Sensorless Control of External Rotor Permanent Magnet Motors

Permanent Magnet Synchronous Motors (PMSMs) have got increasing importance for highly dynamic variable speed drives in the low and medium power range because of their high efficiency and compact construction. In applications which need accurate speed control but limited position accuracy mathematical models can be used to detect the rotor position. Sensorless Control offers many benefits over classical control with speed and/or position sensors like a large potential to save costs, reduced space requirement of the drive, increased power density, increased mean time between failure (caused on reduced amount of components) etc. In many applications fields like traction, direct and capstan drives or also starter generators the need for high torque is the basic issue. Therefore external rotor motors are in use. Because of the stator windings need for space in motor constructions with boundary condition of same volumes the air gap radius of an external rotor motor is larger than a classical type ones. This is the reason for enlarged tangential force on the surface and accordingly enlarged torque production, which varies with the square of the air gap radius. The goal of the project is to combine the advantages of sensorless control and external rotor motors, that means the development of an external rotor permanent magnet synchronous motor with enhanced INFORM-capability ("Indirect Flux Detection by Online Reactance Measurement"). This will be necessary to control the machine sensorless at low speed and standstill. The INFORM-method is based on effects of changing magnetic conductivity. For high speed a back-EMF based model will be used. To get magnetic saliency according to the INFORM-capability an iterative cycle of 2D-field-calculation and geometrical parameter adaptation of the cross section will be necessary. After having done measurements on the prototype a verification of the model will be done and the machine will be redesigned. Afterwards a vector- oriented control structure based on sensorless position detection will be implemented and optimised on several criterions. At the end a highly dynamic, sensorless, robust, external rotor permanent magnet synchronous motor drive with highest efficiency and torque production, will be realised.

Permanent magnet synchronous machines (PMSMs) have got increasing importance within the last years because not only of their high efficiency and compact construction, but also due to the decreasing costs of permanent magnet materials and the quite simple control algorithm compared to the induction machine. In many application fields like traction, direct drives or automotive a high torque production is the basic issue. An external rotor type provides an enlarged air gap radius (inner stator winding), higher tangential force and accordingly higher torque production, which varies with the square of the air gap radius, compared to the inner rotor type with boundary condition of same machine volumes. Since the knowledge of the actual rotor position is necessary for controlling a PMSM, mechanical sensors are in use. Sensorless techniques offer many benefits over those encoders like a large potential to save costs, reduced space requirement, increased power density and increased mean time between failure. For the last 15 years it has been the goal of many researchers to find such sensorless algorithms. In the high speed range emf-based models are state of the art and have been already implemented in industrial drives. In the low speed range and standstill it seems that so called "injection"-methods are the only solution for eliminating the mechanical sensor. The INFORM-method (INFORM = Indirect Flux Detection by Online Reactance Measurement), which was optimized during this project, belongs to this category. Injection-methods are based on asymmetric magnetic machine properties, for PMSMs especially the magnetic anisotropy caused by the geometric saliencies and the saturation of stator teeth and rotor yoke due to the permanent magnets. In this project the rotor geometry of a permanent magnet external rotor synchronous machine was analyzed in order to get a high capability for injection methods. Therefore verification criterions were defined and a finite element based simulation tool was developed. The most suitable rotor geometries were manufactured and the measurement results have been used for the verification of the simulation. The result is a simulation tool that yields an a-priori information about the INFORM-capability of any simulated PMSM. Aside from rotor geometry optimization the INFORM-measurement-sequence has been optimized. At the end a highly dynamic, sensorless, robust, external rotor permanent magnet synchronous motor drive with highest efficiency and torque production was realized by combining the injection-method with an emf-based model for the high speed range.