A New Harmonic Current Injection Technique to Reduce Cogging Torque in Axial Flux Permanent Magnet Motors

Abstract

I. In this paper, a new two-stage harmonic current injection (TSHCI) technique is proposed for permanent magnet synchronous machines to reduce the cogging torque and torque ripple components at low speeds. The developed algorithm is validated for different AFPM motors experimentally. It has been shown that the proposed technique help reduce the cogging components significantly especially at very low speeds.II. INTRODUCTIONThe use of permanent magnet (PM) synchronous motors is increasing in industrial applications compared to conventional AC and DC motors due to their high torque density and efficiency. Axial flux permanent magnet (AFPM) motors move forward in many applications due to their geometric benefits, high efficiency and high power density. Such motors may suffer from high torque ripple in some slot-pole combinations. At high speeds, torque ripple can be filtered by the system inertia. However, it would be imperative to reduce the cogging torque components at low speeds in some applications.Harmonic current injection is used to reduce the torque ripple in some PM motors [1-5]. However, it has been observed that such studies are generally carried out at high speeds [6-7]. In this paper, a harmonic current injection algorithm is proposed to reduce both torque ripple and cogging torque at very low speeds. Two AFPM prototype motors is used so as to validate the developed proposed algorithm. The proposed algorithm is validated with and without harmonic current injection for such PM motors at different speed levels.III. EXPERIMENTAL VERIFICATION OF PROPOSED TWO-STAGE HARMONIC CURRENT INJECTIONThe proposed two-stage harmonic current injection method is used for two different AFPM synchronous motors. In order to adapt the algorithm, shown in Fig. 1, the cogging torque and its harmonics are determined first as shown in Fig. 2. After the harmonic current injection, the variation of cogging component and its harmonics are shown in Fig. 3. It has been observed that the effect of the cogging torque can be reduced from 55.91% to 36.2% with the proposed method. The reduction of the impact of the cogging torque by the harmonic current addition method applied to the q-axis current when the AFPM motor is running at 4.5Nm at 10rpm speed. It has been observed that the impact of the cogging torque can be reduced from 55.91% to 30.9% with the proposed method.IV. CONCLUSIONSIn this paper, two-stage harmonic current injection is proposed and the algorithm is validated for AFPM motors at very low speeds. Torque ripple at low speeds are reduced by more than 51% with the developed method. The validity of the proposed technique is shown in 2 different AFPM motor. **