a complete solution for electromechanical system design

8/3/2019 A Complete Solution for Electromechanical System Design

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A Complete Solution for Electromechanical System Design

Dr. Weizhong Fang, Ansoft Shanghai, ChinaDr. Uwe Knorr, Ansoft Corporation, USAScott Stanton, Ansoft Corporation, USA

Dingsheng Lin, Ansoft Corporation, USA

Abstract

With increased requirements for higher energy

efficiency of electrical drive systems and an in-creased use of electrical drives in transportationapplications a more accurate prediction of the

behavior of electrical machines is necessary.The design of electrical drive systems involvesmany aspects; from very detailed elec-

tromagnetic, thermal and semiconductor levelanalyses to system level mechanical and controldesign tasks. Often these designs are performed

by individual design specialist/groups without anintegral approach taking the various interactionsbetween the system components into account.

This paper will describe a complete design envi-ronment for electrical drive systems from com-ponent to system level design providing an inte-

grated toolset for various drive design tasks.

Integrating the design flow

The design of drive systems involves a wide va-riety of analyses and design methods. They

reach from finite element analysis for electro-

magnetic and thermal design problems over de-tailed electrical simulations including dynamic

and thermal semiconductor behavior to ratherabstract and higher level control design tasks.For each of these levels specific design pack-

ages are available using different numericalmethods. Which method is used depends on thedesign phase and the simulation target as well

as on the availability of models and computationpower.

Today’s technology provides unprecedentedcomputation power already on PCs. This allows

the engineer to re-evaluate traditional designmethodologies and to go to more complete de-sign strategies. For the design of drive systemsfrequently stand-alone tools are used. These

point tools have their individual strengths butusually miss interaction or even interfaces fordata exchange. However often results of a spe-

cific design stage are required in another designstage as input. Depending on the implementedrules in a design group there are different ap-

proaches for the design of products. Usually thefirst step is a selection of an initial design basedon technological specifications and customer

requirements. In terms of a drive system theselection of the electrical machine type and thespecification of the dimensions, winding data,

materials etc. would be a typical approach. Atthis design stage lots of design variants have tobe compared and analyzed. Therefore analytical

solutions in combination with design databasescan be used efficiently. Numerical methods arestill to time consuming to narrow the design

space. After the initial design an implementationspecification is available, covering most of thedesign requirements and specifications.

From here the design paths can vary dependingon the number of design issues covered and

design teams involved. Usually the design pathsplits into different physical domains. In terms ofthe drive system the motor design is transferred

to the electrical machine design specialist. At thesame time system designers can start to evalu-ate circuit topologies and control strategies. For

both levels analytical methods can deliver spe-

cific data sets. While in the machine design ge-ometry and material or winding schemes are

dominant, in the design of the power electronicsand control system lumped parameter modelsare usually used. This splitting of design flows

allows concurrent developments and savesvaluable design time. Each of the two designpaths evolve separately and run several design

cycles. Each of them may include optimizations.

Initial Design

FEA - based analyses State Space

Equivalent CircuitGeneration

Geometry &Material

Parameter

Sets

ParametricSolution

ParameterSets, Lookup

Tables Model Generation

Model Generation

Model Order

Reduction

Model Generation

Circuit, Control,

SystemSimulation



In the following sections solutions of Ansoft´sElectromechanical System Design Suite are

demonstrated for the achievement of differentdesign goals during the design process. As anexample for the design of a drive system a

switched reluctance motor system is used.Switched reluctance drive systems receive con-tinuous attention due to the robustness and low

manufacturing costs. However several issues,such as noise caused by torque ripple preventeda wide range introduction of this motor type so

far. Modern power electronics seem to be apossible solution to reduce torque ripple by us-ing smart control algorithms. The 6/4-pole, 3-

phase SRM motor and drive system used in thispaper has no real industrial background. It wasselected due to its generic design avoiding the

use of proprietary design data.

Design space reduction

First step in a design is the selection of an ap-propriate design for the given specifications.

RMxprt is a tool that helps designers to create awhole machine design out of a number of tech-nological specifications. The software package

supports this by an intuitive and easy to usegraphical user interface. After selecting the re-quired motor type the technical data for rotor,

stator and rating information can be entered ineasy to use dialogs (Fig. 1).

Fig. 1 Design Data Input

Due to the analytical methods used in RMxprtthe analysis runs only several seconds. Bothgeometry and performance data are available as

a result. Fig. 2 shows the flux linkage vs. cur-rent at different positions, Fig. 3 the laminationproposal as it is generated by the software.

These data help engineers to evaluate the qual-ity. The analysis is performed using analyticalmethods and therefore only lasts several sec-

onds. Since the analysis is relatively fast, it al-lows the engineer to modify designs quick and

easy in order to investigate different solutions inan extremely short time. Extensive data evalua-

tion capabilities and automatic report generationsave valuable time while analyzing the data andcreating presentations. RMxprt can also be

linked to Optimetrics – a powerful optimizationtool.

Fig. 2 Design Results- Performance Data

Fig. 3 Design Results - Lamination

The advantage of an integrated design environ-ment comes to full effect when the next design

stages have to be involved. Geometry and mate-rial data can directly be generated Maxwell, theFinite Element Analysis (FEA) based tool for

highly precise electro-magnetic design. Simulta-neously a system level model is created for adirect integration into SIMPLORER – the power

electronics and drive simulator.

System level design path

SIMPLORER – the system level design tool ofthe drive design suite – was specifically devel-

oped for power electronics and drive systemsimulation. Several features support an easyand fast model generation and provide excep-

tional simulation speed even for large-scaleproblems. SIMPLORER is based on a uniquesimulator coupling technology (Fig. 4). Inside

the kernel three different simulation engines are



running concurrently. Each engine was specifi-cally designed for a physical domain that is fre-

quently part of a drive system.

Fig. 4 Simulator Coupling Technology

The Circuit simulator is on non-SPICE based,specifically for power electronics applicationsdeveloped tool. The software provides several

power electronics and drives specific compo-nents such as power semiconductors and elec-trical machines. For each component several

accuracy levels are available. Depending on thesimulation target the user can select the accord-ing modeling level. In terms of power semicon-

ductors ideal switches as well as static, dynamicand dynamic-thermal components are available.Generally models are provided in multiple ab-

straction levels. Using different accuracy levelsthe user can save considerable time during thesimulation process. Ideal and static power

switch models can be used advantageously forsystem design tasks, dynamic and dynamic-thermal models for detailed electrical analysis.

The Block Diagram simulator is based on a dis -tributed integration method. It allows analyzing

complex control structures, mechanics or hy-draulics on a more abstract level. Typical appli-cations are vector control units for drive sys-

tems.

State Machines extend the modeling capabilities

towards discontinuous processes. They can beused very easy for the modeling of controls ofpower electronic systems, especially when it

comes to the generation of pulse patterns for thepower electronic semiconductors. Another appli-

cation could be the measurements of character-istic values online while a simulation is running.These values can be used as cost functions forthe integrated optimizer.

SIMPLORER´s open Simulator Coupling Tech- nology provides a way to co-simulate with exter-

nal tools. A future implementation using thattechnology will be the direct integration of theMaxwell2D transient finite element solver. This

link allows including the FEA machine modeldirectly into the systems simulator and running it

simultaneously.

D1

D2

A

+

IA

Speed

A

+Torque

Vsrc

A

+

D3

D4

IB A

+

D5

D6

IC

A_p B_p C_p

Position

A_m B_m C_m

Torque

+

V position

IGBT1

IGBT2

IGBT3

IGBT4

IGBT5

IGBT6

Fig. 5 RMxprt Model in SIMPLORER

The SR motor model created during the analysiswith RMxprt can be easily loaded into the SIM-

PLORER schematic. By placing an RMxprt sym-bol and assigning the according file name auto-matically an icon with the geometry of the motor

is generated as well as all electrical and me-chanical pins. Thereafter the driver circuitry andthe control are setup with SIMPLORER generic

components. Fig. 5 shows a part of theschematic with the SR motor symbol and the

driver bridge circuit.

Since the model was specifically generated foran electrical machine defined by given geomet-

ric dimensions and material data, there are noparameters to be modified. When the model fileis imported into SIMPLORER, the symbol auto-

matically shows the geometry of the motor. Allpins for electrical and mechanical connectionsare generated. The driver circuit is modeled us-

ing SIMPLORER´s static IGBT models. To seethe effects of the switching behavior of powersemiconductors dynamic IGBT or MOS models

can replace the idealized static model.

The position dependent control of the switches

is modeled based on state machines. Oneswitching cycle can be subdivided into 6 states,each where the according phase is turned on. A

state remains active until the position of the rotorhas passed the off-angle (a_of), which is repre-sented by the transitions between the states.

The position information is derived from the ac-

Simulation Data Bus

Circuit Simulator Circuit Simulator Block Diagram

Simulator Block Diagram Simulator State Machine

Simulator State Machine Simulator

Simulink Simulink MathCad MathCad Maxwell Maxwell

C/C++ C/C++



tual rotor position using SIMPLORER´s inte-grated expression evaluator. In each of the

states the control signals for the phases are setaccordingly (Fig. 6).

PAonSET: VSA:=1

PConSET: VSC:=1

PBon

SET: VSB:=1PBoff

SET: VSB:=0

PAoffSET: VSA:=0

PCoff

SET: VSC:=0

rp-pos1 >= a_off rp-pos1 >= a_on

SET: pos1:=pos1+a_on


rp-pos1 >= a_off

rp-pos1 >= a_onrp-pos1 >= a_off


rp-pos1 >= a_on

Fig. 6 Position Dependent Control

Fig. 7 shows the motor torque and Fig. 8 thethree phase currents at constant speed. The

results correspond to the results of the analyticsolution with RMxprt. The torque waveformshows some little oscillations in the rising slope

of the signal. These short changes are cause bythe analytic nature of the solution when the sta-tor and the rotor tooth are starting to overlap.

Later it is shown, that with an FEA basedequivalent circuit model these short distur-bances are eliminated. The design of the drive

system using the RMxprt models provides re-sults in a very short time with a acceptable accu-racy for many design purposes.

Motor TorqueTorque.I

t

0.30

0

50.00m

0.10

0.15

0.20

0.25

0 1.00e-0031.00e-004 2.00e-004 3.00e-004 .00e-004 5.00e-004 6.00e-004 7.00e-004 8.00e-004

Fig. 7 Motor Torque

Phase Currents

IA.IIB.IIC.I

t

7.50

-2.50

0

5.00

0 1.00m0.10m 0.20m 0.30m 0.40m 0.50m 0.60m 0.70m 0.80m 0.90m

Fig. 8 Phase Currents

FEA based machine design refinement

The SIMPLORER system level model is gener-ated automatically during the analysis with

RMxprt. Since all geometry and material dataare available it is easy to generate a Finite Ele-ment model. On the push of a button all neces-

sary data stored as ready to open Maxwell pro- ject. This seamless integration saves valuabledesign time compared to disconnected software

packages, where all data has to be enteredmanually. Fig. 9 shows the geometry in themodeler of Maxwell2D. All data from RMxprt is

already assigned and ready to use.

Fig. 9 Geometry in the 2D modeler

RMxprt automatically computes the minimumfield domain needed for the finite elementmodel. This saves significant computation time

without loosing accuracy. Fig. 10 shows theresult of Maxwell´s automatic meshing process.No user interaction is necessary.



Fig. 10 Finite Element Mesh

The Maxwell 2D transient solver analyzes the

machine considering eddy current effects, satu-ration, motion effects and losses. The externalcircuit can be defined using the built in sche-

matic editor. All mechanical properties are de-fined in easy to use dialogs. In the result of theanalysis the user gets electromagnetic field in-

formation as well as transient wave forms. Thefollowing pictures show some results of the tran-sient simulation. Fig. 11 presents a filed plot

and Fig. 12 a color shade representation of themagnetic filed density.

Fig. 11 Field Plot

Fig. 12 B Color Shade Representation

Motor designers can derive a variety of designinformation from these graphical displays. Fig.13 shows the transient torque waveform, Fig.

14 represents flux linkage versus time.

Fig. 13 Transient Torque Waveform

Fig. 14 Flux Linkage versus Time

More accurate data for the system levelsimulation

After the detailed analysis of the design someparameters of the electrical machine may havechanged due to geometric and/or material and

other modifications. Obviously an updated simu-lation on the system side will be necessary. Toinclude the results of a FEA analysis there are

different possibilities. The traditional method isto manually extract lumped parameters for con-ventional Park-transformation models. This

process however is time consuming and re-quires linearization that might lead to inaccurateresults under certain circumstances.

A better and easier solution is the automaticgeneration of system level models based on a

state space methodology. This technology canbe used advantageously for several electromag-netic systems, such as solenoids, sensors or

electrical machines. Especially for Switched Re-luctance (SR) machines, where a closed formu-lation based on a set of differential equations is

not possible, this approach is a useful alternativeto create fast and accurate system level models.



Based on the geometry and material definitionsfrom the transient analysis, a 2 or more dimen-

sional parameter sweep model can be gener-ated easily. From the results of this parametricFEA analysis, where the flux linkage for various

rotor positions and current distributions in thewinding is determined and recorded Max-well2D´s equivalent circuit generator derives a

new system level model. Multidimensionallookup tables embedded into the equivalent cir-cuit represent the nonlinear behavior of the sys-

tem. The generation of the model is fairly fastbecause the parametric solution is based onmultiple static finite element computations. This

of cause implies that the system level model willnot contain eddy current effects. However theachieved accuracy is sufficient for most detailed

system type analyses.

If eddy currents are essential, the most accurate

chance to integrate FEA results into a systemlevel simulation is a direct integration of the finiteelement solver into the system simulator. Even if

the FEA based model usually has a very highnumber of equations (usually in the order >10000) with the computation power available

today it is possible to run these problems to-gether. The key for that solution is SIM-PLORER´s simulator coupling technology. The

transient FEA solver and SIMPLORER ex-change simulation data at each simulation timestep.

The equivalent circuit model generated can beintroduced into a SIMPLORER schematic the

same easy way an RMxprt model is loaded. Fig.15 shows the same schematic with the importedmodel.

D1

D2

A

+

IA

Angle

A+

Torque

Vsrc

A

+

D3

D4

IB A

+

D5

D6

IC

IGBT1

IGBT2

IGBT3

IGBT4

IGBT5

IGBT6

N_1N_2 N_3N_4 N_5N_6

N_7 N_8

J

MchRMas1

GAIN

position

Fig. 16 Equivalent Circuit Model

The mechanical load is modeled using a rotating

mass element from the SIMPLORER mechani-cal library. These mass elements have all themajor mechanical non-linear effects already built

in, among others stick friction and backlash. Thesimulation results in Fig. 16 show, that thetorque waveform now does not have the short

disturbance in the rising slope.

Motor TorqueTorque.I

t

0.35

-50.00m

0

50.00m

0.10

0.15

0.20

0.25

0.30

0 1.00e-0031.00e-004 2.00e-004 3.00e-004 .00e-004 5.00e-004 6.00e-004 7.00e-004 8.00e-004

Fig. 17 Motor Torque

Conclusion

The design of electrical machines involves awide variety of design methods and design data.

In order to handle this complexity design toolsmust provide a tight integration and seamlessdata exchange capabilities. With the presented

design suite these requirements are fulfilled.Users can easily transfer data from one designlevel to another and model extraction is sup-

ported.

a complete solution for electromechanical system design

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