Unlike a regular old DC motor, brushless motors require an electronic controller to commutate
the motor. The methods by which they do this differ, but it commonly boils down to whether or not the motor has Hall Effect sensors
placed strategically in the windings to sense the magnet rotor's position so the controller knows which coils to switch. Other strategies include position encoder wheels, but the Hall sensor is by far the most prevalent in small motors. The vast majority of model motors are sensorless
That's the first rule to keep in mind. Because the load experienced by an aircraft increases predictably with speed, and is zero at zero speed, pretty much all aircraft motors (especially outrunners) are just spinning rings of magnets on a stick. That is why they can be made so cheaply. Industrial motors for general motion control like servo systems and robots and whatnot need to deal with constantly varying and transient loads, so they therefore have some kind of feedback built in already.
Usually, the only model motors that come with sensors built in are small R/C car and truck motors like this
.Sensored control, the fundamental method
In a sensored motor (and associated controller), there are generally three sensors positioned at 120 or 60 electrical degrees apart in the motor that output a 3-bit Gray-like
position code. The placement of these sensors depends on the winding configuration, number of slots, and number of magnet poles. For more information on how and why this works, see my hub motor instructable
The advantage of sensored commutation is that the controller always knows where the motor is. Therefore, sensored motors usually have low-speed and stall characteristics more like a classic DC motor. Sensored commutation has its downsides - sensor failure or glitchiness could mean the motor controller stops running the motor. Most inexpensive industrial and commercial controllers, even EV controllers, are just governed by a state table whose input is the 3 Hall sensors, and so sensor failure would mean an erroneous output and non-running motor. There is also the issue of finding the correct combination of Hall sensor leads and motor phase outputs - up to 12 possible ways of matching them together, if your motor doesn't come with a controller.
Finicky details aside, sensored commutation is the way to achieve consistent stall- and low-speed behavior, and "real" vehicle controllers are usually sensor-commutated.Sensorless control, i.e. R/C speed controllers
A sensorless controller has to detect the position of the rotor through some other means. The most common method is to randomly bump the motor (drive two phases) and observe the back-EMF, or generated voltage, profile on the third phase. The slope of the BEMF dictates which direction the motor moved in, and therefore which coils to switch next. Sensorless commutation uses more sophisticated position state estimators which have to have some speed feedback to "pick up". In other words, sensorless motors can't technically move without being already moving. This clearly poses a quandary for vehicles and other inertial loads. If the initial "bump" is not strong enough, the motor will not move enough to generate a meaningful voltage pulse.
For viscous loads like a propeller (load proportional to speed), this is not a problem at all. Therefore, the vast majority of R/C controllers are sensorless.
Sensored ones do exist - again, for cars and trucks, which as you might guess, are just like tiny versions of EVs you personally ride, an inertial load. Sensorless controls for cars and trucks are also commonplace, since modern motors are powerful enough that a pulse on the phases is enough to move a small model.
The biggest implication of sensorless control is the dubious availability of stall torque. This makes the torque equation presented two pages ago a little misleading. You will generally not be able to stand on a vehicle and challenge someone to a drag race. Sensorless vehicles, especially those using R/C parts, will need to be "kicked off" or push started
The only real difference between a sensored motor an a sensorless motor is... sensors
. You can actually take any common R/C outrunner and add Hall Effect sensors to use them with EV traction controllers. Numerous ways exist to append sensor feedback onto your motor: I cover installing sensors within the windings (inside the motor) on my hub motor Instructable; and here are twoways
people have used externally mounted sensors. You would end up with an additional 5 wire harness coming out of the motor, comprising three Hall sensor outputs, logic power, and logic ground.
Hall sensors are typically "open collector" i.e.
they can only sink current. Controllers have internal pull-up resistors built in, so there's generally no need to internally pull the Hall sensor output to the logic rail. However, power supply decoupling capacitors placed right at the Hall sensor leads seem to be helpful. The Hall cable should not be routed parallel or immediately next to the phase conductors, since the high switching currents in the phase wires can cause induced noise problems with the sensor cable.Update June 2013: Outrunner Hall Sensor appendages
I hate to plug myself usually, but in the past few months, I've successfully developed and am currently selling a line of Hall Sensor Boards and Hall Sensor Mounts sized to several of the typical outrunner suspects. Placed externally, they allow a sensored-only motor controller to drive these motors. See the boards
on Equals Zero Designs. I designed these specifically as a "stock solution" for everyone asking me how to add sensors to your outrunners!