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发表于 2010-3-2 15:09
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来自: 中国上海
Influence of Design Parameters on Cogging Torque
in Permanent Magnet Machines
Z. Q. Zhu, Member, IEEE and David Howe
Abstract—The influence of various design parameters on the
cogging torque developed by permanent magnet machines is investigated.
It is shown that the slot and pole number combination has a
significant effect on the cogging torque, and influences the optimal
value of both skew angle and magnet arc, as well as determining the
optimal number of auxiliary teeth/slots. A simple factor, which is
proportional to the slot number and the pole number and inversely
proportional to their smallest common multiple, has been introduced
to indicate the “goodness” of the slot and pole number combination.
In general, the higher the “goodness” factor the larger
the cogging torque.
Index Terms—Cogging torque, electrical machines, machine design,
permanent magnet machines, speed ripple, torque, torque
ripple.
I. INTRODUCTION
COGGING torque results from the interaction of permanent
magnet mmf harmonics and the airgap permeance
harmonics due to slotting. It manifests itself by the tendency
of a rotor to align in a number of stable positions even when the
machine is unexcited, and results in a pulsating torque, which
does not contribute to the net effective torque. However, since
it can cause speed ripples and induce vibrations, particularly at
light load and low speed, its reduction is usually a major design
goal [1]–[4].
In the paper, the effect of the slot and pole number combination
on the cogging torque is investigated, and its relationship
with various other design parameters, such as the width of
the stator slot openings, the magnet arc and the skew angle, as
well as with design features such as auxiliary teeth and slots is
considered, with respect to machines in which the magnets are
mounted adjacent to the airgap.
II. ANALYSIS TECHNIQUES
III. CHOICE OF SLOT AND POLE NUMBER COMBINATION
IV. USE OF AUXILIARY TEETH AND SLOTS
V. OPTIMAL MAGNET POLE-ARC
VI. SKEWING
VII. STATOR SLOT OPENING
VIII. CONCLUSION
Certain design parameters can have a very significant effect
on the cogging torque, in particular the slot and pole number
combination. It also affects the choice of the optimal magnet
pole-arc to pole-pitch ratio, and the optimal skew.
A simple factor has been introduced to indicate
the “goodness” of a slot and pole number combination
from the view point of cogging torque. It can, therefore, beused to aid the selection of an appropriate number of auxiliary
teeth and slots. In general, the higher the value of the largerthe cogging torque. Hence, brushless motors with overlapping
stator windings will generally produce about twice the cogging
torque as equivalent motors with nonoverlapping windings. An
odd number of slots is preferred for 2-pole motors, since is
usually smaller than for an even number of slots.
It has been found that the larger the value of then the
greater is the number of possible optimal ratios of pole-arc to
pole-pitch, while the larger the value of the greater the
number of possible values of optimal skew. |
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