1. Aspects of Permanent Magnet Machine Design
Christine Ross
February 7, 2011
Grainger Center for Electric Machinery and Electromechanics

2. Outline Permanent Magnet (PM) Machine Fundamentals
Motivation and Application
Design Aspects
PM Material
PM Rotor Configurations
Manufacturing Processes
Design Tools

3. Permanent Magnet (PM) Machine Fundamentals
Focus on electronically controlled PM AC synchronous machines
Rotor magnetic field is supplied by PMs
Stator windings are sinusoidally distributed windings, excited by sine-wave currents
“Brushless DC” machines can also use PMs
3-phase
stator windings
Laminated
stator
Four magnets – two pole-pairs, p = 2
Between the slots are the teeth which carry the flux through the winding region;
Flux is predominantly confined to the teeth and links the coils of the winding
4-pole PM rotor
Cross-section of
surface-mounted PM machine

4. PM Machine Theory Output torque is proportional to power
Control instantaneous torque by controlling magnitude of phase currents
Go into d-q equivalent circuit? Torque production

5. PM Machine Control Instantaneous torque control
Servo performance kW
Fast dynamic response
Smooth output torque
Accurate rotor position sensor information needed
Single-phase equivalent circuit

6. PM Machine Control Flux-weakening control Constant power drives
Traction, washing machines, starter/alternators
Require constant output power over a speed range
To operate above rated speed while maintaining rated terminal voltage, reduce flux by controlling magnetizing current
Ideal flux-weakening characteristics
[1] Soong

7. Motivation for PM Machine
Motivation for PM machines:
High efficiency (at full load)
High power density
Simple variable-frequency control
Rotor excited without current
No rotor conductor loss and heat
Magnet eddy current loss is lower than iron loss and rotor cage loss

8. PM Machine Disadvantages
Magnet cost
New magnet manufacturing processes
Magnet sensitivity to temperature and demagnetization
Little control of magnet field
Always have no-load spinning losses
Without control, over speed means over voltage – fault management issues
Limited by flux density of PM
Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air-gap flux density and eventually, core saturation.
Uncontrolled air-gap flux density leads to over voltage and poor electronic control reliability.
A persistent magnetic field imposes safety issues during assembly, field service or repair, such as physical injury, electrocution, etc.
In all cases, high performance permanent magnet materials are always expensive or virtually cartel controlled by a single country.
The mining of high performance permanent magnet materials is environmentally demanding and as a result, the use of permanent magnets is by no means environmentally friendly.
Air-gap depth tolerance improves only 20% over other electric machines before magnetic leakage becomes the same concern for any electric machine.
High performance permanent magnets, themselves, have structural and thermal issues.

9. PM Machine Applications
AC PM machines
Servo control systems
Precision machine tools
IPM – washing-machines, air conditioning compressors, hybrid vehicle traction
DC PM machines
Lower cost variable-speed applications where smoothest output torque is not required
Computer fans, disk drives, actuators
Industrial applications where constant speed is necessary
Differing functionality, precision, automation in motion control
IPM washing-machine motors
[5] Hendershot and Miller

10. Design Specifications
Electrical
Environmental
Ambient temperature
Cooling system
Structure
Vibration
Mechanical outputs
Torque
Speed
Power
Key features of machines
Flux linkage
Saliency, inductances
Assembly process
Magnet cost
Number of magnets
Simplicity of design
Field weakening
Reluctance torque
Field control
Line start, no inverter
Saliency – winding inductances vary as a function of rotor position;
No saliency – if remove magnets, rotor has no tendency to align itself with stator when current is flowing (no reluctance torque)
After this slide, add about general PM design procedure?

11. PM Material Soft magnetic material (steel) – small B-H loop
Hard magnetic material – (PM) – large B-H loop
Choose magnets based on high Br and Hc
Remnant Flux
Density Br
Coercivity Hc

12. PM Material Arnold Magnetics PM Br (T) Hc (kA/m) Cost
Resistivity (µΩ-cm)
Max. Working Temp. (ºC)
Curie Temp. (ºC)
Alnico5-7
1.3 60 47
> 500
Ferrite
0.4
300
low
>10,000
250
450
NdFeB (sintered)
1.1
850
medium
150
80-200
Sm2Co7 (sintered)
1.0
750
Higher than NdFeB 86
Energy product – largest value of product of B and H, measure of magnet performance
Add curie temperature?
Because of high coercivity of high performance permanent magnet materials, such as neodymium, air-gap depth is more tolerable, which puts lower structural constraints on frame and bearing assemblies.
Ferrite – low-voltage DC motors
Samarium-Cobalt magnets (late 1960s) – higher power levels
Neodymium-Iron-Boron (NdFeB) magnets (early 1980s) – increased the cost-effectiveness of high-energy magnets
Demagnetization:
Thermal effects – as temperature increases
NdFeB: both Br and Hc reduce, most prone to demag at high temp
Ferrite: Br reduce, Hc increase, most prone to demag at low temp
Can improve by using higher Hc magnets, increase magnet thickness or airgap
[2] Miller
[3] Hendershot and Miller
Arnold Magnetics

