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Observer Based Sensorless Control
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This report presents a rotor position estimation technique for a five-phase permanent magnet synchronous motor with independent phases, based on a back-EMF observer. The method involves the use of a proper linear transformation which allows representing the five-phase motor by an equivalent two-phase model. Due to its characteristics, the sensorless strategy can be used in multi-phase motors having non-sinusoidal back-EMF shape; such is the case of brushless DC motors used in fault-tolerant applications. After an overview of the back-EMF model for the five-phase motor, the linear transformation and the observer-based estimation technique are used.And the experimental results are also shown.
Permanent magnet synchronous motors (PMSM) are widely employed for their high efficiency, silent Operation, compact form, reliability, and low maintenance. Depending on the application, different types of motors are used, with different rotor structure (surface or buried magnets), winding type (distributed or concentrated), and back-emf shape (sinusoidal or trapezoidal). Recently, multi-phase PMSM with independent phases have been proposed for safety critical applications such as aircraft brakes, spoiler or flap actuators. In these cases, the multi-phase machine is fed by a multi-phase power converter, and the whole drive system must satisfy severe fault-tolerant requirements, which involve the control hardware and the drive sensors too.
2.1. BLDC MOTOR
The brushless DC motor differ from normal DC motor in that it employs electrical commutation of the armature current rather than the mechnical commutation. The construction of brushless DC motor is similar to a permanent magnet synchronous motor.
The five-phase winding is placed on the stator (armature) while rotor consists of Permanent Magnets. The word Brushless DC motor is used to define the combination of motor, its electronic drive circuit and rotor position sensors.
The electronic drive is an inverter which consists of transistors, which feeds stator windings. The transistors are controlled by the pulses generated by rotor position sensors.
Brushless DC (BLDC) motors are preferred, with magnets mounted on the rotor surface and trapezoidal shaped back-EMF. Hall-effect bipolar sensors can be used as primary position transducers, in a quite simple and reliable assessment: each stator-fixed Hall sensor, one for each phase, directly detects the polarity of the undergoing rotor magnets with a proper angular displacement. The digital signals are processed by the controller and the rotor position information is computed with the resolution necessary for the electronic commutation of the motor.
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