Influence of the Amount of Permanent-Magnet Material in Fractional-Slot Permanent-Magnet Synchronous Machines

Abstract

Abstract-The efficiency of permanent magnet synchronous machines (PMSM) with outer rotor and concentrated windings is investigated as a function of the mass of magnets used, keeping the power, volume and mechanical air gap thickness constant. In order to be useful for electric vehicle motors and wind turbine generators, the efficiency is computed in a wide speed and torque range, including overload. For a given type and amount of magnets, the geometry of the machine and the efficiency map are computed by analytical models and finite element models, taken into account iron loss, copper loss, magnet loss and pulse width modulation loss. The models are validated by experiments. Furthermore, the demagnetization risk and torque ripple are studied as a function of the mass of magnets in the machine. The effect of the mass of magnets is investigated for several soft magnetic materials, several combinations of number of poles and number of stator slots, and for both rare earth (NdFeB) magnets and ferrite magnets. It is observed that the amount of permanent magnet material can vary in a wide range with a minor influence on the efficiency, torque density, and torque ripple and with a limited demagnetization risk. A lot of papers deal with rotor losses in the considered type of machines, and give methods to compute them. For example [6] gives a general method for several slot/pole configurations. Polinder [7] studies losses in the solid back iron of PMSM with fractional slot windings. Atallah presented an analytical model to compute magnet loss [8], and Ede proposed a computationally efficient 3D numerical method to compute the magnet loss [9]. IndexAn aspect of fractional-slot concentrated-winding PMSM's that is less investigated, is how much the amount of magnets can be reduced, and how this affects the global machine design and performance. As the price of rare earth magnets is high, PMSM's with small amount of magnets become interesting, on condition that their efficiency and torque density remain high. It is known that very powerful and light permanent magnet machines can be made when a lot of PM material can be used: for example the slotless brushless DC PMSM of [10] has two concentric rotors, both of them completely covered with a thick layer of NdFeB magnets in Halbach array. Alternatives are investigated to reduce the required amount of permanent magnets, or even to have motors without magnets [11]. Nevertheless, [12] compared caged induction, reluctance, and PM motor technologies for the more electric aircraft, concluding that three-phase permanent magnet machines may still be the favorite choice.Therefore, we investigate the impact of the amount of magnets and the magnetic material grade in the stator of the PMSM on the efficiency, keeping the machine volume and torque density constant. Here, we consider not only the efficiency at nominal speed N nom and torque T nom , but also the average efficiency in the speed range 0.25-1.50×N nom and torque range 0.25-1.50×T nom . A number of effects of reducing the amount ...

Integrated Model of Power Electronics, Electric Motor, and Gearbox for a Light EV

“…The analytical model based on [14] is valid for all PMSMs with surface magnets in variable speed drives. However, this model has a few drawbacks, which are not present in the FEM of Section V.…”

“…In [14], the following methodology is described to determine the geometry of the machine and to calculate the efficiency of the motor. The rotor back iron thickness (t ry ) is attained from the maximum of the air gap flux density (B a,max ).…”

“…The analytical model of the motor is extremely fast, but it is not accurate enough to quantitatively predict the efficiency. Therefore a FEM of the , based on [14], is also implemented and validated by the measured results in Section V.…”

“…The FEM of the motor, based on [14], is evaluated for a motor configuration with N p : 7 and N s : 12 and for a given r rotor and GR. The PMSM is simulated using a sequence of static 2D FEM, varying the rotor position through the moving mesh technique.…”

“…These time-harmonics are evidently the only harmonics in the current waveform without PWM because the PWM time-harmonics are considered separately in P rotor,PWM . Detailed information regarding the additional loss terms is given in [14].…”