13. PM Material Chinese dependency No shortage Mountain Pass, CA Idaho
Nd is about as common as Cu
Arnold Magnetic Technologies

14. PM Machine Rotor Configurations
Surface-mounted PM rotor
Maximum magnet flux linkage with stator
Simple, robust, manufacturable
For low speeds, magnets are bonded to hub of soft magnetic steel
Higher speeds – use a retaining sleeve
Inset – better protection against demagnetization; wider speed range using flux-weakening; increases saliency; but also increases leakage
Inset magnets
In embedded type, including small region larger than air gap of non-magnetic material between magnets causes the leakage to not increase
Surface bread-loaf magnets

15. PM Machine Rotor Configurations
Interior-mounted PM (IPM) rotor
IPM Advantages
Extended speed range with lower loss
Increases saliency and reluctance torque
Greater field weakening capability
Reulctance torque comes from the shape of the steel lamination, not from the magnets
Total torque is a combination of reluctance torque and magnet alignment torque
A. O. Smith

16. PM Machine Topology SMPM: IPM: More mechanically robust
Magnet losses can be an issue (not shielded by rotor iron); reduce by segmenting magnets axially or radially or increasing magnet resistivity
IPM:
Better demagnetization withstand
Note that inverter limits the performance of torque and speed with current and voltage
What is saliency, why can it be useful or good? – when rotor is unexcited, rotor will rotate if the stator is excited
Why is field weakening useful – wide range of applications requiring wide speed range at constant power; field-weakening capability allows IPM to achieve this
IPM controller more complex – nonlinear relationship between torque and current,
Obtain maximum torque, phase-shift the current by an angle that depends on the load, even without magnetic saturation
IPM magnets less susceptible to eddy-current losses
Characteristic
SMPM
IPM
Saliency No
Yes
Field Weakening
Some
Good
Controller
Standard
More Complex
[4] Klontz and Soong

17. PM Manufacturing Practices
Realistic manufacturing tolerances
Key parameters – stator inner diameter, rotor outer diameter, no load current, winding temperature
Issues with core steels – laser cutting, punched laminations, lamination thickness
Issues with magnets – dimensions, loss of strength due to thermal conditioning
High speed practice and limits – rotor diameter limits speed
Hybrid Camry PM synchronous
AC motor/generator
ecee.colorado.edu

18. PM Machine Design Process
Design and simulate motor and driver
Separately
Combined
Analytical, lumped-circuit, and finite-element design tools
Different tools are used to trade-off understanding of the design, speed, and accuracy
Finite element meshing,
flux lines and B for
SMPM machine
A.O. Smith

19. Analytical Design Tools
Broad simplifying approximations
Equivalent circuit parameters
Use for initial sizing and performance estimates
Performance prediction
Limitations
Does not initially account for local saturation
Requires tuning with FE results

20. Analytical Design Tools
Core losses
Hysteresis loss
Eddy current loss
Anomalous loss – depends on material process, impurities
Problems with core loss prediction
Stator iron loss: based on knowledge of stator tooth flux density waveforms
Usually assumes sinusoidal time-variation and one-dimensional spatial variation
Flux waveforms have harmonic frequency and rotational component
Use dB/dt method for eddy-current term, frequency spectrum method
Torque, efficiency, inductance
[4] Klontz and Soong

21. Lumped-Circuit Design Tools
Non-linear magnetic material modeling of simple geometries
Need a good understanding of magnetic field distribution to partition
Fast to solve, good for optimization
Limitations
Requires tuning with FE results
Lovelace, Jahns, and Lang

22. Finite-Element Modeling and Simulation Tools
Important aspects – model saturation
More accurate
Essential when saturation is significant
Limitations
Meshing
Only as accurate as model design – 2D, 3D
Not currently used as a design tool due to computational intensity
Nonlinear magnetostatic FE
average magnetic flux density solution
for machine with solid rotor

23. Ideal Design Tool Easy to set up
Models all significant aspects of machine that affect performance – magnetic saturation
Efficiently simulates transient conditions and steady-state operation

24. References
[1] W.L. Soong, “Design and Modeling of Axially-Laminated Interior Permanent Magnet Motor
Drives for Field-Weakening Applications,” Ph.D. Thesis, School of Electrical and Electronic
Engineering, University of Glasgow, 1993.
[2] T.J.E. Miller, “Brushless Permanent-Magnet and Reluctance Motor Drives”, Oxford Science
Publications, 1989.
[3] J.R. Hendershot and T.J.E. Miller, “Design of Brushless Permanent-Magnet Motors”, Magna
Physics Publishing and Oxford University Press, 1994.
[4] K. Klontz and W.L. Soong, “Design of Interior Permanent Magnet and Brushless DC Machines –
Taking Theory to Practice” course notes 2010.
[5] J.R. Hendershot and T.J.E. Miller, “Design of Brushless Permanent-Magnet Motors”, Motor
Design Books, 2010.
Questions?