Build thread: submicro brushless motors

[shagrug]

      On motor no.7 I went the other way, using a lower 20.2 ohm coil. I also used thicker wire this time, the .05mm (.00215"), with 175 turns.  I thought the thinner timer wire wouldn't have enough mass, so I used the thicker gauge. 

       As expected, more current draw- and a good bit more rpm's.  Also a bit of heat- slightly warm at 2.4v, and too hot after 20 seconds of 3.7v running. I understood the heat having talked to heliman (at RCG), who has been trying much lower ohm coils. It reminds me of many years ago making actuators for early micro rc systems (like Mattel). The coils had iron cores, and if you got the coil wrong they could get balsa burning hot. You had to find a balance of coil strength and warmness. I know the warming is a sign of getting the most useable power I can out of this size magnet- a good thing. It's really about right for flying, since the prop would have less load in flight (verses static testing), and should only need full throttle at takeoff and the occasional climbout.

      It has long bushings and reversed rotation, as I was thinking of adding a gearbox.

brushless motor no.7 test run

[shagrug]

      Motor no.8 is identical to 7, except the coil was upped to 28.8 ohms. Again using the thcker .05mm wire, with 240 turns. You can see the difference in copper mass  in the photo, compared to the 20.2 ohm

      This one stays cool at 2.4v, and gets warm (not hot) at 3.7v, as I was expecting. And the lower current draw didn't surprise me.  I was curious to see the rpm difference- and again it was slightly higher.

      These motors are certainly different in regards to brushed motors- which would be different on the rpm levels. I'm getting the feel for the right ohms on the 2.5mm size, and thinking this is close to the maximum power it can make with this simple coil design. I might squeeze more power out by changing the timing a few degrees- that is, moving the chip to over to one side of the coil. (advancing most likely) I'd like to make another test motor with a sliding-rotating paper tube over the coil, and the A1442 mounted to this to find the right spot.

      And these motors run much faster with no prop or load- I did some tests with the dummy prop in the photo, rpm's were over 32,000 at 3.7v. (and much more with no prop at all)  I don't  really have any interest in no load speeds, just  when using the plantraco 32mm prop.  It's a pretty heavy load for the 2.5mm, and easy to relate to pager power.  I favor using it too since most other microfliers will have one laying around to test with too. Using a thrust stand would be good too- but I'm too lazy. Both 7 and 8 are much stronger running  than the 3.2mm shicoh pager, and getting close to the 4mm pager- which I think is very good for the magnet size/motor weight.

      I've set up the rotation /bushings for the gear drive again....but I haven't decided which motor to use yet. Probably I'll use this one, since the current draw is less.

brushless motor no.8 test run

[shagrug]

      I was asked about making the mandrel, so below are some photos....

      I've been milling them from 1/4" brass rod- but you can make them from anything you want.  Plastic or wood might be easier to work with, I'm using the brass so the parts won't wear out. I knew testing would require a few motors, plus I plan to make more.

      The PTFE type teflon heatshrink is a great thing to have, it lets the finished coil slide right off. I bought about 10 different sizes this time (it's fairly cheap), so I can use it for actuator mandrels as well. The smaller sizes range from .002" to .006 in wall thickness- varies with how much you shrink it too. So you need to allow for this thickness when making the mandrel center. The less air gap (between coil sides and magnet the better, I've been using .005" or so. With such a close fit the center hole needs to be dead perfect...you can see more than one hole in my mandrel blank in getting it just right. Cut the heatshrink long, and trim after shrinking. The heatshrink comes in 2x and 4x shrink rates, the thinner wall ones are normally 4x. I make the side washers from .015" sheet teflon (from smallpartsinc. ), cut out with sharp edged tubing. The posts are from 1/32" diameter brass rod, this being the size of the bushings I've been making. The thick brass washers at each end allow me to wrap the start end around a couple of turns, this extra length you will need to test the ohms and hook up to the A1442.

