“The parts which didn't break in the crash were too heavy!”

  A series of articles on electric flight by Rob Ireland

DO IT YOURSELF BRUSHLESS MOTOR   

[My version of a DIY brushless 400-size motor based on an article written by John Rutter in Q&EFI Jan 2005]

Why bother with DIY?

It's a good question! The outrunner type of motor (magnets revolving in the outer casing around a stationary core of windings) is very simple to make and as a result there is now a vast choice of these brushless motors available. There are dozens of manufacturers advertising this type of motor at reasonable prices. There is one 10A motor including speed controller available for less than £20!

There are probably 2 reasons to DIY

One is the same as all other DIY subjects – the strange pleasure which comes from fiddling about with things we are not good at and triumphing in the face of adversity.

The other is the ability to tune the motor to the application. This can be done by altering the number of winds on the core (fewer winds – hotter motor, more winds – slower motor and bigger prop.). On this type of motor construction it can also be achieved by the length of the stator core laminations.

My motor…

I wanted to replace standard Speed 400 motors which are limited to about 10A and only 55% efficient, with a brushless motor having a capability of at least 15A at 85% efficiency. This makes the brushless motor over twice as good as the Speed 400, and it would also be lighter!

John's article indicated that a motor built from 2 stators stacked together would be about right for my requirements. The finished weight was 1.5oz – half the weight of a Speed 400.

Parts required

You will need:

•  A speed 400 motor, new or used.

•  12 Neodymium magnets 12 x 4 x 1.5mm. These are available from George Mizzell, Engineered Concepts, 1836 Canyon Rd., Birmingham , AL 35216 , USA . www.supermagnetman.com These currently cost 60 cents each and delivery is very fast.

•  2 stators (9 pole, 22.7mm diameter) at 85p each from C&K Designs, 14 Queen Elizabeth Dr., Corringham, Essex, SS17 7TH. www.brushlessmotors.co.uk

•  A solder connection plate. This is not essential, but makes the winding connections much neater. 95p from C&K Designs.

•  2 ball races, 0.125” x 0.250”. RS Components part no. 747-614.

•  A drill blank, 0.125” diameter at about £1.12, available from Drill Service (Horley) Ltd., Albert Road, Horley, RH6 7HR, tel. 01293 774911 www.drill-service.co.uk .

•  A length of 8mm OD tube, carbon fibre or aluminium alloy.

•  Insulated winding-wire. 0.4mm diameter suited my motor. 0.5Kg reel available from RS Components, part no. 357-738. If you don't want to experiment with windings, smaller packs are available from C&K Designs.

How to do it……

1. Dismantle a Speed 400

File, prise or grind off the lugs at the brush holder end of the motor can. The brush holder can now be pulled free, with the armature following next. Remove the spring clip which retains the 2 magnets. The clip, magnets and armature can be discarded. Carefully close the magnet retention wings nice and flush with the can. Any protrusion of these on the inside of the can will affect the correct seating of the new magnets.

2. Drill out the Brass Bush

This needs to be done carefully, as this operation will dictate how co-axial the outrunning magnets will be to the propeller shaft. I used a 3.2mm drill in a hand brace. Using the chuck, the drill can be cut into the brass bushes quite easily. Drill the rear bush in the brush holder first then temporarily reassemble the brush holder to the empty can. Feed the drill in through the enlarged brush holder bush and carefully open out the front bush. (For some reason I have photographed this drilling operation back-to-front!).

3. Test Fit the Prop Shaft

This shows the 0.25” drill blank temporarily fitted through the empty motor casing. At this stage the drill blank was to be soldered to the front bush, leaving about 10mm protruding onto which a standard prop adaptor would fit. After spending £20 on various fluxes I still could not solder onto the high-alloy drill blank. I resorted to using a brass collet soldered square to the front of the bush and the prop shaft would be held in place later with the grub screw.

4. Cut to Size

In the above photograph the collet can be seen soldered to the front bush. I ground a small flat into the drill blank for the grub screw to locate against.

The new neodymium magnets will locate against the small dimples which are pressed into the can, which can be seen close to the front face. From the rear edge of these dimples, measure back about 11 to 12mm. This is the cut mark for reducing the can to size.

Using a bench-mounted drill, clamp the prop shaft of the motor in the chuck. With the drill running, use a junior hacksaw to slowly cut through the casing. This gives a nice controlled and even cut. It's best not to cut right through using the power drill, but remove the motor casing from the chuck and finish the cut with the casing sitting in the jaws of a vice. The results are quite good after the ragged edges have been smoothed with a file.

5. Discard More Bits

Here, the can is shown cut into 2 parts. The rear portion and brush holder can now be discarded.

6. Sort out the Sticky Magnets

The neodymium magnets have to be arranged with their poles aligned N-S-N-S-etc and assembled into the motor can in that order. Use a piece of steel to lay out the magnets side by side. If the poles are opposing they will not sit as shown in the photograph. When all 12 are arranged as shown, mark them suitably.

7. Assemble the Magnets into the Motor

During the assembly of the motor, I was able to ensure the prop shaft and stator would be accurately concentric with the revolving magnet casing. The mounting of the shaft using the collet and grub screw now allowed the shaft to be withdrawn thus making the magnet placement much easier than otherwise! To space out my magnets evenly I had to experiment with spacers which took a little time to do. A strip of 1/32” ply was used, onto which I stuck 2 or 3 layers of masking tape. The resulting composite was cut into slivers which were placed one between each magnet (not shown). It is virtually impossible to place the magnets without spacers! When all the magnets are in position, the spacers can be very carefully removed, relying on the balanced magnetic flux to keep all in position. Initial fixing of the magnets can be done with some thin cyano. This was followed by a mix of epoxy with the whole lot kept warm enough to let the epoxy flow nicely. If the magnets are too hot to touch they will lose their strength! Final cleaning-up of the epoxy was done before it had fully set using methylated spirit.

