Circuitry for increasing efficiency of a linear electric generator

#This invention relates to linear electric generators (LEGs) and to circuitry for improving their efficiency. A problem with known LEGs may be best explained with reference to prior art FIGS. 1 and 2. A LEG may be constructed, as shown in FIGS. 1 and 2, so as to have a long stator (e.g., an induction coil assembly 24 of length d1) and a relatively short permanent magnet assembly 22, of length d2. When the permanent magnet assembly 22 moves relative to the induction coil assembly 24 a voltage (and a current if a load is connected across the coil assembly) is induced in the coil assembly. An advantage of systems in which the coil is made much longer than the magnet, as compared to those with long permanent magnet assemblies and short induction coil (stator) assemblies, includes the ability to hold the wiring of the induction coil assembly stationary (i.e., cables are not continually moved or flexed). Another advantage of such systems is that the long coils and short permanent magnet enable the use of relatively simple passive permanent magnet dampers/brakers (e.g., passive dampers can be formed by placing conductive plates at the ends of the travel of the permanent magnets to form a passive damper or brake). Still another advantage of having a relatively short permanent magnet is that big and long magnets present a hazard in that they tend to attract a large amount of debris.

#Although the structure shown in FIGS. 1 and 2 has many advantages, a problem with known linear electric generators having a long induction coil assembly and a relatively short permanent magnet assembly is that the electric current generated in the coils has to pass (flow) through the entire coil assembly (i.e., all the coils) in the stator, as illustrated in FIG. 1. The useful voltage derived from the coils is obtained from those coils and coil sections directly opposite and very close to the permanent magnet assembly. This useful voltage induces a current which flows through the coils. The portions of the stator coils that are not adjacent (or directly opposite) to the permanent magnet assembly (PMA) and those that do not interact with the magnet assembly cause a voltage drop in the coil (i.e., due to the resistance and inductance of the coil) without enhancing the generation of additional current. The voltage drop due to the resistance/impedance of the coils not contributing to the generation of voltage (current) results in significant losses in the power being generated by the LEG.

#A proposed solution to the problem is shown and discussed in a co-pending application titled Coil Switching Circuit for Linear Electric Generator by David B. Stewart et al filed concurrently herewith and bearing Ser. No. 11/030,932 and assigned to the same assignee as the instant application and whose teachings are incorporated herein by reference. The Coil Switching application teaches the use of a switching arrangement for coupling only selected sections of coils of the induction coil assembly (ICA) of a LEG across output lines of the LEG. The selected sections include those sections of coils of the ICA closest to the passing PMA. A disadvantage of the proposed solution is that it requires the use of switches to couple the active coils to the output lines of the LEG and switches to decouple or bypass the inactive coils. For proper operation of the system using a switching arrangement, position sensors, and/or other appropriate means, are needed to sense the position of the PMA relative to the ICA to constantly turn switches on and off in order to ensure that only desired coils are in fact connected in circuit. This disadvantage is overcome in circuits and systems embodying the invention.

#In the accompanying drawing like reference characters denote like components, and

#FIG. 1 is a schematic diagram illustration of a prior art Linear Electric Generator (LEG) permanent magnet and coil assembly;

#FIG. 2 is a schematic diagram illustrating a prior art permanent magnet and induction coil assembly of a LEG;

#Features and various structures of linear electric generators (LEGs) embodying the invention are shown in FIGS. 3-10. In these figures, for ease of description, only one of three possible electrical phases is shown. However, it should be understood that the apparatus may, and typically will, include one or more (e.g. 3) phases. One application of linear electric generators (LEGs) embodying the invention is as a power take off (PTO) device in wave energy converters (WECs) which are placed in a body of water and which include elements (e.g., shaft, 3 and shell, 5) responsive to the motion of the waves in the body of water to produce electric energy. However, it should be understood that a LEG embodying the invention may be used in any other suitable application.

#As shown in the figures, LEGs embodying the invention include a permanent magnetic assembly (PMA) 30 and an induction coil assembly (ICA) 20 separated by a small air gap. In this application it is assumed that the length (d2) of the PMA 30 is smaller than the length (d1) of the ICA 20. In a WEC embodying the invention, the PMA 30 may be attached to (or mounted on) one of a shaft 3 and shell 5, and the ICA 20 may be attached to (or mounted on) and disposed along the other one of the shaft 3 and shell 5 as shown in FIGS. 4A and 4D. In systems embodying the invention, the shaft or the shell may move relative to the other, or both may move relative to each other.

#The PMA 30 is typically constructed of multiple pairs of “north” and “south” polarized magnets mounted on the surface of a ferromagnetic material structure (e.g. steel) with the poles oriented perpendicular to the line of the air gap. These magnets comprise a magnetic “pole pair”. The magnetic circuit may be comprised of a pair of magnets, “air” gaps, a stator yoke, and a magnet backing plate, the latter two items being constructed of ferromagnetic material.