Powering a BattleBot - Brushed vs. Brushless Motors

Let’s take a closer look at the differences in these motors, their merits, as well as some rules of thumb on selecting the right motor for the job.

Brushed and brushless motors both belong to the category of “permanent magnet direct current” motors where coils of wire attract and repel against magnets. In order to achieve continuous rotation or “commutation”, the wire coils have to be turned on and off in a precise sequence. In a brushed motor, this is done mechanically using spring loaded brushes rubbing against a spinning “commutator”.  As the copper windings spin in the motor, the commutator directs current to different windings.

A brushless motor does away with the brushes and instead has its windings electronically pulsed. In order to know when to pulse each winding, most motor controllers measure the voltage generated by the magnet spinning across the windings to determine the magnet’s position. 

The elimination of mechanical brushes, which are often the weak link in a motor, has a number of benefits:

1.) Brush housing assemblies are mechanically complex and prone to fail from big shock loads.

2.) Brushes are made from soft carbon alloys which wear out and need to be replaced on a regular basis.

3.) Spring loaded brushes bounce in big impacts which can cause arcing.

4.) Brushes arc when transitioning between commutator bars which limits their operating speed.

5.) Extra electrical resistance from the carbon brushes and friction from sliding contact increases heat buildup and lowers motor efficiency.

One of the main benefits of eliminating brushes is that it allows the motor to handle more peak current and spin much faster. The only real limits are those set by thermal factors (magnet wire insulation burns, magnets demagnetize) or mechanical factors (magnets flung into oblivion from spinning at 50,000rpm, which isn’t an uncommon speed). Without the brushes, heating comes primarily from winding resistance I2R losses (milliohms), oversaturating the iron stator with magnetic flux at extremely high winding currents, and parasitic eddy current losses generated from spinning fast. Motor efficiency is also improved by being able to dynamically adjust when the phases are energized (motor timing) for optimum performance which results in less waste heat. It’s even easier to dispose of the waste heat because a brushless motor has the heat generating windings on the outside where it can be directly synced to the environment, whereas a brushed motor’s heat path has to travel through magnets. These factors compound such that a brushless motor can typically do the same job as a brushed motor in half the weight.

Comparison of Perm 132 brushed motor and TP100 brushless motor:

In combat robots with a fixed weight budget, the size reduction also leverages additional weight savings because less armor required to protect the motor. Or, you could just be one of those gutsy competitors who leaves it all hanging out.

Nightmare flaunts its big ol’ brushed motor

A word of caution regarding budget friendly hobby motor ratings:  
Merchants know that hobbyists are obsessed with getting the most power at the lightest weight. Since brushless motors have so little electrical resistance (milliohms) and can spin so fast (50,000rpm), they can generate massive power briefly (milliseconds) which tends to become the “max power rating” that goes on the listing. This misleading practice—along with a general lack of solid engineering data—makes it hard to properly size hobby brushless motors for continuous operation. Most hobbyists rely on comparing their desired setup with what has worked for others in the past. 

Fortunately there are a few rules of thumb when sizing power dense hobby motors for new applications:

1.) Expect a brushless motor to generate 2-5 watts per gram of motor weight continuously. Motors that achieve closer to 5 watts per gram do so using the best materials, lots of forced air cooling (like an r/c airplane prop wash) and razor thin safety factors.

2.) Derate any listed max voltage and current equally to hit that 2-5 watts per gram rule.  Remember: Power = Current * Voltage.

3.) Usually you need the torque constant (oz*in/amp) to figure out things like gear ratios in order to move a robot or spin a weapon without exceeding the motor’s current rating. Unfortunately hobby motors almost never list this. However, voltage constant Kv (RPM/volt) is always listed.  The good news is that due to the laws of physics for permanent magnets, the torque constant (oz*in/amp) = Kv (rpm/volt) / 1352.

These same tools can help size brushed motors as well. There are also great online resources. Check out the Team Tentacle Torque / Amp-Hour Calculator for robot drive trains or the Team Run Amok Excel Spinner Spreadsheet. Note that the spinner calculator needs some modifications to account for the current limiting that most brushless motor controllers have to do on large inertial loads. 

