Abstract:
The soft start of a permanent magnet coupler of the type in which magnetic flux from permanent magnets on a rotary magnet rotor bridges an air gap between the magnets and an electroconductive element on a rotary conductor rotor to magnetically couple said rotors together responsive to relative rotary motion between said rotors, is adjusted by changing the amount of flux which bridges the air gap. This is accomplished by attracting some of the flux to magnetic material selectively positioned on said magnet rotor adjacent one or more of said magnets.

Description:
TECHNICAL FIELD 
     The present invention relates to magnetic couplers having cooperating permanent magnet and conductor rotors separated by air gaps. 
     BACKGROUND OF THE INVENTION 
     Permanent magnet couplers of the general type disclosed in U.S. Pat. No. 5,477,094 have a magnet rotor and conductor rotor interacting to transmit power between input and output shafts. The magnet rotor has a plurality of circumferentially spaced permanent magnets having their opposite poles separated by air gaps from electro-conductive conductor elements provided by the conductor rotor on opposite axial sides of the magnet rotor. For many applications it is preferred that there be a &#34;soft start&#34; at start up, meaning that initially during start up there is a noticeable rotational slip between the rotors which gradually reduces to a minimum in a few seconds as the output shaft builds up speed from zero to a maximum. Less starting torque is required to drive a given load if the coupler is matched to the load such that there is a soft start experienced during start up. Also, there is less shock to the power transfer system when soft start is provided. 
     It is preferred that the soft start of a coupler be adjustable and that this be readily accomplished in the field after installation of the coupler. This permits a given magnetic coupler to be suitable for maximum performance in a variety of load applications without a need to change the number of magnets in the magnet rotor or change the air gaps. It also makes it possible to field adjust soft start of a coupler if the load at the installation is changed. 
     One approach to achieving soft start permanent magnet couplers is to vary the air gaps between the magnets on the magnet rotor and the conductor elements on the conductor rotor in a manner that can be easily accomplished at the use site without disconnecting the coupler, moving the prime mover or load, or loosening or adjusting hubs. However, this has required that the magnet rotor have two coaxial rotor components that are slide-mounted relative to the hub of the magnet rotor such that they can be moved axially relative to one another toward and away from the axially-spaced conductor elements on the conductor rotor so as to vary the air gaps equally between the conductor elements and magnets, and thereby vary the soft start. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a soft start adjustment apparatus and method that is relatively simple to utilize in the field after installation of the coupling and does not involve air gap adjustment. The method involves varying the magnetic flux which bridges the air gaps between the magnets and the conductor elements. This is accomplished by selectively shorting out some of the flux by applying one or more flux shorting elements of magnetic material to the conductor rotor to extend, for example, between adjacent magnets. These shorting elements can take the form of U-shaped mild steel clips fitting over the rim of the magnet rotor and firmly fixed in position. Each clip shorts out part of the magnetic flux emanating from the adjacent magnets which otherwise would bridge the air gaps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a longitudinal sectional view through a coupler containing the invention; 
     FIG. 2 is an end view of the magnet rotor, as seen from the left side in FIG. 1, with one of the magnets partly broken away and containing two pairs of flux shorting clips; 
     FIG. 3 is a cross-sectional view of the magnet rotor taken as indicated by line 3--3 in FIG. 2; and 
     FIG. 4 is a perspective view of one of the flux shorting clips. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings there is illustrated a coupling of the type disclosed in U.S. Pat. No. 5,477,094, and namely one in which a conductor rotor 10 providing an axially spaced pair of circular electroconductive elements 12-13, and a magnet rotor 14 providing a plurality of permanent magnets 15 each having its opposite poles spaced by air gaps 16-17 from the electroconductive elements, are mounted by hubs 18-19 on input and output shafts 20-21. The electroconductive elements are preferably copper and are fixed to backing member 22-23 of a suitable magnetic material such as mild steel which are held in spaced relation by tubular spacers 24 extending between the electroconductive elements and by bolts 26 passing through the backing members 22-23, the electroconductive elements and the spacers 24 to nuts 27. The backing member 22 is mounted by bolts 28 to the hub 18. As an alternative arrangement, the members 22-23 may be non-magnetic and backing rings of magnetic material may be mounted between the electroconductive elements 12-13 and the members 22-23. 
