MAGNETIC DRIVE COUPLINGS

where n is the normal harmonic number and Nmis the total number of magnets. The finite thickness of the current sheet can be reduced to zero and the choice of xo is arbitrary so that
An axial force is also present behveen the copper and magnet discs during normal operation which is a function of the slip speed of the coupling. The net axial force acting along the input and output shafts will be equal and opposite for a symmetrical coupling. The magnetic coupling however offers the additional benefits of requiring no electrical contact and being reasonably
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MAGNETIC DRIVE COUPLINGS A.C.Smith, S.Williamson, A.Benhama, L Counter and J.M.Papadopoulos Brook Hansen, UK and Rexnord Technical Services, USA
INTRODUCTION
All field quantities are assumed constant in they- direction. Current flow in the xdirection (copper overhang) is included using classical Russell-Norsworthy functions 121.
A range of magnetic eddy current couplings has recently been launched in the USA. They offer the principal benefits of efficient operation, vibration isolation, no maintenance and seizure protection and have found application in blowers, compressors, conveyors and pumps. The principle of operation itself is relatively straightforward: one shaft is connected to a disc containing a number of embedded magnets of alternating polarity. These are magnetised in the axial direction. The opposing shaft is connected to a pair of steel discs that have a simple copper sheet attached to the inner surfaces. The magnet disc is situated symmetrically between the two steellcopper discs as shown in Fig 1. If the magnet disc has a velocity relative to the copper sheets, a current pattern will be produced in the copper discs. This current distribution will result in a circumferentialforce such that the copper sheet will endeavour to catch up with the magnets. The normal operating torque will result in a small slip speed between the two halves of the coupling.
Ninth international Conference on Electrical Machines and Drives, Conference Publication No.468, 0 IEE, 1999
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=kxBiblioteka -1Figure 2: Layer model Assumptions Layers 2 and 3 (steel and copper) and layers 7 and 8 are assumed to be moving at velocity v with respect to the magnets (stationary). All layers are considered to be homogenous and their material properties represented by conductivity U, and a constant permeability Each magnet will be representedby an equivalent current sheet and the complete magnet assembly assumed to be symmetrical in terms in the magnet dimensions, position and spacing. The magnets are also assumed to have a linear demagnetisation curve.
Material conductivities are temperature dependent using simple thermal coefficients. Excitation Layer The usual procedure for dealing with permanent magnets with a linear demagnetisation characteristic is to replace them with an equivalent current sheet. The magnitude of this current sheet is obtained from the coercive force and the depth of the magnet. The equivalent current density distribution for the base symmetrical unit of a pair of magnets (One pole pair) at X=G is shown in Figure 3 where b is the notional current sheet thickness and w is the axial depth of the magnet. The effective magnet mmf. F is given simply as ( H , -magnet coercive force) F = H,w where w is axial depth of the magnet layer.
Figure 1 : Magnetic Coupling
Layer Model
The linearised layer model is created in physical terms by imagining that the coupling is cut radially and rolled out flat in the circumferential direction. The mathematical formulation will be expressed in a rectangular co-ordinate system to reduce complexity. The basic model is shown in Figure 2.
tolerant of misalignment. In this situation, the misalignment will result in a net positive or negative axial force acting along the axis of the drive depending on the slip speed of the coupling. The objective of the paper is to describe a simple linear analysis based upon a layer theory approach that can be used as a routine design optimisation tool. This method was used extensively in the sixties and seventies for the analysis of linear induction motors. It accommodates the eddy current patterns flowing in the copper sheets in a relatively straightforward fashion and is ideally suited to this type of coupling because each layer is homogeneous. The normal equivalent-circuit based approach would be difficult and inaccurate for this type of device. The paper will contain a description of the layerbased analysis suitable for routine design of permanent-magnet eddy current couplings. The paper will also include practical measurements from a prototype coupling and a discussion of the various design issues of which misalignment is one. LINEAR FIELD ANALYSIS The model is based upon a layer theory approach which historically was developed to examine the behaviour of linear machines. These type of machines commonly incorporate a sheet copper secondary similar to that used in the eddy current coupling. The model assumes that each permanent-magnet disc in the coupling is faced by a copperktee1 sheet on either side. Any further magnet discs are then considered to be identical and therefore all force components should simply be multiplied by the number of additional magnet discs.