[shagrug]

      My new gearbox frame is made from .020" thick "Garolite", a product I've seen several times before but never worked with. The Plantraco fiberglass control horns and wheels are made from it- and it's fairly strong and lightweight material. I counted 5 layers of fiberglass strands sandwiched inside, with a smooth outer surface on both sides. It is available from:
http://www.mcmaster.com/#catalog/115/3561/=5czjtb
      (Many thanks to Nick for this link)
     
      I first tried cutting it with my 4" blade table saw- then found a sharp pair of tinsnips works fine. The material cuts and machines easy, much like the .016" aluminum I used on the first gearbox. I found the weights very close between the .020" garolite and .016" aluminum too- only 3 grams difference per square foot. I would describe it as "fiberglass board", somewhat like unclad circuit board. And rigid- not bendable like aluminum.

      The finished gearbox mounted on motor no.8 weighs 0.37g, slightly heavier than the first due to the bigger coil. It runs nice and smooth too,  with no problems. When I get a prop made for it I'll do a test run to check the current draw again. I'd like to know before I mount it in a plane, to be sure the coil won't get hot and to know the load on the lipo..
     

     

[Blue Note]

That's awesome work.  Inspires me to try something myself, someday.  Love the avatar picture too.  Very fitting.

[shagrug]

      Thanks Blue Note.....but I can't take credit for the avitar, Kerbob kindly made that for me.

      Here is a test run of motor no.8 with the gearbox added and the 32mm plantraco prop- it will give a idea of rpm's and current draw before and after gearing. The motor stays cool at 3.7v now, so it is underpropped I think. I'm working on a 2.5" diameter prop now that should be better.

motor no.8 gearbox test with plantraco prop

[Valkyrie_1520]

Dan,

I have a stupid question. How do the RPM's of the electric motors compare to the RPM's of glow engines. With the RPM's that you were showing on this last test, it would seem that the RPM's are sufficient for a scale application with a glow engine. I prop my 4 cycle glow engines to get about 9,500 RPM's. My 2 cycle engines are propped at about 11,000 RPM's. Is there a guide somewhere that says an 'x' KV motor should turn 'x' RPM's?

Please pardon my ignorance. I'm still learning the electrics. 
 

[shagrug]

Hi Tim,

      Well, my experience is the electrics generally run higher rpm's and smaller lower pitch props than glow engines with regards to direct drive. Gear drives add torque and slow the prop down which makes them better for scale planes. I'd say the gearing makes it more efficient since run times usually go up, on the same battery. With different windings you can change the torque and rpm levels, and higher loads always give more heat to deal with.

      Glow engines (both 2 and 4 cycle) have a whole lot more torque for their size/weight, and I've only seen a few geared glow engines on planes, mostly just helis. Prop speed goes way down since the engine is more about muscle than speed. But again this is just generally speaking- you can get glow motors up to 60,000 rpm and more too.

      The brushless motors are stronger though, and changing things. I only started flying brushless about a year ago, so I'm still learning about them. I'm sure there are formulas to figure out what a motor can do in regards to kv-rpm, but they will be ballpark figures. Most motors produced now usually list tested rpms/amps with a popular size prop (or more) for comparison, which I like to see. I still usually just feel the heat on whatever motor I'm using (brushed or brushless) to judge if it's overloaded or not. I never like to get brushed motors too hot, they won't last long (as the brushes/commutator wear faster).... the brushless ones are more tolerant heat wise-but you still have a point of too hot/damage.

      I'm not sure if I'm answering your question.

[EricJ320]

Kv=rpm/v, or rpm per volt, so assuming you have a 1000kv motor, and loaded voltage was 11.0v, the motor should turn 11,000rpm with no load.  The Kv really doesn't tell you much about how a motor will perform, it's too dependent upon the size prop you want to turn to get the desired thrust and speed you're looking to achieve.  Kv really is best to help you decide which motor you should be looking to use with the prop you want to turn.