Because they spin so fast, brushless motors take a lot of mechanical gearing to get bring them down to useable speeds for applications like drivetrains—which can quickly erode the weight savings and add cost. One solution is to use a form of brushless motor called an outrunner.

In theory, the lack of brushes makes brushless motors very robust, though in reality a combat tune-up is necessary.  Outrunners need to have the magnets in the bell potted with extra epoxy or risk throwing a magnet. The rear end of an outrunner should be supported with an extra bearing, otherwise it's just a big cantilevered spinning ball of magnets, which tends to bend from impacts. Inrunner motors are more robust and contained, although they may have poor bonding of the steel stator to the aluminum motor shell. If this bonding breaks loose, it will cause a meltdown. Caustic Creations used extra epoxy on Poison Arrow to bond the windings to the can and a compression clamp around the motor housing for extra security.  Inrunners tend to have the highest power output because the copper windings are bonded to the motor, allowing for heat to be dissipated quickly, but they spin fast and require a lot of gearing. Outrunners with high magnet pole counts spin much slower due to "electrical" gearing—the electrical commutation goes through multiple cycles for every mechanical shaft rotation.  

The downside to brushless hobby setups is they're typically not as responsive as brushed to abrupt speed and direction changes which impairs control and maneuverability. This is mostly a byproduct of the simplistic programming of hobby ESCs (Electronic Speed Controls) that run "sensorless" mode.  Sensorless is a misnomer; a sensorless ESC actually measures the back EMF voltage from the unpowered 3rd wire to figure out where the rotor is relative to stator to properly commutate the windings.  A back EMF voltage is only generated if the motor is turning, so in order for a sensorless ESC to know how to sequence the 3 phases, the motor has to be spinning at a minimum velocity. If the motor spins below the minimum, the ESC can only guess at the proper sequencing.  

During startup a sensorless ESC sequences the windings at a set rate and throws in a lot of amps to kick the motor to that minimum velocity. The algorithm is tailored for the relatively light loads of an airplane prop or RC car. If the operator applies a slowly accelerating load, the algorithm won’t function well, causing the motor to cog or a total ESC meltdown. This is why people say brushless has poor stall torque and generally recommend high KV motors with large gear ratios—the backlash/slop in multiple stage gear reduction gives the motor a chance to pick up some momentum before encountering load.  

Engineers can get away with a low KV drive motor coupled to a single stage VersaPlanetary gearbox. Higher-end ESCs are smart enough to detect a stall and terminate startup before meltdown. They also come with lots of programming options and can be better tuned to high inertia loads encountered in robot combat. Another fix is to use "sensored" motors and ESCs which employ Hall-effect sensors placed near the magnets to provide position feedback even at rest, but those are difficult to come by and add complexity to your build. Many sensored ESCs will actually revert to sensorless at higher speeds or if sensor failure is detected.

It's always a good idea to purchase a programming card with the ESC to facilitate tweaking and testing parameters quickly. Proper settings are key to robust brushless operation and vary significantly with different ESCs. Brushless ESCs used for drive motors that have to quickly reverse are of the RC car variety and have programmable reverse delay. Default reverse delay usually takes a couple seconds to activate, which is terrible for tank/skid steering.  Zero reverse delay is often referred to as "crawler" mode (intended for RC trucks crawling up boulders—which requires sensitive low speed throttle control and quick reverse).  True current/torque limiting that comes with some RC car ESCs, such as the Castle Mamba Monster and XL2, is very effective when used in addition to the common current shutdown feature, which completely turns off the ESC at a set current. It’s always best to have redundant safety measures. When a brushless ESC eventually does blow up, it tends to internally short and start a battery fire, which is why you should put fuses on all ESCs.

Brushless ESCs suffer from the same inflated power claims as motors do. Fortunately, both are similarly overrated—a 300A 30V brushless ESC should pair well with a 200A 22V motor. You can let the ESC handle 20-50% more voltage/current than the motor, but don’t get carried away. Brushless ESCs are optimized for a particular motor size.

So when you see Poison Arrow in action, now you know how Caustic Creations got it to move so fast and attack so fiercely. And if you’re looking to power your own build, hopefully you’ll have a bit better idea of what kind of motor works best for you.