Thread: Diy Brushless Spindle
Thanks Adam , Neil! I think I will give it a go, It looks very interesting! :-)
I want to change out my wood router for a brushless spindle as I’m having to replace the brushes all too often.
What you guys have done here is very interesting but I’m not sure how to size the motor / what motor to get.
Therefore I have a few questions:
Q1, if the RPM/v is rated at 295 does this mean for every volt it will increase by 295 revs, i.e. for 20v the speed would be 5,900 revs?
Q2, would you read the power output in the same context as you would a wood router, i.e. if both were 1000w would both be deemed to have the same output?
Q3, Any recommendations for motor to cut wood (Oak) ? At the moment I do a lot of 3D profile work thus cut at around 2.5m per min.
Q4, should I give any consideration to the speed control other than it meets the spec of the motor, ie is there a rule of thumb for the control to have say a 10% high power rating?
Many thanks in advance Paul
1) Yes that's right.
2) Hard to say - the ratings for the brushless motors are a bit optimistic since they assume excellent cooling from the very high airflow present in a model plane. What power rating the motor is will be dictated by the speed rating since 'hobby' brushless motors with a similar kv specification tend to have similar power ratings. That's unless you introduce a belt into the system...or couple multiple motors on one shaft, but that's probably taking it too far for machining wood.
3) Clearly depends on the tool diameter, but I'm guessing you'll want about 12000rpm for oak? So look for a motor with around that speed rating. Ideally you don't want to have to use 'high voltage' (i.e. above 25V) ESCs as they're very expensive. So roughly 12000rpm/25v=480kv, or more if run from a lower voltage...
Your next problem is the shaft diameter - the one above is only 6mm which makes it a bit useless unless you have a lathe to modify/make a collet chuck...
5) Easier to advise once the motor is chosen, but it's sensible to get a bigger one than rated to be sure. They're pretty cheap from the site I linked to above.
Jonathon, thanks for the speedy reply.
I’ve just order from Hongkong the 100a motor control plus a drive module so that’s dealt with. I’m not too concerned about the shaft / collet holder / bearings as I will hand that off to my brother to sort (although he does not know it yet!) and he’s tooled up for the job.
As to the power supply, seems your suggesting a 24v, how about the amps? I’m not sure how the PWM supply to the motor is translated for the source.
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.
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...
Power supply is tricky (unless you have a large budget) since 70 amps is a pretty large current:
£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:
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:
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)?
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.
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).
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.
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