The power a motor can deliver has more to do with size than anything else in a brushless motor.  You could take a 1000kv motor in two sizes, say a 28mm 150watt and 35mm 400watt, theoretically they will both turn the same rpm unloaded, and may use close to the same prop size.  But the power extracted will be much higher on the 35mm motor, but furthermore, it is capable of turning a larger prop.  It's a heavier motor, with more momentum and larger windings.  Because of that, it can put out far more power without doing damage to the motor.  It can take more heat, but heat is wasted energy, and means the motor is operating more inefficiently.  So while touching a motor to see if it's operating at an acceptable  temperature is ok to make sure it's not hurting it, it doesn't mean it's operating at it's peak efficiency.

Kv is a bad place to start when choosing a motor.  You should start with size, or more importantly power.  Knowing how many watts you want for a specific model, then find a motor in that power range, then based on speed and thrust you want, decide what prop you will need and how fast to turn it, then choose the kv in that sized motor you've found to turn the chosen prop.  In other words, start with the plane, then a motor size, then the prop, and then choose the kv of the motor.  After that you can decide on the ESC and battery that will provide the appropriate amps that motor/prop will need.  It can be very difficult sometimes, throw into it different cell counts, and the choices become near limitless.  But as Dan pointed out, many of the manufacturers put out data that makes it much easier, and there are many calculators out there that can help as well.

Does that help, or just make it all more confusing?

BTW, Dan, very impressive work, as always!

[shagrug]

Hi Eric-

      That's a good explanation, many thanks.

      I would be more inclined to say the heat is a byproduct of work performed, rather than the heat being wasted energy. 

      All motors produce some heat (even unloaded)....And a bad motor design or other problem can waste energy and produce excess heat too- so it works both ways.

      Dan

     

[Valkyrie_1520]

Thanks for the info guys. I didn't mean to hijack the thread.

This is some great work Dan!

[EricJ320]

I would be more inclined to say the heat is a byproduct of work performed, rather than the heat being wasted energy. 

      All motors produce some heat (even unloaded)....And a bad motor design will waste energy and produce excess heat too- so it works both ways.

I totally agree that heat is a byproduct of the power produced.  In a perfect world, every amount of energy would be put to the prop, heat is energy that is lost through resistance within the motor.  No motor, including internal combustion engines are perfectly efficient, in that some power is lost creating heat instead of horse power.  The same is true for electric motors.  Think of kv as gearing in an IC engine, staying in 1st gear will get you from A to B, but the motor is not working very efficiently, will run at a high rpm, will get very hot, and may lead to early failure.  Picking the wrong Kv can do all the same things, it may do the job, just not very well, and possibly at a cost to the motor itself.  If a motor is running very hot, it's just not the right motor and/or Kv for the application.

Ok Dan, the thread is yours, back to your amazing work!
Eric

[shagrug]

      A couple of test props, with blades made from a plastic "AA" size battery package blister and hard balsa for the hub. The smaller is 54mm (2 1/8") diameter, the larger is 63mm (2 1/2"). Weights are .06g and .07g, pitch is just eyeballed. The bigger draws .02 more amps at both settings.

      I like the smaller one's shape better, plus I need smaller for a plane I'm working on. I'll need to paint the blades, next thing to do....

      I decided later today to make a quick thrust test, and made the rig shown in the last photo. I got .84g thrust at 2.4v, and 1.36g at 3.7v. Running at 4.2v gave 1.54g, which I think is very good.

gearbox test with 54mm prop

[Kerbob]

Nice use of the AA battery packaging for the props.  Also like your thrust measuring device.  Whats the plane going to be?

[shagrug]

Hi Kerbob-

I think it will ultimately end up in something like this.....

I picked it for several reasons- quick and easy to make, good wing loading, easy to change motors, etc., a good durable testbed. The all up weight w/10mah lipo will be around 2.4g (give or take 0.1). I'm using a rabbit rx and some of my actuators, busy winding them now. I'm pretty sure it will fly, but not sure how aerobatic it will be as a 4 channel. It might get a stronger motor later,  depending on how it goes.

One of these motors might also work well in the blue pitts you like- I've been eyeing that plan again. I'll see how this one does first, since they are close in size. But I've got several other scale plans I've been studying.I spent part of last night spraying inkaid, so I'll be ready for whichever plan finally I decide on.