Politecnico Di Torino Porto Institutional Repository [proceeding] Hybrid Sensorless Control of Axial Flux Permanent Magnet Motor Drives, including Zero Speed Hybrid Sensorless Control of Axial Flux Permanent Magnet Motor Drives, including Zero Speed

The sensorless control of an axial flux permanent magnet motor drive is proposed and tested. The motor is not purposely designed for sensorless control and shows a very small inherent saliency. This significantly affects the saliency-based position estimation in the low speed region. Other nonidealities, such as the non-sinusoidal back-EMF waveforms and possible misalignment between stator and rotor are also evidenced. A robust sensorless control scheme is proposed, able to deal with these non-idealities and with a rather simple implementation. The position estimation is based on a closed loop hybrid observer of the permanent magnet flux linkage. Experimental results report torque and speed sensorless control. Introduction Axial flux permanent magnet machines are disk-shaped machines characterized by their compactness and high torque density [1]. The short axial length can facilitate the mechanical integration into compact drivelines, while the specific shape makes them suited for high pole numbers. For these reasons they are well suited for direct drive applications and have been particularly proposed for electric and hybrid vehicle drivetrains [2] and wind generators [3]. Mounting motion sensors in a highly integrated environment can pose significant challenges. Sensorless control can be an adopted primary solution or an emergency backup, to improve the reliability of the drive. Other benefits include cost reduction and improved reliability due to the often challenging environmental conditions. As for a radial flux synchronous PM machine, the motor position can be estimated at standstill and low speed by tracking the saliency by means of signal injection [4-5] and at higher speeds by model-based methods, such as those relying on backelectromotive force (EMF) integration [6]. Hybrid methods mix the two types of estimation over the speed range of the drive [7]. Most of the referenced works refer to radial flux motors, with few exceptions [8]. Despite the commonalities, axial flux machines can be more challenging to control than radial flux machines due to several inherent phenomena. These include the variable saturation level along the machine radius (i.e. from the inner to the outer diameter of the machine), the significant 3-D effects and the airgap length generally has a significant variation due to the complex mechanical structure and significant axial forces. A robust hybrid, sensorless control scheme for a double-sided axial flux PM machine is proposed within this work, capable of controlling the machine from zero to maximum speed. The position tracking is based on rotating voltage injection at standstill and very low speed and on back-EMF integration at higher speed. As already mentioned, the control is tested on a machine with very little saliency (Ld = 1.055 mH, Lq = 1.0 mH) and with significant non-idealities. At higher speed, a model based scheme is used for the position estimation. The back-emf of the tested motor are non sinusoidal, and this produces a position estimation error and in particular a significant ripple on the estimated speed. The control robustness is thoroughly tested and implementation details is provided. A first test shows the performance of the sensorless hybrid observer in torque control mode from standstill to 30% of the base speed, at no load and at load. A second experiment shows the operation in closed loop sensorless speed control. The effects of rotor misalignment and magnetic field harmonics in the presented waveforms are evidenced. Axial flux machine The motor under test is a double sided machine, with toroidal stator windings. The machine is totally enclosed and water cooled and has been designed for hybrid traction. Its ratings are presented in the “Experimental results” section. The two rotors are offset by half a stator slot pitch for mitigating the cogging torque, as represented in the schematic layout of the machine reported in Fig. 1. Fig. 1: Schematic representation of the double sided, axial flux, surface mounted PM synchronous machine under test. The two rotor sides are offset by half slot pitch. The fundamental saliency is very low despite having soft magnetic pole-pieces on the d-axis to increase the machine inductance as discussed in [9-10]. Fig.2 shows the basic rotor structure in which the soft composite pieces have been introduced in the inner ring of the two rotor plates. The soft magnetic composite is Somaloy 700 [11]. Fig. 2: Sketch of the two part rotor double-sided axial flux PM machine. Hybrid rotor position observer Low speed, saliency-based estimation The rotor position estimation at low speed is based on the injection of a rotating voltage signal in the stationary frame [4-5]. Rotor part A Rotor part B Water jacket Stator part B Stator part A The injected voltage, high frequency component is: t j inj HF s inj e V v ω ⋅ = , (1) where the vector amplitude is Vinj = 45 V and the angular frequency is ωinj = 500·2π rad/s. Under the assumption of a sinusoidal fundamental saliency, the resulting current vector consists of a positive and a negative-sequence component: ( ) ( ) ° + ω − θ ° − ω ⋅ + ⋅ = 90 2 90 , t j n t j p HF s inj inj e I e I i (2) where θ is the rotor electrical position. The amplitude of the negative sequence component depends on the motor saliency, according to: