Brushless Motors -
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 updated 2022 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 -
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.
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 -
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.
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.
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.
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.
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-
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. In 2022 a smaller ESC was produced, about 23 x 15mm and weighing about 2.1gm. They current version’s processor runs at 49mHz which allows the motor to run at up to around 110,000rpm.
The ESC’s processor only runs when the hand controller is delivering enough voltage to the track, a key part of the development was getting them to work down to a low track voltage. The processor shuts down when the track voltage is below about 1¾ volts which happens 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 2021 there was 100 millisecond delay in restarting the processor which was perceptible to some drivers. The code has now been changed to overcome that.
These ESCs have reverse polarity protection built in so will not be damaged if the car drops into a lane backwards or a power supply is hooked up backwards while testing. Some other makes of ESC will fail if reverse polarity power is connected.
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 Richard credits Stefano Mirabelli with the recommendation for that size of motor. They come in different lengths, Bob and Richard concentrate on the 1106 size, the fourth number is the length of the stator stack (so 1106 has a 6mm long stack, 1103 has a 3mm long stack etc.) shorter stacks give less torque.
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. The bare motor weighs 7.2 gm. Complete with pinion and wiring to the connector they weigh 8.1gm. 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.