     The magnet rotor 14 comprises a disc 30 of non-magnetic material having a plurality of generally rectangular openings 32 extending axially therethrough and radially oriented. These openings 32 are arranged in a circle and are equally spaced apart. The magnets 15 are mounted in the openings 32 such that adjacent magnets have their poles reversed. Bolts 33 connect the hub 19 to the disc 30. 
     When the input shaft 20 is rotated the magnetic flux emanating from the magnets 15 bridges the air gaps 16-17 and eddy currents are responsively induced in the rotating electroconductive elements 12-13, thereby creating a ring of poles with each pole having a polarity the same as the polarity of the adjacent pole of the related magnet. As a result the magnet rotor 14 and related output shaft 21 are rotated together with the input shaft 20 and conductor rotor 10, but with some rotational slip of the output shaft relative to the input shaft. This slip is normally a maximum at start-up when the output shaft is loaded. 
     In accordance with the present invention the effective magnetic flux output (strength) of the magnets bridging the air gaps 16-17 is adjusted so that the slip is radially reduced over a start-up period of several seconds as the load is taken. This enables less horsepower for start-up to be necessary for a given load. Adjustment of the effective flux output is accomplished by shorting (shunting) part of the available magnetic flux so that it will not bridge the air gaps 16-17. This is done by providing one or more removable flux shorting members 34 of magnetic material on the magnet rotor 14 between magnets. 
     Each flux shorting member 34 may comprise a generally U-shaped clip shaped to fit over the rim of the disc 30 of the magnet rotor 14 and presenting a pair of clip flanges 34a-34b extending from a central web 34c between two of the magnets as shown in FIG. 1. These clip flanges fit into a pair of recesses 36 formed in the end faces of the disc 30 starting at the periphery. This periphery is preferably formed with equal flat faces 30a each interrupted by a pair of radially extending threaded holes spaced apart circumferentially of the disc 30, 38. Each recess 36 has an outer circumferential portion 36a from which a plurality of tapered extensions 36b with rounded noses 36c continue radially inward between the openings 32 for the magnets. As shown in FIG. 2 and 4, each of the clip flanges is generally T-shaped providing each flange with a pair of arms 34d-34e and a central leg 34f. The clip web 34c has a pair of tapered holes 39 arranged to register with a selected pair of the radial holes 38 in the disc 30 so that the respective clip can be held in place by a screws 40 each with a beveled head. The ends of the noses 36c of the tapered recess extensions 36b may have an undercut complemented by a recessed end portion 34g on the clip legs 34f so that the clips interfit with the disc 30 in a manner preventing spreading of the clip legs apart. 
     If, after the coupler has been installed at a job site, it is determined that a softer start is desired, a pair for example, of the flux shorting clips 34 are installed at diametrically opposite locations on the magnet rotor by applying them to the magnet rotor through one of the gaps between the spacers 24 in the conductor rotor. The installed clips preferably engage the adjoining magnets and attract magnetic flux therefrom. To that extent the clips weaken the flux bridging the air gaps 16-17. If this is found to provide an insufficient reduction in effective flux, a second pair of the clips 34 is installed at diametrically opposite locations and preferably midway between the first installed pair for balance as indicated in FIG. 2. If an odd number of flux shorting clips is to be used a non-magnetic clip of the same weight can be used to even out the number of clips to weight balance the magnet rotor. Adding of shorting clips is continued until the desired soft start is achieved. 
     It is important to note that the installation or removal of the shorting clips can be achieved when the coupler is in a fully assembled condition, thus making it possible to do so when the coupler is ready for start-up. The shorting clips do not require special skills for installation, and the desired soft start for a given load condition can readily be achieved by the described simple trial and error procedure in practicing the method of the present invention. 
     The described flux shorting apparatus for performing the method of the present invention is not intended to imply that the method is limited to use of this apparatus to vary the magnetic flux bridging the air gaps. For example, flux shorting elements may be slide mounted on or in the magnet rotor to be adjusted radially between inactive positions radially inset from the magnets to active positions adjoining some of the magnets. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.