# Torque Characteristic Analysis of a Transverse Flux Motor Using a Combined-Type Stator Core
## 1 Introduction  
A traditional motor drive system is combined with an electromagnetic component (e.g., DC motor, induction motor, permanent magnet synchronous motor, and switched reluctance motor) and mechanical component (e.g., gear box, transmission). Some drawbacks are added into the system by using mechanical components, such as friction loss, maintenance cost and noise. Therefore, the direct drive motor system is proposed to avoid problems. In a direct motor drive system, the motor is coupled with load directly, and the direct drive motors are usually needed to own the features of low working speed and high output torque in most direct-drive applications (e.g., electrical vehicle, wave energy conversion, ship propulsion, railway traction, and wind power generation).  
A series of motor technologies has been investigated for direct-drive applications, such as the multi-pole permanent magnet synchronous motor (PMSM) [1], the brushless DC motor (BLDC) [2], the permanent magnet vernier motor (PMVM) [3; 4; 5] and the transverse flux motor (TFM) [6; 7; 8; 9]. Among those motor technologies, the transverse flux motor has a unique transverse main flux path that is perpendicular to the rotate direction. In contrast to the radial flux technologies, the feature of the transverse flux path makes the electrical load and the magnetic load be decoupled from each other, which improves the torque density and the feasibility for multi-pole design. Nevertheless, the structure of TFM is usually complex as the 3D flux path, and the use of a silicon steel sheet is laminated. The soft magnetic composite (SMC) is a reasonable material for TFM [10]; however, the mechanical strength and magnetic characteristic of SMC is lower than a silicon steel sheet.  
Various transverse flux motor structures have been proposed and researched by the scholars around the world [11; 12; 13; 14]. They can be classified into synchronous type and reluctance type [15]: (1) if the permanent magnets are fixed on the rotor side (including mounted on the rotor core surface or buried into the rotor core), the motor can be classified as a transverse flux permanent magnet synchronous motor (TFPM) [16; 17]; (2) if there is no permanent magnet on the rotor side, the motor can be classified as a transverse flux reluctance motor (TFRM) [18]. The TFPM can obtain larger torque density than the TFRM; meanwhile, the manufacturing cost of TFRM is lower and reliability of TFRM is higher. Thus, the cost and the characteristics of motors should both be considered.  
In this paper, a transverse-flux motor using a combined-type stator core (CS-TFM) is proposed. The CS-TFM is a kind of external rotor motor. The rotor is comprised of a rotor teeth core and non-magnetic sleeve. The stator is comprised of a core, permanent magnet and armature winding. Both the rotor core and the stator core can be fabricated by a silicon steel sheet, which can improve the mechanical strength and simplify the processing technique. Both permanent magnet and armature winding are fixed on the stator side, and the rotor structure is simple and reliable. The torque density of CS-TFM is improved more than TFRM by using a permanent magnet, and the cost is lower than TFPM as the PM volume of CS-TFM is smaller.  
In Section 2, the detailed motor structure and working principles are introduced. In Section 3, the theoretical expressions of electromagnetic torque and cogging torque are deduced. In Section 4, the basic characteristics such as permanent magnet flux linkage, no-load back electromotive force (no-load BMF), cogging torque, and load torque are analyzed by 3D FEM in detail. In Section 5, the influence of structure parameters on the torque density is analyzed. In Section 6, the torque densities of different direct-drive motor technologies are calculated and compared, and a summary is included in Section 7.