Speed Controllers - Making Sense Of It All
I’ve noticed a few new builder threads lately where, in the component list, a brushless speed controller is chosen to control a brushed motor. While those of us who have been competing for a while know that won’t work, it’s not always obvious when someone is starting out and it can be a bit of a daunting area, so I thought I’d write a quick post to help out and link to some suitable and commonly used parts.
The Basics
I’m not going to into too much technical detail here, just keeping it simple. The key difference between brushed and brushless motors is the number of connections it has. Brushed motors usually have two connections, tabs or wires on it; brushless motors have three. This is because the copper windings in brushless motors are arranged in phases. If you take a battery and touch the positive and negative to the two connections on a brushed motor, the motor will turn. If you try to do the same to a brushless motor, it won’t work.
Brushed: http://za.rs-online.com/largeimages/F2483716-01.jpg
Brushless:https://ae01.alicdn.com/kf/HTB12aJfL...tor-b-font.jpg
For the moment, and when starting out, brushed motors are easier to use for drive motors. Brushless motors are becoming more and more suitable for use as drive motors in featherweights, but it can take a bit of experimenting with the computer code used on the speed controller and often the motor itself needs a bit of modifying. When starting out, all of that can seem very intimidating, as you don’t want to risk damaging expensive components if something goes wrong, hence why easy to source parts such as cordless drills are great for providing an almost-ready-to-roll drive system.
Speed Controllers
Because brushed and brushless motors are different, they require different types of controllers. Again, the main way to tell them apart is in the number of connections, or wires.
Most speed controllers have wires coming out either side of a circuit board, often protected by some sort of casing (though occasionally all wires can come out the same side). On one side, there’ll usually be a red and black wire – to connect to the positive and negative of the battery/power supply – and a thinner 3-strand wire with a small connector on the end; this connects to the radio control receiver to send the speed controller the signals received from the transmitter.
On the other side of the circuit board/casing, you’ll either have two wires coming out, or three wires.
If it’s two wires – it’s a brushed motor speed controller: http://www.mtroniks.net/images/XL/Marine20HV.jpg
If it’s three wires – it’s a brushless motor speed controller: http://drone-about.fr/wp-content/upl...10/ESC-25A.jpg
That’s literally the main difference between each. Two wires, it goes to the two connections on a brushed motor; three wires, it goes to the three connections on a brushless motor.
Due to brushless motors becoming the standard motor used in RC hobbies, it’s becoming more and more difficult to find suitable speed controllers for brushed motors, so here is a list of some suitable controllers. The key figure in almost all speed controllers is the Amp (A) rating. For motors such as cordless drills, you want to be looking for controllers around the 10-15A continuous mark as a minimum, and the higher the better.
Dimension Engineering
Sabretooth 12A: http://www.technobotsonline.com/sabe...er-for-rc.html
Sabretooth 25A: http://www.technobotsonline.com/sabe...er-for-rc.html
The Sabretooth controllers can be a bit hit and miss sometimes. Some people have run them without issue, others have had them fail despite not exceeding their limits. They can seem a bit pricy but it’s worth noting that these are dual motor controllers, so they can control two motors, meaning you only need one for a two-wheel-drive robot.
Robot Power
Scorpion XL (12.5A): http://www.robotpower.com/products/s...n_XL_info.html
Scorpion XXL (20-25A): http://www.robotpower.com/products/s..._XXL_info.html
http://www.robotshop.com/uk/scorpion...or-driver.html
Again these are dual controllers, and again they are a bit pricy. They do seem to have a better track record compared to the Sabretooths (I have used a couple of XLs without issue) and they have built-in current limiting, meaning you’re less likely to blow them up by drawing too much current. They’re also America-based so you usually have to factor in shipping and customs, however the XXL is available from a UK supplier, see the second link for it.
At this point it’s worth me saying that the Scorpion XL/XXL and the Sabretooth controllers don’t have wires coming in or out of them. Instead they have screw terminal blocks that you can connect your wires into.
Botbitz/Ranglebox
30A ESC: http://botbitz.com/product/botbitz-3...ed-controller/
http://ranglebox.com/product/botbitz-30a-brushed-esc/
85A ESC: http://botbitz.com/product/botbitz-8...ed-control-v2/
http://ranglebox.com/product/botbitz...2-brushed-esc/
Both of these are single channel controllers, meaning they can only control one side of drive. You would need two for a 2WD robot.
To confuse things a bit, these are actually sold originally as brushless controllers, but the guys at Botbitz wrote new code for them to convert them to brushed controllers! Botbitz is based in Australia so you need to factor that in when buying them. Ranglebox is a UK distributor of the Botbitz products, making them available closer to home.
eBay/Miscellaneous
320A ESC: http://www.ebay.co.uk/itm/320A-7-2V-...AAAOSw~OVWwaAK
There are various types of brushed speed controller on eBay with no particular branding. The 320A rating of this model isn’t really accurate, but would be more than suitable for featherweight motors. However, while they are cheap, they aren’t always as streamlined or as precise as the more expensive options. Key issues tend to be that reversing capabilities aren’t great, the deadband (the amount you have to move your stick on the transmitter before the speed controller starts turning the motor) can affect operating performance and there is no braking, so when you take your finger off the stick, the motor coasts to a stop rather than stopping immediately. All of these issues tend to make the robot very difficult to control. Our best advice with these speed controllers is don’t buy them. Save your money and get one of the more expensive controllers that will do a better job and be more useful in the long run.
FeatherTwo
Rory Mangles, of Team Nuts, also makes featherweight brushed speed controllers for a very reasonable price: https://docs.google.com/document/d/1...X9DFu5AJk/edit
These are dual ESCs so, again, only one is needed. You might be thinking ‘why are there four wires coming out one side of the circuit board?’ These are just the wires that go to the motors. The two reds go to one motor, and the two blacks to the other.
Don’t Be Fooled!
Beware of some brushed speed controllers such as this one: https://hobbyking.com/en_us/turnigy-...ushed-esc.html
They look suitable and are incredibly cheap, but they’re designed for use in planes and some RC cars where reverse isn’t required, so they only run in one direction. Not ideal for a combat robot!
Hopefully that gives those of you starting out some useful information. If you’re ever unsure of your component selection though, always post your thoughts here before buying, and we can advise you if they’ll work or if you need to be looking at different parts.
Happy roboteering!
