rotor design for sensorless position estimation in permanent-magnet machines

NCKU Servo Control Lab / Electric Motor Technology Research Center

Rotor Design for Sensorless Position Estimation in

Permanent-Magnet Machines

Rafal Wrobel, Alan S. Budden, Dan Salt, Derrick Holliday, Phil H. Mellor,

Andrei Dinu, Parmider Sangha, and Mark Holme

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 9, SEPTEMBER 2011

Page(s): 3815 - 3824

學生 : 馮謙詠

指導老師 : 王明賢

1


Outline1.Abstract

2.Introduction

3. Design Considerations

a. Manufacturing Considerations

4. FE Design

a. FE Modeling

b. Saliency Analysis

5.Conclusions

6.References

2


AbstractA high-frequency injection sensorless rotor position estimation algorithm is

incorporated directly into the finite element design process to realize a

permanent-magnet (PM) machine that is suited to zero- and low-speed

sensorless control.

The machine design is tightly constrained by an existing stator assembly, only

enabling the redesign of the replacement PM rotor.

3


IntroductionSensorless position estimation in PM machines is reliant upon the existence of

a measurable electrical parameter that is related to rotor position. When the

machine is operating at speed, the electromotive force (EMF) generated across

the machine supply terminals provides a suitable rotor-position dependent

quantity .

When the rotor is stationary or operating at low speed, however, no rotor-

position-dependent parameter is readily available, and more complex methods

that typically depend upon detection of the machine’s rotor dependent

inductance profile are used .

Typically, a PM machine may exhibit some degree of magnetic saliency which

will cause a rotor-position-dependent variation in the inductance of the stator

windings.

4


Design ConsiderationsThis machine comprises a concentrated wound stator with 1.5 slots per pole

and a uniformly magnetized rotor with NdFeB magnet segments bonded to the

surface of a magnetically permeable steel hub, as shown in Fig. 1.

The interior PM (IPM) rotor topology exhibits more favorable saliency

characteristics and was therefore chosen as the basis for the development of a

new rotor.

5

Fig. 1. Baseline concentrated wound surface magnet topology.


Manufacturing Considerations

6

To simplify manufacture, the proposed rotor would use soft magnetic

composite (SMC) pole pieces with rectangular PMs arranged in between a

“spoke” pattern.

The proposed rotor design is shown in Fig. 2, where the composite pole pieces

and PMs are bonded to a nonmagnetic steel hub and the structure is secured

using a thin sleeve.

Fig. 2. Proposed IPM arrangement.


FE DesignThe manufacturing considerations described in Section 3-a dictate that torque

and magnetic saliency are key design parameters.

In addition, the predefined stator structure and air gap and the proposed rotor

structure and materials limit the design variables to the depth and width of the

rectangular magnet segments.

7


FE ModelingThe magnet depth is expressed in terms of the ratio of the inner dimension

to the outer radius , and the width is expressed as the ratio of an angle

subtended by the magnet to the pole pitch , as shown in Fig. 2. These

quantities are represented in the set of dimensionless parameters x, as shown in

(7), which is used to specify the motor structure used in the FEM analysis

Torque is calculated using the coenergy method shown in (8), where is the

magnetic coenergy, Θ is the angle defining the relative position between the

rotor and the stator, and ΔΘ is the angular rotation between field solutions

8

mmR

R

p

R

Rm

p

m , (7)

W


FE ModelingNote that (10) is the inverse of the more usual definition of the saliency ratio

and reflects the inverse saliency that is characteristic of the IPM machine

Both the torque and saliency ratio calculations are repeated for different values

of x, with target values of = 1 to ensure the correct torque performance

and = 1.1, 1.2, 1.3, . . . , 2.5 to ensure a degree of rotor saliency that is

suitable for sensorless control.

The FE analysis incorporates an objective function, defined in (11), which

seeks to minimize the error between the calculated and target values of torque

and saliency

9

.

id

iqi L

Lx (10)

ixTTi


FE Modeling

10

Fig. 4 shows that, in general, the torque developed by the motor increases as (magnet width) increases and as (magnet depth) decreases. The saliency surface, shown in Fig. 5, is more complex and shows no simple trend in relation to changes to and .

pm

pm

rm RR

rm RR


FE ModelingTable I presents the target torque and

saliency ratio values and the

corresponding values resulting from the

FE design process which are also

highlighted on the surface plots of

Figs. 4 and 5. It is important to note that the

saliency ratios shown in Table I result from

calculation at a fixed rotor position

and that, beyond the limits of

= 0.73 and = 0.93, none of

the resulting rotor versions meets the design

specification.

11

pm pm


Saliency Analysis

12

Fig. 9. Estimated rotor position for rotor version 1.

Fig. 11. Estimated rotor position for rotor version 14.


Manufacture Of Rotor Version3

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Fig. 14. IPM rotor version 3. (a) Rotor components. (b) Assembled rotor priorto the addition of the containment sleeve.


ConclusionA high-frequency injection sensorless position estimation algorithm has been

directly incorporated into the FE design procedure for an IPM machine,

resulting in a machine that exhibits the required electrical characteristics while

being tailored for sensorless control.

The design was highly constrained such that the rotor was required to fit

within an existing stator and to produce the same torque as the original motor

The experimental results verify the machine performance and operation under

the control of an injection-based sensorless rotor position strategy.

14


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Thanks for your attention.

rotor design for sensorless position estimation in permanent-magnet machines

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