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Brushless Basics

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Brushless motors – a basic guide

Page no.1 - who. why and waht do we need to know?

Judging by the questions asked on the forum we could all do with a run through the basics.  I may be well placed to start laying this out because:
•   I don’t know the answers – but would like to
•   I have, use and make Brushless motors (but for aircraft and Higgins Hellkittens)
•   I like, myself, to know the basics before spending any money

It would be a good idea to make this as broad and informative as possible.  Comments, additions experience and suggestions are genuinely welcome.

An area I know nothing about is boats and propellers!  If anyone knows the relationships between props, power, pitch, boat speed and the precession of the equinoxes I will undertake to try and produce a calculator to make some of the things calculable!

What are they, exactly?
Well; they are electric motors, same as the ones we are used to, and have the same basic habits meaning:
They must run at design speed to be efficient
If overloaded (too big a prop) they will slow down and absorb more power until they find a speed where the power input matches the load.
Conversely, if the prop is small they will spin faster and take much less current (and often be more efficient)

They are Alternating Current (AC) motors! 
So how do they run off our batteries, then?  Brushless motors need a special ESC which chops up the DC voltage into 3-phase AC and feeds it into the motor wires.  Speed control comes from the way the ESC drives the phases round, and the VERY cunning bit is the timing.

Brief diversion into 3-phase AC  -optional reading
The brushless motor has three sets of windings (one per phase).  The rotor has a set of magnets (now always rare-earth magnets) which are not quite lined up with the stator poles (often there are 9 stator poles and 12 magnets).  When one phase is energised it attracts the set of magnets which are nearest and pulls the rotor round, then the next phase is energised with the same results, then the third phase.  Repeat thousands of times a second.

The speed of rotation of the phases is governed by the throttle setting, and the timing of when the current in a phase is switched on is also important.  The required timing changes with the speed (like in a car engine), the load, the type of motor and various other things.  Some ESCs are adjustable in timing, and several have a few settings to select among.  I believe that ALL brushless ESCs are delivered with a nice average set of settings and will operate well when plugged in to nearly any motor and will start and run with any average load.
They are also generators, just like DC motors.  If they are driven by their load they push power back into the supply.  The power will appear as (surprisingly) 3-phase AC, and when you see small wind turbines this is what most of them are doing and using!

What happens if I plug a brushless motor directly to a battery?
Don’t try this at home!Now that you have read the description of how they operate you will be able to work out the answer. 
If you do it you will have connected one of the phases across the battery – the rotor will kick instantly till the magnets are next to that set of poles.  Then nothing more will happen until smoke curls out of the motor because you still have the battery connected to three coils and they are getting HOTTER.
With a high-power battery the motor will be damaged if it goes on for too long (a second or two depending on the wind (but your could always rewind it rather than scrapping it!).

Types of Brushless MotorsThere are only three types that the modelling fraternity will use, and boat modellers will probably only meet one type.  However some of us may wish to drive huge paddles directly and efficiently so I will mention, both types:
1.   Brushless motors (there really doesn’t seem to be another name)
2.   LRK motors
3.   Printed motors

If I define and dispose of LRK motors quickly.  They are a family of brushless motors designed for high magnetic efficiency.  Because of this they are also electrically efficient.  They tend to be large diameter and flat (pancake shape) and therefore produce huge torques, but seldom high revs (and will run off many ordinary brushless ESCs).  Used for application like electric bicycles and other vehicles.
For more information Google “LRK motors” or look through
LRK are the initials of the gentlemen who have developed them.

Printed motors – instead of physically winding coils on the stator it is possible to use a printed circuit to act as the current carrying conductor.  I have not met one of these motors in the flesh, but the gent from Model Motors Direct has referred to them in a couple of years ago in one of the comics.  Printed circuit board being what it is the coils have to be spread over an area, so these motors are going to be large diameter and very flat (usually there is a printed board each side of a rotor with magnets inserted into it or bonded on)  So printed motors are round and very flat, and should be high torque, low current devices (the printed tracks are not going to carry as much current as a wire)
Should be cheap. Though!.

“Chapters” I have in mind to follow on with include: (but are not limited to)

1.   Inrunners/Outrunners differences, similarities
2.   Matching an existing motor
3.   Gearboxes, belt drives, etc
4.   Kv for fun and profit
5.   Why would I want to change to a brushless?
6.   Boat speed, motor revs, voltage?
7.   Mounting the beasts
8.   Reversing – how do I do this
9.   Speed control
10.   Why would I be interested in 4-D flying?? :}

Comments, contributions etc, welcomed
It is always valid to guide me to say more, less or nothing at all

Sorry no illustrations yet - other folk have found and posted diagrams of how they work, etc - I will try to break up slabs of text with relevant piccies


Martin [Admin]:

Well I've certainly hit the old    button to make sure I don't miss anything here!
Thanks Andrew.   :-))

Intro, feedback etc.
First;  sorry about the gap before getting this to you.  My day job got manic but we have now shipped the bits that make up my project so now there are some gaps in the day.

