm^3/min to the 740w motor requirement. Can you please explain or point to a source of information. Frankly I’m surprised a 740w motor will cut that much, I actually cut with a 6mm cutter and no deeper than 2mm with a 500w router (anything else frightens to much!!), so what would the motor requirement be for 18cm^3/min (6*0.6*2.5*2=18).


Thanks paul

[Jonathan]

I'm actually running a 120A, 130kV, 6.5kW rated (yeah right) motor from a 100A ESC, currently on my micro lathe but I have used it on the router - somewhere in this thread. This motor is good up to 50V, so I should use a HV ESC to get the full speed but I've not got round to buying one. The rigidity of the micro CNC lathe limits the power draw somewhat as it's not rigid enough, so it will only draw a few hundred watts. If I use the motor at a low speed with a big drill it could be more. I leave the PSU at about 24.5V, but it's really not critical so long as it's not above 4.2 volts per cell (i.e. 6*4.2=25.2V). It's nice to have the lab PSU, but it's not worth buying one just to power a spindle. Down to about 23V would be fine as far as the battery's concerned.

The listing for the ESC states 22.2V - this is because they expect people to use Lipo batteries, which are nominally 3.7 volts per cell so a 6 cell battery is 22.2V. However when fully charged the same battery will be 4.2*6=25.2V, so the ESC will withstand 4.2V. Since it's a cheap one it would be prudent to run a little below that.

I read somewhere that hardwoods take about 7W/cc/min to cut, so 108*7=756W. That makes 18cc/min about 18*7=126W. Bear in mind since this is motor output power we need to include the efficiency of the motor.

The motor I linked to was based on 12000rpm, but with a 6mm cutter you could be using almost twice that? I've cut some hardwood, not sure what it was, with a 6mm single flute carbide cutter at 6m/min and 6mm depth of cut and 20000rpm. However if you're machine can only manage 2.5m/min then a lower spindle speed is fine, but perhaps aim for 18000rpm? Although having said that the motor is most efficient when at maximum speed.

Anyway, if it's 126W and even if we double it to make sure, at 250W then you've got plenty of options for the PSU. At that power the battery is hardly necessary, but it would make a very robust system if you did have one. I'd mount the battery and ESC on the Z-axis as close as you can to the motor to keep all the wires short, then have long wires from the PSU to the motor. This will make the ESC run more efficiently, i.e. cooler and in extreme cases can make the difference between it surviving and not.

[m_c]

Jonathan, have you ever had any problems with cell balance using a LiPo?
I'm just wondering how actually using a multicell pack without balancing it works out longer term, as my current project needs a 2cell pack, and I'm trying to decide if I really need to include a balancer circuit.

[Jonathan]

Jonathan, have you ever had any problems with cell balance using a LiPo?
I'm just wondering how actually using a multicell pack without balancing it works out longer term, as my current project needs a 2cell pack, and I'm trying to decide if I really need to include a balancer circuit.

Not had a problem, except when the battery is already on it's way out. With a 2 cell battery I doubt you need to bother with balancing.

[m_c]

Not had a problem, except when the battery is already on it's way out. With a 2 cell battery I doubt you need to bother with balancing.

It's one of these things that gets mentioned alot, but not sure how much difference it's going to make in practice. I'm probably going to at least add the circuitry to the PCB as I've spare ADC pins on the uC anyway, and might built a couple prototypes with and without the extra components just to see what happens. I'm aiming for longetivity, so it may be a case of every little bit helps!