Beginners guide to Brushless Motors

Brushless Motors - How do they Work

The theory

Where to start? Oh Well - here we go - skip this bit if you wish but do read the do's and don'ts!!....

When you pass a current through a wire a magnetic field is produced, the field wraps round the wire and "travels" in a particular direction. Reversing the current reverses the field. Packing more wires together increases the field strength - and packing the wire in a torroidal shape further concentrates the field and increases the field density. This is governed by a rule!! Fleming gave us the left hand rule so that we can remember what goes on in motors - and a right hand rule for generators to remeber I use the fact that g for generator only occurs in right hand - so the left hand rule is for motors!!.

If you create a shape with your left hand as in the picture and point the Index or Second finger in the direction of the Current flow(The scientific symbol for current is I so use whichever you can remember) (positive to negative) then the First finger points in the direction of the flux (North to South) and the ThuMb points in the direction of the motion

If you wrap your coils around an iron core then the magnetic flux can "flow" more easily and you can increase the effect. Now mount a number of these on a spindle with the core attached at one end and you have the midlle of a motor. Put this in some bearings, add some magnets round the outside and we are nearly there. Apply a current through the coils on the spindle and due to the force generated the spindle will rotate until it finds a balance point... not much good as a motor. But wait - switch the current round and the spindle will move again. So all we need to do is keep doing this and we have a motor. Thats what the brushes and the copper strips are all about - when the motor coils are about to find equilibrium the brushes cross to the next copper segment and swith the current round. A coil that was attracting now repels and vice versa. Get the relation ship wuth the magnets right and the motion will continue. Adjusting the relative position of the brushes can make the motor(thaat's what it is now) run more efficiently or more powerfully - or even not very well at all).

Here is a very simple motor that you can make at home. In this case the flux wraps round the wire. You can see how the brushes and commutator change the direction of current flow. This motor may need self starting. To ensure self starting you need to have at least 3 coils in a motor. you don't need to have the split rings, cleaning the insulation off the wire will suffice. A cocktail stick up the middle can serve as a spindle and it can be supported by bent wire posts. The brushes can be more bent and stripped wire.

Not a lot to do with a brushless motor I hear you say - well wrong. A brushless motor is arranged differently - inside out you might say - and this gives us a new set of problems. If the coils do not move then how do we do the commutation (change the current flow)? Well we could use a commutator with a few more brushes and it would do the job. But there are more moving parts and just plain more parts - so it is more expensive less efficient and less reliable - so I guess we don't do that!! Fortunately electronics comes to our aid we can do the commutation electronically with banks of transistors to do the switching and a bit more electronics to decide when. It's the second bit that has limited progress as the elctronics to detect the position of the magnets and switch the current round has been rather large - until the microprocessor arrived. So we had to wait a little while before it became possible and until the technology was cheap enough to make it viable. A couple of things helped. The motors give greater power in thea similar size (or same power in a smaller motor) - this means less weight, they are more efficient than brushed motors (no brush losses or drag) and they are more reliable. small powerful and reliable - just what you need in space and fighter planes!!

The first brushless motors to appear were multipole and there was little choice, they used a magnetic detector to sense where the magnets were and they were fine - loads of power - but the controllers were bulky and the sensor wires were vulnerable to damage. The next idea was to use an unpowered coil of the motor to detect the magnets going past and use that to estimate when they are in the right position to swap the current round - it works but has a downside in that the speed has to be fairly stable to do this effectively and when the magnets are moving slowly you cannot detect them so at slow speeds you have to guess!!. snd that's hwre we are today.

Do's and Don'ts

Controller Set Up

Motors with different numbers of coils need different timing set ups in the controller in order to work properly. Your controller will probably not be set up for your motor and you must check the set up before you use it in ernest. Each controller has it's own way of setting up - usually telling you what is going on by making the motor beep. FAILURE TO GET THE SET UP CORRECT WILL USUALLY END UP WITH THE CONTROLLER BLOWING UP. Fortunately controller remeber the set up - but you will need to change it if you use a different motor SO KEEP THE INSTRUCTIONS. Please do beware - manufacturers like plettenberg make motors with 2,4 and 6 poles - all need different settings. Do not expect to get a warranty replacement for your mistake - the damage done is like a fingerprint (you know exactly what caused what!!

Wires

Brushless motors rotate very fast - so the controllers need to make a lot of calculations every second and they have to switch the juice on and of very fast. Now all those wires have a property called inductance. Inductance resists the flow of current - and the effect is worse the higher the frequency - SO LONG WIRES ARE BAD. Schulze recommend a maximum current path of 8 inches. The current path includes - the controller power leads, the battery power leads and joint between cells. So on a 12 cell pack with a joining wire of 2 inches plus 10 0.75 inch bars, plus the battery leads at at 2 inches and you have a 13.5 inch current path. TOO MUCH!!!!!!!!! KAAAAABOOOOoooooooom!!!!!!!!!!!! AND THAT'S NOT COUNTING THE SAFETY LOOP!!. Problems with long wires can be characterised by the controller pulsing.

So use end to end soldered cells and keep joins by battery bars to a minimum.]

Must have longer wires!

The effects of the wires, described above, happen even on short wires and so the controllers have a backup supply in the form of two large capacitors - these supply the current until it arrives from the battery. The longer the wires the harder these capacitors have to work - they are the first thing to go characterised by the ends expanding.

If you must use longer wires then you MUST add extra capacitors - not just any capacitors though - they must be super low ESR ( sometimes described as low resitance or impedance) and you need 440uf worth for every 4 inches of extra wire. One manufacturer recommends the use of 220uf capacitors in pairs. AsTec can supply suitable 440uf capacitors.

The grey bar on the side of the capacitors is close always close to the -ve lead. The capacitors are voltage sensitive - wrong way round and BANG. the lead end of the capacitors must be as close as possible to the controller. You must ensure that all joints are insulated and we recommend that you cover with heat shrink.

NB doing this may invalidate your gurantee - check with the manufacturere first. We (AsTec) can accept no responsibility for any problems resulting from this.

Safety Loop

You do not need one for if the controller gets wet but you do need one to protect against radio noise. If a brushed controller fails or gets wet the FET's usually go short and the motor will run. With a brushless controller failure it will burn out - the motor will not run - it can't as the commutation has gone.