2.5 State of the art
2.5.1 Application of SMC in RFM
The applications of SMC in almost all kinds of RFM have been researched. It has been shown that a direct replacement of laminated steel core with SMC core leads usually to poorer performance due to the disadvantages of SMC. More at-tentions should be paid to the electromagnetic design, the manufacturing and the assembly of electrical machine so that the advantages of SMC can be exploited to achieve an improved performance. Some important literatures depending on the topologies of RFM are summarized in Table 2.1.
Machine topology Literatures
DC motor [37–39]
Universal motor [40–44]
Reluctance motor [45–51]
Induction motor [45, 52–60]
PM excited RFM [12, 45, 61–76]
Table 2.1. Application of SMC in RFM
One of the possibilities to make use of the manufacturing flexibility of SMC is to design an armature with overhangs, as illustrated in Fig. 2.15. The special geometry leads to an increase of air gap axial length, a better utilization of PM and an improvement of torque density [38, 39, 44].
Boss Overhang
(a) Armature of a PM excited DC motor [38]
Overhang
(b) Electrical excited DC motor [44]
Figure 2.15. Armature of a DC motor made from SMC with overhang
The design flexibility and isotropy of SMC are further exploited in [41]. As shown in Fig. 2.16, the stator core is made of SMC and separated into poles and two half-yokes which enables an application of bobbin winding and realizes significantly simplified assembly.
Figure 2.16. Stator of an universal motor made from SMC [41]
Theoretically, the reluctance machine isn’t suitable for the application of SMC because the high permeability of soft magnetic components is required to gener-ate large reluctance force. However, it could also be interesting in some special cases, especially when the simplifications of manufacturing and assembly are de-sired. For instance, the stator of a new DC field multi-tooth switched reluctance machine is illustrated in Fig. 2.17.
2.5 State of the art
DC field winding
Stator
Rotor
Armature winding
Figure 2.17. Stator of a switched reluctance machine made from SMC [41]
Similar to switched reluctance machine, soft magnetic material with high per-meability is demanded in ASM. A direct replacement of the laminated electrical steel with SMC for induction machine leads to higher iron losses and lower effi-ciency, especially at high slip. However, some aspects of ASM such as manufac-turing, thermal performance and recycling of copper can be improved through the application of SMC.
For instance, the SMC stator core of an ASM as traction motor is divided into many separate parts to enable an assembling of pre-formed coil [57, 59], as illustrated in Fig. 2.18. Furthermore, the tips of stator teeth are axially extended to enlarge air gap surface. The measurement shows that the efficiency under rated conditions and peak output power increase by 3.9 % and 5 % respectively.
Furthermore, it should be noted that for the inverter-fed ASM with SMC stator core, the losses increased by the inverter harmonics are even lower than that of conventional AMS [53, 56].
In general, the PMRFM is more suitable for the application of SMC. It is less sen-sitive to the low permeability of SMC than the armature magnetized machines due to the large effective air gap caused by PMs. Furthermore, the isotropy and manufacturing merits of SMC can deliver additional considerable benefits. Thus, the utilization of SMC in PMRFM attracts a lot of attentions in the past time.
(a) Assembled stator (b) Yoke (c) Tooth(SMC)
Figure 2.18. Stator of an ASM made from SMC [57]
A PMRFM comprising an inner rotor with surface mounted PMs and an SMC stator with concentrated winding was reported in [67] where a number of design features can only be realized with SMC, as illustrated in Fig. 2.19. Firstly, the concentrated windings combined with the directly molded tooth with rounded axial ends can reduce copper losses, easier winding process and better thermal contacts. Besides, the armature core is segmented and split into core back sec-tions and teeth, which allows the use of preformed coils with a high filling fac-tor. In addition, the tooth tips and stator core are axially extended over end windings to reduce the stator back iron thickness and to enable a better use of PMs. The prototype has superior higher efficiency than the conventional ma-chine, which indicates the feasibility and potential of SMC applied in low-cost and mass-produced PMRFM.
(a) Core components and coil (b) Assembled tooth (c) Assembled stator
Figure 2.19. Stator of a PMRFM made from SMC [67]
2.5 State of the art
Another interesting design of PMRFM is presented in [75, 76] which is similar to a hybrid stepping motor, and the core components are made of SMC, as shown in Fig. 2.20. The axially magnetized PMs aren’t mounted on the surface of the rotor but inserted between the SMC rotor cores. In addition, the complex SMC stator with multi-tooth is axially extended to reduce the copper losses.
Compared with the PMRFM with a conventional laminated core, the machine shows higher efficiency at high speed.
(a) Laminated core (b) SMC core
Stator
PM Aluminium Rotor Casing
(c) Prototype
Figure 2.20. A novel PMRFM with SMC cores [76]
All of the above machines are experimental prototypes developed in laboratory.
However, some commercial applications of SMC in the electrical machines can also be found in market. An industrial servo motor was developed by ABB which achieves a size reduction by 1/3 and meanwhile improved performance compared with the previous design. Besides, a PMRFM with an inner SMC stator used in anti-lock braking system was developed by Aisin Seiki Co. Ltd.
which realizes 36 % reduction in axial length and 17 % reduction in weight. The application of SMC in a DC pump motor was performed by Laing GmbH to achieve a lighter stator with simplified structure and less number of components.