Project 2
Abstract—Brushless DC (BLDC) Motors are receiving
significant attention from industries since the last
decade. They are extremely reliable and weightless than
other machines of comparable power outputs; also they
are suitable for high speed applications such as in CNC
milling machines. Although the design procedures of the
machine are available as a concatenation of many
different sources, the need for an unified, step-by-step
design procedure from first principles of
electromagnetic is an absolute requirement. The
objective of this project is to design an Internal
Permanent Magnet Brushless DC (IPMBLDC) Motor.
Once it was determined what type and size of material
available for stator and rotor, a reasonable size and
power output estimated. Using general radial flux
concept the design was refined. Software tool, RMxprt of
ANSYS is used to build geometry of motor and to
calculate basic design parameters, and then this design
is taken into Maxwell 2D environment to further
analyze it.
Keywords—BLDC motor, ANSYS, Permanent Magnet,
Motor Parameters.
I. INTRODUCTION
As its name suggests, a brushless motor is a motor
without brushes unlike the conventional DC, Slip Ring
Induction and Synchronous motors. Brushless Permanent
(BLPM) motors are finding wide applications in industries
in recent few years and people are trying to replace rugged
Induction motors with BLPM motors. One of the main
advantage exhibited by BLPM motors which overcomes IM
is that, its torque per ampere doesn’t vary during
operation and it’s easy to control, especially in high
precision motion control applications. Another plus point
regarding BLPM machines is that, there is nothing like slip
as in the case of IM [1] [2]. Fig. 1.1 shows a schematic
diagram of a general BLDC motor.
Fig. 1. Schematic of a BLDC motor [3]
There are several different configurations of BLDC
motors depending upon the position of permanent
magnets and stationary coils viz, exterior rotor and
interior rotor configuration.
A. Interior Rotor Motor:
If an application requires rapid acceleration and
deceleration of load then the torque to inertia ratio should
be as high as possible. This indicates use of interior rotor
motor configuration. This motor has the closest
configuration to that of the classical AC motor.
B. Exterior Rotor:
If an application requires constant speed at medium to
high speed it may make more sense to use an exterior
rotor motor. The most common example of this type is
hard disk drive motor.
II. FUNDAMENTALS OF MAGNETIC FOR BLDC DESIGN:
A. Motor size
This is a very preliminary design requirement before
starting design of any motor. The answer for this query is
usually given by, Equation (1).
π=ππ·2πΏ (1)
In this equation, T stands for torque given by motor, k
is a constant, D is rotor diameter and L is the axial length of
motor. It can be understood from Eq.1 that torque, in turn
power output of motor increases if its rotor diameter or
axial length are increased. Now if the length of motor gets
doubled, power output has to get doubled. On the other
hand, if diameter of rotor gets doubled, there is more room
for magnets available on rotor because increasing
diameter of rotor increases circumference of rotor
considerably. Considering this fact, in case of motors used
for submersible pump application is having much more
axial length compared to their rotor diameter. This is done
just to improve power output from motor.
B. Basics of Magnetic Circuits
A magnetic field is described by two important
quantities, B magnetic flux density, which is magnetic flux
available per unit area, whose unit is wb/m2or Tesla and
other one is H magnetic field intensity, which is change in
the magnetic field, whose unit is A/m[10]. These two
quantities are collinear for all magnetic materials. When
magnetic materials are operated at very large operating
point, the relation between these two quantities can be
stated by Eq.(2), μ is known as permeability of the
material.
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B = μH (2)
Magnetic circuit analysis plays around two
fundamental equations, one relates magnetic flux density
to magnetic flux (B to ∅) and the other one relates field
intensity to magneto motive force (H to MMF). Now the
total flux in a given volume can be given as integration of
all flux densities in the given area, it is given by Eq. (3),
where A is cross sectional area. This forms first
fundamental equation.
∅
= π΅π΄ (3)
Now the second fundamental equation is givenby, Eq.
(4), where F is magneto motive force, whose unit is A and π
is length of material, whose unit is m.
πΉ = π»π (4)
Substituting Eq. (3) and Eq. (4) into Eq. (2) and
rearranging the terms gives us Eq. (5)
∅
= ππΉ (5)
In Eq. (5), P is known as permeance of material and it is
given as Eq. (6), whose unit is wb/A or henries H.
Materials having higher permeability have higher
permeance and promote greater flux flow through them.
π = ππ΄
π (6)
Eq. (5) is analogous to general Ohm’s law, which is I = GV.
Flux ∅ is analogous to current I, MMF is analogous to
potential difference V and conductance G is analogous to
permeance P. Inverse of permeance is known as reluctance
R, it is given in Eq. (7).
