A Novel Direct-Drive Dual-Structure Permanent Magnet Mach

ANSYS 2011中国用户大会优秀论文

A Novel Direct-Drive Dual-Structure Permanent Magnet Machine

Shuangxia Niu, S. L. Ho and W. N. Fu

The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong By incorporating the merits of fractional-slot concentrated windings and Vernier machine structure, a new multi-pole dual-structure permanent magnet (PM) machine is proposed for low speed, direct-drive applications in this paper. In the outer stator, a fractional-slot concentrated winding is adopted to reduce the slot number and stator yoke height, hence saving space and improving torque density. In the inner stator, a Vernier structure is used to reduce the winding slots, thereby enlarging the slot area to accommodate more conductors, thus the inner stator space is fully utilized. Consequently, the merits of these two structures can be ingeniously integrated into one compact PM machine and the torque density is improved, cogging torque is reduced and the control flexibility with two sets of independent stator windings is increased. By using time-stepping finite element method with curvilinear elements for moving between the stator and the rotor, the steady state and transient performances of the PM machine are simulated and the validity of proposed dual-structure PM machine is verified.

Index Terms—Curvilinear element, dual-structure, finite-element method, fractional-slot concentrated winding, permanent magnetic motor, Vernier structure.

I.I NTRODUCTION

IGH TORQUE, low speed permanent magnet (PM)

machines have a wide range of applications in direct-

drive systems, such as in wind power generation, electric

traction of electric vehicles and robots. In such low speed

direct-drive machines, the pole number of the machine is

usually large. For conventional PM machines with integer slot

windings, high pole number implies high slot number and for

machines with a limited outside diameter, large slot number

will result in low copper fill factor, long slots, narrow tooth

width, and high magnetic flux leakage. In order to avoid such

undesirable effects, fractional-slot concentrated windings are

used [1]. In fractional-slot concentrated windings, each coil is

wound on one tooth and the slot number per pole per phase is

generally less than one. Compared with integer slot windings,

the end winding is shortened and manufacture cost is reduced.

Another method to reduce the slot number and produce high

torque at low speed is to adopt a Vernier machine structure. A

Vernier PM machine with concentrated winding can produce

specific space harmonics in the airgap magnetic field with low

armature pole pairs and slot number [2].

In this paper, the fractional-slot concentrated windings and

Vernier PM machine structure are ingeniously incorporated

into a compact dual-structure PM machine. A time-stepping

finite-element method with curvilinear elements for the

modeling of the rotation movement is developed to simulate

the dynamic operation of the proposed machine.

II.P ROPOSED M ACHINE

A.Machine Structure

The proposed dual-structure PM machine has 11 pole pairs with 24 slots in the outer stator, 1 pole pair with 3 slots in inner stator, 11 pole-pair surface mounted PMs on the inner and outer faces of the rotor, as shown in Fig. 1.

(b)

Fig. 1. Proposed machine. (a) Cross sectional view. (b) Structure.

1)Outer stator

For the outer stator, the fractional-slot concentrated windings are used, and the slot number

s

N and the pole number p

2 satisfies the following relationship

2

2±

=

s

N

p(1) In the outer stator, 24

=

s

N and 22

2=

p. This combination of slots and poles in the outer stator has the following merits [3]:

(1) The fractional-slot concentrated winding arrangement in the outer stator can shorten the end winding, thus improving the utilization of copper materials, and reducing the copper losses. The efficiency of the machine is increased.

H