Calculation of ironless Permanent Magnet couplings using semi numerical magnetic pole theory method

This paper deals with a way of computation of the mechanical behavior of Permanent Magnet synchronous couplings . This method is based on the calculation of the forces between the magnets of the device. The formulation of these forces is based on magnetic pole theory. The computation is done using a semi-numerical integration method. This method has been validated in test cases and appears to be very advantageous in terms of calculation time and precision. So this solution appears to be a good way to study and design this kind of devices. keywords : Permanent Magnet, Modelling, Force, Torque, Coupling Introduction Synchronous Permanent Magnet couplings are used in many applications to transmit a torque from one rotor to another without any contact and without any friction 1 (Elies,1998),(Giannini,1982). These devices are very useful in sealed equipment to avoid contamination between two different media. They are used when a high efficiency is wanted because they allow to eliminate the mechanical losses. They can also be used in many secure equipment to avoid a failure due to a torque overload. The main design specifications for this kind of structure are its pull-out torque which is the maximal torque which can be transmitted by the coupling and its mechanical behavior in terms of stability (forces and stiffnesses on the rotors) (Yonnet,1981). This paper deals with a way of study of ironless couplings. These ironless couplings have a very small inertia and can be advantageous in many applications (Charpentier,1999). A classical way to study this kind of devices is the 2D or 3D Finite Element method which allows to compute the torque and forces exerted on the two rotors (Feirera,1989)(Wu,1997). However this solution is very heavy in terms of calculation time. The calculation precision with FE method is very sensible to mesh density and to the formulation. So this type of calculation can be not advantageous in terms of precision to calculate the characteristics of the couplings. So it can be difficult to do a systematic study of these devices using FE analysis. Some authors have developed analytical and semi-analytical models to study the synchronous PM couplings (Overshott,1989),(Furlani,1995),(Furlani,1996). This paper deals with an interesting way of calculation of the characteristics of the 2 ironless PM couplings. This method is very general and is based on the calculation of the forces between the magnets of the structure. This calculation is done using a semi-numerical method based on magnetic pole theory. The obtained results are then compared with a classical finite element method and appears to be very efficient to study this type of devices in terms of precision and calculation time. Presentation of the devices The synchronous Permanent Magnet ironless couplings consist of two rotors. The first one is the leading rotor and the second one is the led rotor. This kind of structure allows to transmit a torque from one rotor to the other without any contact. The two rotors are build with parallelepipedical Permanent Magnets stuck in ironless cores. Two configurations are the more often encountered in industrial applications. The first one is a cylindrical air gap coupling where radially oriented magnets are stuck in an inner and outer rotor and where the air gap is cylindrical. The second one is a plane air gap coupling where the two rotors are separated by a plane air-gap and the magnets are oriented axially. Figure 1-a presents an example of a cylindrical air gap coupling with 10 poles. This coupling is shown in the zero torque stable position. In Figure 1-b an example of a 6 poles plane air gap axial coupling is presented. 3 This coupling is also shown in its zero torque stable position. Leading rotor Leaded rotor Cylindrical air gap coupling (10 poles)