Ac 2011-1731: Permanent Magnet Linear Alternator Mag- Netic Field Analysis

Permanent magnet linear alternators are energy converting devices that have been under research and development for years. This type of alternators have advantages of high efficiency and no need of liner motion to rotation converting, when the driving force is in linear oscillation. Such applications can be found when ocean tide, solar power, and swing motions are used for generating electric power. Magnetic field analysis is essential for electric machine designs. Results from the analysis provide the magnetic field distributions in a machine. Magnetic field analysis may help the designer to modify their design for getting a better product. Magnet field analysis may also be used for finding torque, force, power losses, and induced voltage, which are the primary parameters of electrical machine performance study. This paper presents the magnet field analysis of a cylindrical permanent magnet linear alternator. This alternator has four permanent magnet rings mounted on a plunger (shaft), which oscillates linearly when the machine is working. The magnetic polarities (N pole or S pole) of the rings are in radial direction and the polarities alternate from one ring to the next on each side of the plunger. Two of the magnet rings are on each end of the plunger and a magnetic loop is formed by the permanent magnet rings, plunger, stator core, and the air gap between plunger and stator core. The stator core is an iron structure surrounding the plunger, magnet rings, and the stator winding. When the plunger is at one end position of its oscillation, one stator pole faces to an N pole of a magnet ring and the other stator pole faces to an S pole of another magnet ring. When the plunger moves to the other end position of its oscillation, the magnet ring polarities face to the stator poles are switched. Therefore, the magnetic flux in the loop changes its value and direction for each cycle of the plunger oscillation. The stator winding, which is a circular coil around the plunger and inside of the stator core, experiences the flux changes and generates AC power. In this research, the magnetic filed study was conducted with Maxwell software, which is a product of ANSYS Company. The magnetic filed distributions for different positions of the plunger and different space gap between magnet rings were investigated. The results from the research are presented and discussed in this paper. Introduction In a cylindrical rotating alternating current (AC) generator, the magnets, which could be either permanent magnets or electrical magnets, are mounted on a rotor. When the rotor is rotating, these magnets create a rotating magnetic filed. The windings in stator slots cut the rotating magnetic field and produce induced voltages. Depending on the winding structure, the induced voltages can be either single phase or three-phase. When an electrical load is connected to the generator, the mechanical power used for driving the rotor is converted into electrical power and delivered to the load. Figure 1 shows a three-phase rotating AC generator 1 . Generators like this one have been used at most of the applications where an AC generator is needed, because majority of the power for driving generators are from a rotating machine, such as hydraulic turbine or a diesel engine. Figure 1. A three-phase AC generator When the driving power is from a linear oscillating source, the linear moving force has to be converted to rotating torque before it can be used for driving a rotating generator. Such conversion involves extra mechanical parts and power losses. In an application like this, a linear alternator or generator could be more efficient, because no linear force to rotating torque conversion is needed. Linear alternators had been under research for many years 2,3,4,5 . When more and more research is conducted on renewable energy and green energy, linear alternators will get more attention, since some of the new energy sources come with oscillating force in small stroke. One of the examples is generating electrical power with ocean tide energy 6 . In this paper, operation of a linear alternator is reviewed and magnetic field study on this machine is presented. The computer magnetic field study of this research was conducted as a graduate student project. Through this project, the involving graduate students not only leaned how to use the Maxwell software for creating an electrical machine geometry, adding permanent magnet and current to the machine, and interpreting the calculation results, but also got an in-depth understanding on the linear alternator operation and design as well as what the machine may do in utilizing renewable energy. Principle of Linear Alternator Figure 2 is a cross section view of a single-slot single-phase cylindrical permanent magnet linear alternator. This alternator has two parts: a stator and a plunger. The stator consists of a stator core, made of silicon-iron, and a circular stator winding embedded inside of the stator core. The plunger is a piece of silicon-iron tube with four permanent magnet rings mounted on it. Figure 2. A linear permanent magnet alternator This permanent magnet linear alternator's magnetic flux loop, the dash line path labeled with ABCDEF, and dimension symbols are shown in Figure 3. When the linear alternator is in operation, its plunger reciprocates with the stroke length: