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Modular PM Machines Based on Soft Magnetic Composite (SMC )

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### Modular PM Machines Based on Soft Magnetic Composite (SMC)

Prof. T.A. Lipo

Wen Ouyang (Ph.D Candidate)

University of Wisconsin-Madison

April, 2006

Introduction

- This study is to investigate the benefits of soft magnetic composite (SMC) material in electrical machine design for a typical drive system.
- A modular machine structure is adopted for flexible machine geometry benefiting from the SMC fabrication process.
- For each SMC module, concentric winding is designed to minimize the end winding section and simplify the fabrication process.
- The machine performance is discussed briefly with illustratioin of a prototype Surface PM machine design.
- Healthy and faulted operations are also briefly investigated.

Soft Magnetic Composite (SMC)

- Soft Magnetic Composites (SMC) are composed of surface-insulated iron powder particles.
- SMC can be compressed to form uniform isotropic components with complex shapes in a single step.
- SMC makes it possible to define a magnetic field in three dimensions, thereby permitting the designer to build an electric motor beyond the restrictions set by the traditionallamination technology.

Electrically Insulated Fe-powder Particles

Typical SMC micro-structure

Soft Magnetic Composite (SMC) Magnetic Property

Technology improvement narrows the gap between steel and SMC.

SMC (Somaloy500) Material Properties

Core Loss (W/kg) Measurement from Ring Sample (OD55 ID45 H5 mm)

(according to CEI/IEC 60404-6)

Hz

Tesla

SMC Iron Loss Characteristics (2)

Although the hysteresis loss of SMC is higher than conventional lamination, better eddy characteristics makes it suitable for applications with high frequency excitation.

- Compact winding structure with the minimum end winding section.
- Higher slot fill with simpler winding and less insulation.
- 3D structure provides extra flux path by the extension of pole tip.
- Concentrated winding further reduces the machine volume.
- Machine can be assembled by the SMC modules, which simplifies the machine fabrication process.

Module segment

Additional Advantages

- Reduced copper volume as a result of increased fill factor and reduced end winding length,
- Reduced copper loss as a result of the reduced copper volume,
- Unity iron stacking factor,
- Reduced high frequency tooth ripple losses since the SMC has essentially no eddy current losses,
- The above bulleted items suggest a potential increase in overall efficiency,
- Potential for reduced air gap length as a result of the tight tolerances maintained in manufacturing SMC material,
- Reduced axial length-over-end-winding dimension as a result of the compact end winding,
- Absence of phase insulation as a result of using non-overlapping windings,
- Potential elimination of the ground wall insulation since the SMC stator itself acts as an insulator,
- No need to stress relieve the stator lamination after punching and assembling the stack, a relatively costly and time consuming task, (stress relief is, however, included as part of the manufacture of the SMC part),
- Reduced conducted EMI when machine is used with inverter supplies since the stator SMC body acts as an insulator and does not conduct current to ground,
- Reduced bearing currents in the presence of PWM waveforms again because of the use of SMC which acts as insulation against this type of current flow,
- Modular construction allows the possibility of easy removal of an individual modular unit for quick repair or replacement,
- Stator is easily recyclable since the stator can again be compressed back into powered form with pressure and the copper windings readily removed.

Disadvantages

- Relatively high hysteresis loss (low frequency loss),
- Slight penalty a result of smaller saturation flux density,
- Relatively brittle material,
- Producibility of structures to meet close specs not yet mastered,
- Size of producible structures are limited.
- Lower relative permaability (700 vs roughly 3000)

Module Shape Parameter (γ) Dependancy Analysis

Two Phase

Three Phase

With higher γ, which means larger slot opening, the fundamental suffers with increased harmonic components.

Module Shape Parameter (χ) Dependancy Analysis

Two Phase

Three Phase

With a rectangular design (χ=0), the fundamental reaches maximum while the same occurs for the harmonic components.

Module Shape Analysis Comments (Trapezoidal)

Comparisons of two phase and three phase design with trapezoidal pole shape

Module Shape Comparison (Trapezoidal and Sinudoidal)

Comparisons of two/three phase design with trapezoidal/sinusoidal pole shape

- The general-purpose sizing equations have been developed and takes the form of:
- Or
- With the definitions of the variables in both equations listed below:

PRrated output power

K = Ar /As, ratio of electric loading on rotor and stator. In a machine topology without a rotor

winding, K=0.

m number of phases of the machine

m1 the number of phases of each stator (if there is more than one stator, each stator has the same m1).

Keemf factor that incorporates the winding distribution factor Kw and the ratio between the area spanned by the (salient) poles and the total air gap area.

Machine Design Equations (continued)

Kia current waveform factor in order to indicate the effect of the current

waveform, where:

the current i(t) and Iphmax are the phase current and the peak phase current, Irms is the rms current.

Kpwelectrical power waveform factor,

where fe(t)=e(t)/Epk and fi(t)=i(t)/Iphmax are the expressions for the normalized emf and current waveforms. e(t) and Epk are the phase air gap EMF and its peak value. T is the period of one cycle of the emf.

KL , defined as the aspect ratio coefficient

o , the diameter ratio

the machine efficiency,

Bgmaxflux density in the air gap

A total electric loading, including stator and rotor loading

Nt the number of turns per phase,

f the power supply frequency

p number of machine pole pairs

Finally, the machine power density for the total volume can be defined as:

where Lt is the total length of the machine including the stack length and the protrusion of the end winding from the iron stack in the axial direction.