8. The Stator

Shown above are the 2 stators, each about 5mm thick. They have been cyno'd together with the lobes lined-up and the bores concentric with the mounting tube. In this case the mounting tube is a piece of aluminium alloy which will take a bearing race in each end of it. About 35mm long is adequate for its purpose. Carbon fibre tube is equally useful for this and is available with an outer diameter of 8mm which is an accurate match to the bore of the stator. Fit the ball races into each end of the tube, using the drill blank again to ensure that everything is running true. Apply a small amount of cyno or epoxy to the outer race if necessary and allow to set.

9.  Winding the Stator

Start by numbering each lobe of the stator with an indelible pen. I will also consider this numbered face as the front and the opposite side will be the rear. There are 3 separate pieces of wire which have to be wound onto the stator to produce the 3 phases. Each wire will need to be wound round 3 lobes. I had decided to build this motor with 13 winds in a delta connection (the end wire of each phase is connected to the start wire of the next phase). The first phase is wound starting with stator lobe 1 and will progress onto lobe 4 then lobe 7. It is important to make all the winds in the same direction and to wind the same number of coils onto each lobe (this is surprisingly difficult to do!).

The stator can be held in one hand while winding, which helps.

Beginning at the front of lobe 1, neatly wind on 13 turns of wire starting from the front. Having completed the number of winds, pass the wire then around the front of lobe 2, through the slot between 2 and 3, behind lobes 3 and 4 returning it through the slot between 4 and 5. Complete the winding of lobe 4. From the final wind on lobe 4, pass the wire in front of lobe 5, through the slot between 5 and 6, behind lobes 6 and 7, returning it through the gap between lobes 7 and 8. Complete the winding of lobe 7. The first phase is now wound.

Take a second length of wire and proceed with winding phase 2. Start by loosely twisting the start of this wire to the end wire at lobe 7, then wind lobes 8, 2 and 5 in that order, following the same plan as for phase 1.

This is phase 2 successfully wound.

Now take the third length of wire for phase 3 and loosely twist it into the end of phase 2 (lobe 5). Wind lobes 6, 9 and 3. The end wire of this phase will be connected to the start wire of phase 1.

10. Almost Done

The 3 pairs of wire ends have had the insulation scraped off, and twisted together. These have been fitted through the holes in the solder connection plate.

With the drill blank fitted, slide the bearing tube over it and carefully fit the wound stator and solder connection plate over this. Careful – when it gets near the magnets they will grab it! With the bearing tube fully into the motor can and the stator sitting in its “magnetically balanced” position, run some cyno between stator and bearing tube, and between solder connection plate and bearing tube.

The completed stator can be carefully withdrawn and the wires soldered through the holes in the plate.

11. A Slight Adjustment!

On trying to assemble the motor I found that the stator and rotor were rubbing! This was rectified by mounting the stator assembly in the bench drill and filing some material off the crests of the stator lobes. Use magnets that are 1.5mm thick and you will avoid this problem!

12. A Finished Motor

Now running with satisfactory clearance, the connection wires for the speed controller have been soldered onto the solder connection plate.

For small motors where the stator is only one section of 5mm length, John indicates that the magnetic field alone is sufficient to counter the thrust at the propeller and prevent the motor pulling itself apart. For this one with a 10mm stator I will cyno a 10mm length of brass tube to the rear end of the drill blank leaving a whisker of clearance between the tube and the rear ball race. This will be permanently fixed, but this motor can be disassembled from the front if the stator windings need to be modified at any time.

For mounting in a model, carbon fibre tube can again be use. With an inside diameter of 8mm and a saw cut through the wall for a distance of about 12mm in, the CF tube can be clamped to the bearing tube with a small hose clip.

Do an initial test run on the motor with no prop fitted. It should run-up smoothly and quietly with no vibration. Direction of rotation is easily changed by un-plugging any 2 of the 3 wires and swapping them over.

13. Bench Testing

One of the likely uses of this size of motor will be with 7 cells (where 7 nicads or NiMH compare favourably with 2 LiPoly cells) or 8 cells.

To this end the testing was conducted using a 7-cell and an 8-cell nicad pack with suitable props.

DIY BRUSHLESS OUTRUNNER 400/10-13 WIND

(HACKER OPTO 40-3P SPEED CONTROLLER)

[GR = Graupner, MA = Master Airscrew, MS = Microspeed]

Conclusion

The motor is fairly cheap to build and was certainly satisfying in the end result. The 13 wind version seems suitable for what I need it for although I can easily rewind the stator to make changes later.

Any further motor construction will be even cheaper (as I have some material left over) and quicker to build.

Because the motor is a Speed 400 diameter, and also because the wires exit the motor in a position where they cannot be rubbed by the rotating can, it is ideal for the dozens of planes drawn up for Speed 400's. The Typhoon range for instance, has similar performance characteristics, but the diameter is much greater and the wires exit near the front mounting plate and usually have to be routed past the rotating can. This increases the mounting diameter to an even greater extent.