π
= 1/π = π/ππ΄ (7)
Now in terms of reluctance, Eq. 2.5 can be re-written as,
πΉ=
∅
π
(8)
Eq. 1.8 is analogous to ohm’s law, but current flow through
resistance constitutes energy dissipation while flux
through reluctance constitutes energy storage.
C. Sources of magnetic field
There are two sources of magnetic fields one is an
electromagnet in which a wire is wrapped around a soft
iron piece and other one is a permanent magnet.
D. Concept of Flux Linkage
The way in which electrical energy gets converted into
mechanical energy is very closely dependent upon how
much flux gets created because of magnetic field, also the
way in which this flux links with the materials available in
its vicinity.
III. DESIGN PROCEDURE FOR BLDC MOTOR
The approach here is to begin with basic motor
physical constraints and magnetic circuit describing flow
of flux, from which operating point is to be decided. Then
motor dimensions and current required to generate
specific motor output power at rated speed with desired
back emf is to be found [4].
A. Radial flux topology
Radial flux topology is most commonly used strategy in
motor construction. With reference to Fig. 3.1, some of the
strengths and weaknesses can be given as,
1) Strengths
ο· Rotor-Stator attractive forces are balanced around
the rotor so there is no net radial force on rotor.
ο· Heat produced by the stator winding is readily
removed because of the large surface area around
the stator back iron.
ο· Except for skewing, the rotor and stator are
uniform in the axial direction.
ο· The rotor is mechanically rigid and easily
supported on both ends.
2) Weaknesses
ο· For a surface mounted magnet rotor, it is not
possible to use rectangular shaped magnets; at
least one surface must be arced.
ο· If the motor is to operate at high speeds, some
means of holding the magnets to the rotor are
required, thus sleeve or strapping adds to the air
gap length.
ο· The air gap is not adjustable during or after motor
assembly.
ο· The adhesive bonding the rotor magnets to rotor
back iron forms another air gap since it is
nonmagnetic.
IV. MOTOR DESIGN:
In this focuses on basic design of motor in RMxprtTM of
Ansys Maxwell. Fixed parameters available from below
Table used as input to this software. A brief introduction to
RMxprt is given here first, and then details about design
are given.
A. RMxprtTM
ANSYS RMxprt is a template-based design tool.
Designers of electrical machines and generators can
enhance their design with this tool. Together Maxwell and
RMxprt create a truly customized machine design flow to
meet market demand for higher efficiency, lower cost
machines. Using classical analytical motor theory and
equivalent magnetic circuit methods, RMxprt can calculate
machine performance, make initial sizing decisions, and
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perform many analyses in a matter of seconds. RMxprt can
then automatically set up the complete Maxwell project (2-
D/3-D) including geometry, materials, boundary
conditions including the appropriate symmetries, and
excitations with coupling circuit topology for rigorous
electromagnetic transient analysis. Design sheets list all
the relevant input and calculated parameters and a
graphical display of waveforms, such as current, voltage,
torque, and back emf. It uses classical electric-machine
theory in combination with a magnetic circuit approach to
calculate performance.
B. Stator design
Table I. shows analysis setup parameters for design.
Based on these parameters RMxprt will solve for other
parameters. The diameter of lamination available is stator
outside diameter.
TABLE I. GENERAL INPUT PARAMETERS
SR.NO.
PARAMETER
VALUE
1
Rated Output Power(W)
500
2
Rated Voltage(V)
230
3
Number of Poles
4
4
Given Rated Speed(rpm)
3000
5
Frictional Loss(W)
10
6
Windage Loss(W)
2
TABLE II. STATOR DATA
Sr.
No.
PARAMETER
VALUE
1
Number of
Stator Slots
12
2
Outer
Diameter of
Stator(mm)
108
3
Inner Diameter
of Stator(mm)
62
4
Length of
Stator
Core(mm)
55
5
Number of
Conductors
per Slot
110
6
Average Coil
Pitch
4
7
Stator Slot Fill
Factor(%)
40.6927
The stator parameters are shown in Fig. 4.1 and Table 4.2
shows parameters.
Fig. 2. Stator data
C. Slot parameters
Details about slot dimensions are given in Fig. 4.2 and
Table 4.3.
Fig. 3. Slot Dimensions
TABLE III. SLOT DIMENSIONS
Sr. No.
PARAMETER
VALUE
1
hs0(mm)
1.5
2
hs1(mm)
1.0
3
hs2(mm)
9.4
4
bs0(mm)
2.5
5
bs1(mm)
11.4777
6
bs2(mm)
16.8367
7
rs(mm)
3
V. PARAMETRIC SOLUTION
Rmxprt has one tool called parametric solution by
which software can be requested to solve one parameter
with respect to another parameter. Here conductors per
slot (cps) were solved in a given range in order to get the
number at which maximum efficiency and output power at
rated speed can be fetched. Results are shown in Fig. 3 and
Fig. 4.
