Development of industries has rapidly increased the demand of motors and varied the usages and applications of them. Recently, the environmental problems and the distribution of our high dependency on petroleum energy have become the biggest social issues. Above all, EVs (Electric Vehicles) have attracted many people as it contributes to environmental preservation and high energy efficiency. Small-capacity motors presently use DC motors and BLDC (Brushless DC) motors are used for special purposes, such as controlling. These BLDC motors have many advantages over the small-scale DC motors and have the potential to be used for various fields in the future. BLDC motors, using permanent magnet excitation type, are classified into radial flux and axial flux types. Radial flux type have the airgap surface perpendicular to the axis to generate torques from the interaction of fixer coil and magnetic flux within the airgap. On the other hand, axial flux type generates torques for the motors from the magnetic flux perpendicular to the axis. Axial Flux Permanent Magnetic Motor has higher density of energy compared to general motors and slimmer structure with shorter axial length structurally. Also, it can drastically decrease the overall weight of the motor as it can directly attached onto the wheels. Generally, the output of BLDC motor is importantly decided by the performance of the controller and the torque and efficiency of the motor. Recently, many studies are being conducted actively to apply AFPM motors on electric cars for direct driving. The development of rare-earth magnets (Nd-Fe-B) with high density of energy has accelerated the expansion of the application of these motors.
Structure and Control of Coreless AFPM Motors AFPM motors are brushless and are powered by switching on DC voltage. Centered by the fixer of the motor, the rotors are arranged to use permanent magnets on themselves and the electric coil is wired on the sides of the fixer. Generally, the motor is powered by three-out-of-two phase connection method on the coil of the fixer sequentially. Fix 1 illustrates an inverter circuit of the motor and the sequential connection section of each phase. BLDC motor adds a terminal that senses the location of the rotors, instead of the brush or the commutator, for the rectifying action and the voltage on the armature coil through the inverter following this data on location. The location-sensing terminal uses hole-sensors or encoders, and recently also sensorless BLDC motors partially.
The rotors of the AFPM motors are two discs with permanent magnets to work as fans and are structurally fitted to remove heat generated by the lost energy of the motor. The magnetic circuit of the motor is decided by the materials of the permanent magnet, the arrangement, the size of pole area, and the length of airgap. Motors usually have fixer coil and fixer wire cores forming the passage of the magnetic circuit, but using coreless can increase the ratio of number of coil turns per volume and torques per unit weight.