SRM as Rare-earth-free Alternative for PMSM

Today, with the on-going development of rare-earth magnets like NdFeB, permanent magnet synchronous motor (PMSM) uses has become more widely used. NdFeB is a strong magnet [1]. By using NdFeB, a compact, robust, high-torque, and high-temperature-tolerant motor can be produced [2].

However, rare-earth materials such as neodymium (Nd) are only available in certain places. Therefore, some countries have to import them. In the present, China has the most rare-earth material supplies, about 23%, and 95% of the world’s neodymium production. On top of that, China limits its rare-earth material exports in order to supply the country’s own needs, which results in neodymium’s high price [3]. On another note, rare-earth materials processing poses risk to people and environment’s health because it uses toxic substances [2].

Some motor technologies have been developed as alternatives to rare-earth PMSM, such as: squirrel cage induction motor (SCIM), synchronous reluctance motor (SynRM), and switched reluctance motor (SRM). Let’s focus on the last technology, SRM.

SRM is a brushless AC with salient poles at stator and rotor. Without permanent magnet and windings, the rotor is made of laminated iron. The stator has windings which will be excited to produce magnetic field. Rotor will rotate towards the excited windings to reach the position of least reluctance. To make the rotor keeps spinning, windings excitation is done in turn from phase to phase [4].

The stator has a pair of poles situated at opposite positions for each phase [5]. The number of rotor poles is different to the stator, such that wherever the rotor rests, it will always be misaligned to the next excited stator pole. The 6/4 configuration (six stator poles and four rotor poles) is the most commonly used. The other possible configurations are: 4/2, 8/6, 12/8, 16/12, and 32/24 [6].

Fig. 1 Diagram SRM 8/6

Fig. 2 SRM 6/4 from Kaskod Mtronix

Construction of SRM is simple, therefore it reduces fault risks. For example, the each stator winding is independent. In the case where fault occurs, such as short circuit, motor can still work as long as there is at least one functioning pole pair [6]. The windingless rotor also has less inertia, so the torque at start up and acceleration capability are high. Because the heat that occurs will be concentrated at stator, cooling system is simple, too [4].

SRM has not been employed for commercial electric vehicle as of now. Some of SRM’s weaknesses are high torque ripple, high vibration and acoustic noise, and complex control caused by nonlinear behavior and a lot of iron losses due to high flux frequency. Because of that, SRM efficiency is relatively low. Moreover, driving SRM requires a unique inverter [4][7].

In [7], strategies were discussed to design an SRM that is competitive with PMSM for uses in HEV (hybrid electric vehicle):

1. Increase the number of stator and rotor poles,

2. Use low-loss silicon steel with 0.1mm width, and

3. Use current mode operation in high speed.

Employing the abovementioned strategies, efficiency, torque density, and operation range of SRM improve.

In [8], a design of SRM was analyzed and compared to PMSM used in third-generation Toyota Prius 2009. The SRM was designed to have the same outer dimension as the PMSM, but weightier due to the bigger rotor diameter. Torque density was improved by optimizing stator and rotor structures. In terms of torque density, efficiency, and operation speed, SRM held a candle against the target PMSM. However, improving the SRM’s power density was proven to be the challenge.

Research on improving SRM performances, especially the unique inverter requirement, suppressing the vibration and acoustic noises, and lessening torque ripple, is continuing. Some researchers suggested using permanent magnet in the stator structure to increase torque density and efficiency as well as decrease the torque ripple [4]. In [9], motor configuration with more rotor poles than stator poles was proposed. Comparing the prototypes, it was showed that using more poles in the rotor than in the stator could increase the peak and average torque with lower torque ripple.

References:

[1] Adam Hill. (2018, Maret) wiseGEEK. [Online]. http://www.wisegeek.org/what-are-neodymium-magnets.htm

[2] Kristin Lewotsky. (2012, Juni) Motion Control Online. [Online]. https://www.motioncontrolonline.org/content-detail.cfm/Motion-Control-Technical-Features/Motor-Designs-Mitigate-High-Neodymium-Prices/content_id/840

[3] Research and Markets. (2013, Oktober) [Online]. https://www.prnewswire.com/news-releases/global-permanent-magnet-market-report-2013-2018-ndfeb-ferrite-smco-alnico-226977661.html

[4] Jordi-Roger Riba, Carlos Lopez-Torres, Luis Romeral, and Antoni Garcia, “Rare-earth-free propulsion motors for electric vehicles: A technology review,” Renewable and Sustainable Energy Review, vol. 57, pp. 367–379, 2016.

[5] Hoe Seng Ooi. Dr. Ooi. [Online]. http://dr-ooi.com/Basic_of_SR_Motor.htm

[6] Leslie C. Langnau. (2000, Mei) Machine Design. [Online]. http://www.machinedesign.com/motorsdrives/are-switched-reluctance-motors-you

[7] Akira Chiba, Kyohei Kiyota, Nobukazu Hoshi, Masatsugu Takemoto, and Satoshi Ogasawara, “Development of a Rare-Earth-Free SR Motor With High Torque Density for Hybrid Vehicles,” IEEE Transactions on Enery Conversion, vol. 30, no. 1, pp. 175–182, Maret 2015.

[8] Katsuhiko Urase, Kyohei Kiyota, Hiroya Sugimoto, and Akira Chiba, “Energy Efficiency Comparison of SR and IPM Generators for Hybrid Electric Vehicle,” in 2013 IEEE Energy Conversion Congress and Exposition (ECCE), Denver, 2013, pp. 5085–5091.

[9] Piyush Desai, Mahesh Krishnamurthy, Nigel Schofield, and Ali Emadi, “Switch Reluctance Machines with Higher Rotor Poles than Stator Poles for Improved Output Torque Characteristics,” in Industrial Electronics, 2009., Porto, 2009, pp. 1338–1343.

P.S.: this was written in Indonesian for a class assignment. Paraphrasing the English source was already a strenuous task. I don’t know why I decided to retranslate it into English just for this post. But anyway, enjoy ^^ and tell me if I got some of the facts wrong.