Analysis of torque generation and control in brushless permanent-magnet DC sensorless motors

Torque generation in a brushless permanent magnet motor is due to the interactions between the flux density in the air gap and the current flowing through one or more energized coils. Brushless motors usually incorporate a device to detect the flux changes of the rotor for energizing corresponding phase coils. The most common and widely used position detector is the Hall sensor. However, it is liable to generate malfunction because of the heat and is prone to failures in manufacturing and in use of motors. Also, it can not be used where the small size is of significant importance because it is a detrimental factor of reducing the air gap length.; This dissertation presents a magnetic field analysis technique for permanent magnet axial gap motors and position detection schemes which do not require position sensors. The magnetic field is modeled by a permeance method of the corresponding Maxwell's equations and boundary conditions and is solved using the simultaneous over-relaxation method for getting numerical solutions. Based on the flux density distribution in the air gap of an axial gap motor, the inductance variation of each phase due to the flowing current and flux changes is examined. The inductance bridge systems are used for determining the relative position of the magnet rotor because the inductance variation of each phase results in the difference voltage between phases when applying a reference signal. The position detection schemes are implemented in an example of the motor drive system. The torque and speed characteristics are investigated with a proportional gain to the error signal between the reference speed and the actual rotational speed. Also, the speed characteristics with the pulse width modulation method are considered.; The current flowing through the phase coils experiences resistive loss and generates heat that is a detrimental factor of motor performance. This heat causes the motor temperature rise that results in the magnet degrade, resistance increase, and power drop. Therefore, the torque and speed characteristics are changed due to temperature rise. The torque constant and back electro-motive force constant variations are considered to get the torque and speed characteristics in the thermal state. The lumped parameter model is presented for determining the thermal characteristics of brushless DC motors. The output speed of the drive system is dropped due to the temperature rise.