Every once in a while, I discover a new technology that immediately exposes the existing technology as nothing more than a dirty hack. DC brushless motors are not entirely new, but the technology is relatively new to me. I’m going to take some liberty here and presume that you, dear reader, are aware of some basic stuff about magnets, and currents in coils producing magnetic fields.
So we have our standard ‘brushed’ DC motor, whereby mechanical contact of ‘brushes’ (actually usually chunks of carbon) causes the fields in the coils of the motor to change direction as the shaft of the motor rotates. The problem is that the brushes wear out; they have less than perfect contact (especially in new motors where the carbon brushes usually start square, and wear down into the shape of the shaft); and the field direction is not optimal at many stages of rotation.
Then I discovered direct current (DC) brushless motors. I specify DC because that is the power we get from batteries in radio control applications (and AC is easy – just use the 50Hz alternating current to drive two different coils). DC brushless motors require an electronic speed controller to distribute the current to three (or any multiple of three) different coils. But which coil and in what order? This is where things get sexy.
Early brushless motors used a detector (either optical or mechanical) to determine the position of the motor shaft, and hence the position of the magnet poles relative to the coils. The output of this detector was fed into an electronic circuit, which used the information to energise the correct coils in the correct order, in turn pulling the magnets and hence the motor shaft around. The latest brushless solution appeals to the physicist in me: in exactly the same way that the coils make the magnet move, so does the moving magnet induce a small current in the coils. Modern electronic brushless speed controllers use a feedback loop to detect the small induced current in the coils, and read this as a shaft position, which in turn determines which coils to energise next. Ingenious! No shaft position sensor is required, so the motors can be simple, light, and efficient.
The motor pictured (which will be installed in a new RC plane), is even more bizzarre, being an outrunner or rotating can motor. The shaft is attached to the can (the blue part) of the motor, and the magnets are also attached to the inside of the can. The silver bit on top with the wires attached is fixed, and the windings inside the motor are also fixed. Pretty strange, but it allows the use of larger magnets, more coils (or poles, 12 in this case), and provides increased torque and inertia. This means that an outrunner brushless motor can usually be used to drive a smallish propeller directly, rather than using a reduction gearbox required in most small RC aircraft, saving more weight.
Stay tuned for the plane build and motor installation posts, possibly post-Christmas.