Range of sizes
I have not spoken really of size, but brushless motors are made from roughly one gram weight (a few milliwatts) to about 20 megawatts – cruise ships have 4 or 6 of these!  It is likely, therefore that we fall in the range of possible ratings!

Where will we find them?
1)   Electric scooters, motor-bikes and assisted bicycles increasingly use brushless for the same reason that we might – power not wasted as heat in the motor stays in the battery and can be used.  The top-of-the range models nearly always use brushless motors and Lithium cells in case any of us are looking for the ultimate boat power source.  They seem to live in the 400W to 5kW range.  Naturally they use brushless speed controllers (normally with potentiometer speed control, but we know how to add a servo to drive the arm of the pot, don’t we?
2)   Computers
CD-ROM drives and Hard Disc drives are now always (I think) brushless.  In fact using a CDROM motor raw material has led to the “CD” motor  which was , and probably still is the most common brushless motor for electric flight in the “parkflyer” size range.  The CD motor is an excellent example of the Outrunner type (see below)

Shape, and how it affects shaft speed
In the first spiel I realise that I spoke of large diameter motors inevitably running (relatively) slowly.  There are two factors at work here:
1) Torque – the “Twisting power of a motor – we measure this crudely by trying to hold the shaft when the motor is running.  If the force to rotate the motor is generated at a large diameter, the motor will have lots of torque (and power is torque multiplied by revs)
So large diameter motors are likely to produce lots of torque.
2) Strength.  Brushless motors never have rotating windings (as far as we are concerned), but they certainly do spin their magnets on the rotor.
Inrunners have cylindrical rotors with the magnets embedded in the rotor, and the limit of rotational speed is governed by the strength of whatever is retaining the magnets into the rotor, and the diameter of the rotor.  If it spins too fast the magnets will fly outwards with centrifugal force and act as an effective, but noisy brake
Outrunners have a “bell” shape with a cylindrical drum (with magnets in the inside surface) which spins round the stator windings.  Again, centrifugal force attempts to “open” the mouth of the drum and the material of the drum usually steel has to prevent this happening – so there is a practical limit on the revs possible

Essence of Brushless chapter 2

The types of brushless that modellers will come across

Inrunners have their magnets embedded in the rotor which runs inside (hence the name) the stator.  There is an animation of a secctioned inrunner running in:

I see that this animation runs - there is a lot of good general motor information on the site

Inrunners always have the rotor shaft running in two bearings, almost invariably ballraces, so the shaft is well supported.  The shaft end pokes out of a stationary cylindrical motor – exactly the same general format as a DC motor

This little fellow is 13mm diameter and can have a KV up to about 7000 - so on 10V it spins at 70,000 RPM

Recognition features:
Cylindrical case – often the same general proportions as a DCmotor
Relatively small diameter – often about three times as long as the diameter
Case remains stationary
Case sometimes includes cooling – fins, or water-cooling manifold

Watch out, by the way, for the cooling fins – if they run round the case this way

They were intended for helicopters and there just isn’t the same draft in a boat!

Because the case is stationary – these are the only brushless motors which are easily interchangeable with our conventional DC motors, and a lot of them are described as “400 replacements” or “600 size”  be aware that this means physical size, and they will indeed fit in the motor mount, but are capable of producing anything up to 10 times the power and may do so at astronomical revs

Outrunners need a bit of understanding – they too have a stationary stator but it lives inside the rotor.  So the rotor and its magnets are rotating round the outside of the stator and on end of the rotor is closed and is a tight fit – often a press fit on the drive shaft.  The drive shaft runs on bearings inside the stator and can extend out of both ends of the motor to drive whatever we like.

The rotor is a drum or bell shape with one open end, and the magnets are bonded into the inside surface of it – see picture below
So Outrunners are produce their torque at a much greater diameter than inrunners – they produce loads of torque, but you don’t really want to run them at very high speeds, because of the open-ended stator and the relationship of the magnets to the bearings.

Note the generally big diameter, squat shape - the motor mount is at the left, and EVERYTHING to the right of the wires rotates

Some good outrunners are made so that the relationship of the stator and shaft can be changed

On this motor the shaft can be moved to stick out of either or (as shown) both ends of the motor.  The shaft, gold star on the left of the picture, annd black cylinder all rotate, and in this case the motor mount is the gold component at the right hand end.

More as it happens - next chapter we get numerical!

as always  - comments and contributions welcomed


Now that should help a boat move right along

Even this dummy can understand so far, keep it coming, thanks, Ian.


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