electric motors ocw
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Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6.685 Electric Machines Class Notes 7: Permanent Mag net “Brushle ss DC” Motors February 11, 2004 c 2003 James L. Kirtley Jr. 1 In tr oduction This document is a brief introduction to the design evaluation of permanent magnet motors, with an eye toward serv o and driv e applic ati ons. It is org ani zed in the foll owing manne r: Fir st, we describe three different geometrical arrangements for permanent magnet motors: 1. Surface Mounted Magnets, Conventional Stator, 2. Surface Mounted Magnets, Air-Gap Stator Windin g, and 3. Internal Magne ts (Flux Concentra ting). After a qualitative discussion of these geometries, we will discuss the elementary rating param- eters of the machine and show how to arrive at a rating and how to estimate the torque and power vs. speed capability of the motor. Then we will discuss how the machine geometry can be used to estimate both the elementary rating parameters and the parameters used to make more detailed estimates of the machine performance. Some of the more involved mathematical derivations are contained in appendices to this note. 2 Motor Morphol og ies There are, of course, many ways of building p erman ent magne t motor s, but we will consi der only a few in this note. Actually, once these are under stood, ratin g evaluations of most other geometri cal arrangements should be fairly straightforward. It should be understood that the “rotor inside” vs. “rotor outside” distinction is in fact trivial, with very few exceptions, which we will note. 2.1 Surface Magnet Mac hines Figure 1 shows the basic magnetic morphology of the motor with magnets mounted on the surface of the rotor and an otherwise conven tio nal sta tor windin g. This sk etch does not sho w some of the important mechanical aspects of the machine, such as the means for fastening the permanent mag net s to the rotor, so one shou ld look at it with a bit of caution. In addition, this sk etc h and the other sketches to follow are not necessarily to a scale that would result in workable machines. This figure shows an axial section of a four-pole ( p = 2) mac hi ne. The four magnet s ar e moun ted on a cylindrical rotor “core ”, or shaft, made of ferromagne tic material . Typ icall y this would simpl y be a steel shaft. In some appli cat ions the magn ets may be simply bonded to the steel. For applications in which a glue joint is not satisfactory (e.g. for high speed machines) some sort of rotor banding or retaining ring structure is required. 1
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