Introduction
In recent years, permanent magnet electric machines have gained significant popularity across a wide variety of industrial applications due to their numerous advantages including high power density, high-efficiency level, and high reliability. However, recent economic crises have made motor costs a crucial competitive factor. The price of magnets has increased due to the unstable supply of materials [1]. This rise in cost has negatively impacted permanent magnet machines, reducing their cost-effectiveness criteria.
To address this limitation, alternative topologies such as consequent pole machines (CPM) are investigated and became viable candidates for Surface Permanent Magnet motors (SPM). In fact, as their name indicates, in CPM, the magnet poles with the same magnetizing direction are arranged in an alternating pattern, with ferromagnetic pieces acting as additional poles. As a result, the motor is constituted by half the amount of magnets as shown in Figure 1 (b).
(a) (b)
Fig. 1. 2D Rotor Configuration of the SPM (a) and the CPM (b)
Motor Design Specifications
In what follows, two electric motors sharing the same geometrical parameters but differ by the number of magnetic poles are investigated: the SPM characterized by 10 magnets and the CPM hosting 5 magnets. The stator is kept the same for both structures, characterized by a double-layer concentrated winding configuration. The rotor and the stator cores are made up of M36 while the magnet material is N4212. The motor specifications are illustrated in the table below.
Table 1. Machine Specifications
EMWorks solution and results
Transient magnetic study of EMWorks2D for SOLIDWORKS coupled to motion analysis is used in the following steps to simulate and compare both model performances under no-load and on-load conditions based on 2D configuration.
No-Load Analysis
Fig. 2. Cogging Torque of the SPM and the CPM
The cogging torque is a significant parameter evaluated in open circuit conditions, arising from the interaction between magnets and the stator teeth. This torque is considered undesirable since it can lead to increased ripples during on-load operation. Figure 2 presents the predicted cogging torque for both topologies. It is evident from the graph that the peak-to-peak cogging torque of the CPM is noticeably reduced compared to the SPM and this result is expected as the magnet amount is reduced to half.
Fig. 3. Phase Back-EMF Waveforms of both SPM and CPM Concepts
Figure 3 shows that both back EMF waveforms are adopting a sinusoidal shape at a speed of 500 rpm. One can notice that the maximum back EMF produced by the SPM machine is 17.7 % higher than the CPM topology.
On-Load Analysis
Fig. 4. Phase Flux Linkage Waveforms of both SPM and CPM Concepts
Figure 4 illustrates the waveforms of phase flux linkage for both machines under on-load conditions. To achieve this, the stator winding is supplied with a current equal to 20 A. The maximum flux value of the SPM is 0.186 Wb. On the other hand, in the CPM, where the iron pieces function as south poles, the maximum flux value reaches 0.175Wb.
Fig. 5. Electromagnetic Torque of the SPM and the CPM Machines
The on-load performance of the consequent pole magnet machine is evaluated by predicting the electromagnetic torque. The torque angle is set to the value that corresponds to the maximum torque per ampere operation point. Figure 5 displays the predicted characteristics of the developed electromagnetic torque positions for both machine topologies. The average torque for the SPM is 22.7 Nm, while in the case of the CPM, it is equal to 17.9 Nm.
Fig. 6. Flux Line Distribution for the SPM under No-Load Condition
Fig. 7. Flux Line Distribution for the CPM under No-Load Condition
Figures 6 and 7 illustrate the magnetic flux line distribution and the flux density mapping of the considered topologies. One can notice that the SPM defines a higher saturation level compared to the CPM as it exhibits a double amount of magnets. The flux lines exhibit similar behavior for both machines, as the total number of slots and poles remains the same despite the change in the pole material.
A comparison based on finite element analysis between a conventional surface-mounted permanent magnet motor and a consequent pole machine is established at open circuit and current excitation modes. With the EMWorks2D platform, the electromagnetic specifications such as the torque, the back EMF, the flux linkage, and the flux density are predicted, analyzed, and compared. It has been concluded that the consequent pole machine is an efficient alternative to conventional motors as it satisfies interesting performance levels with a reasonable price.
References
[1] Ding, Kaihong. “The rare earth magnet industry and rare earth price in China.” EPJ Web of conferences. Vol. 75. EDP Sciences, 2014.