Motors

Brushless Motors - How do they work?

Brushless and brush slot car motors depend on the same basic principles.    Both have electric currents that needs to be reversed at the right time so the coils of wire rotate relative to permanent magnets.  The key differences are:-



© Copyright British Slot Car Racing Association 2021   Photos copyright C.Frost    All rights reserved

How is it wired?

At first sight it seems a bit complicated, but let’s have a go at explaining what’s going on in the diagram.

Firstly a normal guide picks up dc power from the two track braids and two wires take dc into the car just as in conventional slot cars.


The ESC takes the dc and switches it between 3 connections - somewhat like the way a 3 pole brushed motor switches between 3 com segments.  The ESC is programmed to switch between the 3 wires which gives some scope for adjusting the motor characteristics.




The 3 wires to the motor feed power to the brushless motor’s 9 poles. Obviously each pole cannot be fed individually from just 3 wires. It works by feeding power to groups of 3 poles. The 3 poles shown in blue are connected in series between wires 1 and 2 : The 3 poles shown in pink/red are connected in series between wires 2 and 3 : The 3 poles shown in green are connected in series between wires 3 and 1.  What’s the result of all that compared with a 3 pole brushed motor (other things being equal)? This brushless motor rotates through a smaller angle each time the current is reversed but produces more torque because it has more poles attracting and repelling the magnets.

Efficiency

Brushless motors have been common in radio controlled model cars for years.    Generally brushless motors convert more of the electrical power into mechanical power at the wheels so produce less waste heat.  That efficiency is a big advantage if your electrical energy is stored onboard - as in the batteries of radio controlled cars.    
There’s normally plenty of electrical power to spare on a slot car track, so assuming you can get rid of the waste heat the motor’s efficiency is much less important .   However brushless motors don’t have a commutator to retrue or brushes to wear out.

Cost

A conventional slot car motor is replaced by two parts – the brushless motor itself and the ESC to drive it.    A brushless motor without the ESC is as useless as a conventional motor with no commutator or brushes.  The brushless motors used in drones are cheap and readily available, major investment is necessary for  the quantity production of the specialised ESC electronics needed to make them work.  The somewhat different ESCs used in radio control are relatively cheap.  The relatively small quantities likely to be produced for slot cars will make slot racing ESCs more expensive.  

It also possible that standard drone motors will be a starting point for the development of brushless motors specifically for slot racing.  If that happens it’s likely specially developed for slot racing motors will be more expensive than standard drone motors.

How does it drive?

Ross Grogan, James Cleave and Will Stemman  tried some of Bob and Richard’s brushless cars in September 2021 on the Nationals/ 2019 World Championship track.  

Ross and Will found a perceptible delay between coming off brakes and onto power when entering corners.  Bob hadn’t found that delay and commented that perhaps Ross and Will are exceptional drivers.   Changes to the ESC code have been made to reduce the delay in coming back on power after braking.   .

Ross was  taking some high speed sections of the track on part throttle with brushless that would be taken at full power with a conventional Eurosport.   James commented on the torque didn’t seem to drop at high speed  unlike a conventional car  where there’s a considerable reduction in torque at higher speed.

Future Development   

We  must remember these are early days for brushless slot cars, these characteristics could change with development. How the car feels at high speed is certainly something that can be improved with development.  Is the conventional car cornering better so full power can be used in places where the brushless car won’t take it .... so the conventional car will lap quicker? Is the brushless car producing more power at high speed which would enable it to lap quicker than a conventional car?   Could chassis be developed to make good use of more power or would you end up with a hard to drive overpowered embarrassment?

How much performance will develop isn't entirely predictable and inevitably a lot of it will be depend on testing trial and error and yet more testing. However, some of the potential can be calculated scientifically.   The current brushless motors and gear ratios give the cars a slightly lower centre of gravity, less gyroscopic assistance in corners, and less flywheel effect.  An article explaining that in more detail will appear later.

Chris Frost


The boards Bob and Richard are using were developed by Peter Smit, a Canadian who specialises in designing ESCs mainly for drones. Bob contacted him some time ago and they are working together to develop a reliable and useable design. At the 1/32 Nationals in August 2021 there were only 3 of these ESCs in existence .

 Pete has designed a new, smaller version which is being tested and they are working on the initial bugs that crop up as testing continues on the old design.

When everything is finalised the details will be open source and available for anyone to build. One thing that Bob is keen for Pete to work on is a fixed rpm design which cannot be re-programmed, the idea of this is for it to be used for controlled racing like the FK classes at the moment but as everyone would be rpm limited it would make a more level playing field and not dependant on how good a motor is.  Before anybody asks where can I buy one, Bob says he is not offering to build one for anyone as the components are so small and fiddly it is painstaking work.

Here are photos of the top and bottom of one of the boards Bob used at the 2021 Nationals.  The ESC boards used a the Nats have components mounted top and bottom and are about 29 x 15mm and weighs about 2.8gm.

The ESC’s processor only runs when the hand controller is delivering enough voltage to the track.  That works just fine when you are on power, but it does mean the processor shuts down in braking and when the track power is turned off.  When the processor is shut down the board goes into brake mode, this works with the adjustable brakes of a conventional slot car controller just like any other slot car.  However when the driver comes off the brakes the processor need to restart before the motor can start producing any power.  When tested in September there was 100 millisecond delay in restarting the processor which was perceptible to some drivers. The code has now been changed to overcome that.

 

Let’s concentrate on  the inner workings  of the outrunner 1100 size brushless motors Bob Budge and  Richard Mack are using in  their slot cars.   Thanks to Bob and Richard for all the help with this article

These motors have two parts as shown in the illustration. The rotor carries the magnets, this rotates round the outside of the coils and they don’t have sensors.  Outrunners come in many different sizes, Bob and Richard have chosen this size which has a 14.2mm outside diameter and weighs 7.2 gm. The 2mm shaft is convenient as it’s a standard size for slot car pinions.  A shaft protruding out the right end to take a pinion has to fitted, the standard drone motors come with the shaft extending out the wrong end.  Richard tells me that’s not a difficult job.

We are used to feeling the magnetic attraction when turning over a motor, a conventional slot car motor will stop in one of 6 positions, this type of brushless motor  will stop in one of 36 positions

Outrunners aren’t the only type of brushless motor, another common type is the inrunner where the magnets rotate inside of the coils.   Inrunners have been tried but Richard tells me they weren't successful.

To do the same job as a conventional slot car motor you need both the brushless motor and the electronics to drive it.     The electronics are doing someting similar to what the brush gear and commutator do in a conventional slot car motor.

Electronics

Brushless motors have been used in radio controlled racing for years, and there have been slot car experiments using off the shelf radio control electronics in the car and full power on the track all the time.   The key difference in Bob Budge’s cars is they are driven with a normal slot car controller which gets away from all the issues of having power on the track braids all the time. The clever bit that makes that happen is the electronic speed control (ESC) board. I’ve been told the first slot racing ESC were developed for drag racing in the USA, apparently the  development and introduction dates back to 2014.