Five Phase Machine Concept Structure

- Five phase machine design offers independent phase control of each module with the integration of switching devices of each module.
- Fault tolerant capability ( up to 2 phase fault ) makes it a potential candidate for applications with critical requirements.
- Higher torque density can be achieved compared with typical induction machine.
- Modular design makes it possible to replace fault modules conveniently when necessary.

Machine assembly and module profiles

Two main design methodologies are applied in this project: 1) Analytical. 2) FEA.

The analytical method is based on a closed form analysis of the machine equations.

Advantages: 1) Concise formula for the machine performance.

2) Explicit dependency of machine design parameters.

3) Easy for optimization.

Disadvantages:1) Errors associated with nonlinearity and complexity of

the structure.

FEA method is based on the numerical method analysis derived from Maxwell equations.

Advantages: 1) Very accurate solution for the machine performance.

2) Direct geometry modeling and analysis.

Disadvantages:

1) Computation cost, especially for 3D.

2) Difficulty to achieve global optimization.

Machine Optimization Method (FEA)

- Response Surface. ( Design of Experiment )

pros: 1) Simple algorithm.

2) Global optimization.

3) Parameter impact information can be obtained.

4) Practiced quite a lot from aerospace engineering, such as plane wing

shape design. But few reports on machine design in IAS since 2000.

cons: 1) If the parameter number is 10, the sampling points

for the initial solution space will be 3^10=59049, which is 41 hours

CPU time if each point FEM simulation takes 1 minute.

2) Data analysis method is necessary to reduce the polynomial error.

3) High number of parameters (over 15) will take too much time on the

solution space construction, resulting in an unfeasible approach.

The coefficients are evaluated by regression.

The error of the model is less than 1% of the FEM prediction in most cases.

Machine Parameter Extraction

- Does it is necessary to consider all the machine geometry parameters?
- Machine main parameters could be controlled under 10 without sacrificing the effectiveness of analysis (Does the radius of slot corner matter much?).
- The main machine parameters:

Stator side: (1) Stator outer diameter (2) Stator inner diameter

(3) Yoke thickness (4) Slot width (5) Slot opening

Rotor side: (1) Rotor outer diameter.(rotor inner diameter does not matter much)

(2) 2~4 parameters for surface PMs or 4~6 parameters for IPM.

Air gap: This is a very sensitive parameter, can be fixed based on

mechanical suggestion. Thus, the rotor outer diameter is

dependent on the stator inner diameter if air gap is selected at the

very beginning, which reduces the rotor side parameters!

- Thus, it is very practical to control the machine parameters with level of 10 variables

Example of Stator Module Structure

The machine stator module can be defined by six main parameters:

1) Stator out radius. 2) Stator yoke thickness. 3) Stator inner radius.

4) Tooth span angle. 5) Tooth body width. 6) Tooth tip thickness.

Example of Surface PM Rotor Structure

The PM span and PM thickness are key parameters, with extra 1 or 2 parameters necessary if the PM is not a regular shape.

Example of Interior PM Rotor Structure

The bridge width is fixed due to the saturation and stress consideration.

5 parameters are necessary for the definition of a typical IPM rotor structure.

Optimized Results (SPM)

Optimization for maximum torque and acceptable efficiency

Optimized Results (IPM)

Optimization for limited field weakening capability and torque capability

Inductance Dependence on Rotor Position (IPM)

Due to large tooth piece design, the machine inductance is inherently dependent on

rotor position, the associated energy variation produces cogging torque.

Higher back EMF limits the speed range of SPM rotor design due to the DC bus voltage.

Optimization of module structure to minimize the back EMF harmonics is one of the optimization objectives.

SPM and IPM Rotor Concept Comparison

Back EMF waveforms

Back EMF Harmonics

THD_SPM=9.87%

THD_IPM=7.16%

SPM Five Phase SMC Drive System Mathmetical Model

- Terminal voltage equation:

where v, e, i, λ denotes the vector of phase terminal voltage, back emf, current, and flux linkage:

- Torque equation from the idealized energy conversion:

SPM Starting Process Simulation (Open loop I)

Torque Response

Speed Response

- Fixed inverter excitation frequency (20 Hz)
- Purely sinusoidal current waveform, light load (up) & heavy load (down)

SPM Starting Process (Open loop II)

Speed Response

Torque Response

- Variable inverter excitation frequency (10Hz ~ 90 Hz) in 0~1 sec.
- Sinusoidal current supply, light load (up) & heavy load (down) for fan load.

SPM Starting Process (Closed Loop)

Speed Response

Torque Response

- Variable inverter excitation frequency with feed back from rotor position.
- Sinusoidal current supply, fan type load simulated.

SPM Phase Loss (1 phase out)

Speed Response

Torque Response

- One phase open circuit at t=0.5 sec.
- Close loop control assumed (rotor position feed back).
- Rotor speed reduced due to the torque loss.
- Torque pulsation increases due to unbalanced operation.

SPM Phase Loss (2 phases out)

Speed Response

Torque Response

Adjacent phases out

Non-adjacent phases out

SPM Torque Simulation (FEA) on Phase Loss

Average Torque

Torque Characteristics

- Briefly, the average torque is proportional to the phase number in healthy condition.
- Cogging torque and current ripple is the main source of torque pulsation if the machine is healthy.

SPM One Phase Short Circuit (simplified assumption)

Phase Current

Speed & Torque

- One phase terminal short circuit induces huge torque ripple.
- Fault phase current depends on the turns number involved in the short circuit and machine speed.

Summary

- Soft Magnetic Composite (SMC) materials have attractive magnetic characteristics for high frequency (high speed) motor designs
- SMC materials have opened up the door to machine designs with 3 dimensional flux paths.
- The chief limitation is currently the permissible size of SMC components.

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