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Fig. 4. Parametric solution of output power v/s cps
Fig. 5. Parametric solution of efficiency v/s cps.
A. Rotor design
Rotor has internal permanent magnets as shown in Fig.
4.5. This rotor pole template is available in RMxprt. Only
the magnet dimensions and rotor outside diameter are to
be input.
Fig. 6. Rotor geometry
Fig. 7. Cross section view of motor
Following figures represent some of the plots got after
simulating design in RMxprt. Only important plots are
given here.
TABLE IV. ROTOR PARAMETERS
Sr. No.
PARAMETER
VALUE
1
Minimum Air Gap (mm)
1
2
Inner Diameter (mm)
18
3
Length of Rotor(mm)
110
4
Stacking Factor of Iron Core
0.95
5
Type of Steel
M43-26G
6
Bridge (mm)
0.5
7
Rib (mm)
10
8
Mechanical Pole Embrace
0.6
6
Max. Thickness of Magnet
(mm)
3.5
7
Width of Magnet(mm)
20
8
Type of Magnet
NdFe30
TABLE V. OTHER PARAMETERS OF MOTOR
SR.
NO.
PARAMETER
VALUE
1
Residual Flux Density (Tesla)
1.1
2
Coercive Force (kA/m)
838
3
Maximum Energy
Density(kJ/m3)
230.45
4
Relative Recoil Permeability
1.0446
5
Demagnetized Flux Density(T)
0.2023
56
6
Recoil Residual Flux Density(T)
1.1
7
Recoil Coercive Force (kA/m)
838
8
Total Net Weight(kg)
3.5834
9
9
Air-Gap Flux Density(T)
0.4931
4
10
Average Input Current(A)
2.5322
11
RMS Armature Current(A)
2.3209
12
Armature Thermal
Load(A2/mm3)
70.319
5
13
Specific Electric Loading(A/mm)
15.728
6
14
Armature Current
Density(A/mm2)
4.4708
1
15
Frictional and Wind age Loss(W)
12.107
1
16
Iron-Core Loss(W)
20.521
6
17
Armature Copper Loss(W)
45.618
1
18
Transistor Loss(W)
3.7336
1
19
Total Loss(W)
82.292
8
20
Output Power(W)
500.11
3
21
Input Power(W)
582.40
6
22
Efficiency(%)
85.870
2
23
Rated Speed(rpm)
3020.0
4
24
Rated Torque(N.m)
1.5813
5
Fig. 11. Plot showing air gap flux density
Fig. 12. Plot showing Induced coil voltages
VI. MAXWELL 2D SIMULATION
RMxprt gives crude motor design and performance
based on given input dimensions. In order to check
performance of motor thoroughly, it is necessary to run a
finite element simulation in software like Maxwell in 2D.
The geometry prepared in RMxprt is taken into Maxwell
2D environment. There software itself creates meshing
which is necessary for FEA, also it automatically chooses
boundary conditions in which geometry is to be solved.
Winding excitations are also assigned automatically here
and software itself decides symmetry in geometry and
accordingly does operations and shows motor, like shown
in Fig.12.
Fig. 13. Sector of Motor in Maxwell 2D
Following are some analysis done in Maxwell 2D by
running the simulations and saving fields at an
appropriate step time. Fig. 13 shows lines of flux in the
stator and rotor and Fig. 14 shows field intensity in stator
and rotor and Fig. 15 shows flux density in stator and
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Fig. 18. Motor winding currents
Fig. 19. Torque output of motor when magnet width is
increased
VII. CONCLUSION
This paper has demonstrated an approach to brushless
DC motor design and to analyze it. It is seen that as per
given dimensions of laminations and magnet, the motor is
able to give a power of around 500 Watt with considerably
lesser torque ripples. RMxprt tool of ANSYS solves basic
motor design equations and gives crude design data. This
data should not be considered as optimized motor design
parameters. It is necessary to undergo simulation through
Finite Element Analysis tool of ANSYS that is Maxwell 2D.
According to this design, motor gives considerably good
efficiency at rated speed and torque. Also ripples in torque
are within limits and currents in winding are not
exceeding beyond rated value. it can be concluded here
after doing this project that, it is not always necessary that
results obtained from RMxprt should match up with the
results got by simulating same model in Maxwell 2D.
Another point to be noted here that, in case of internal
permanent magnet motor, width of magnet plays
important role in deciding torque ripples. Magnet should
cover maximum area so that flux coming from magnet
should cover maximum air gap area. Again it is worth to be
noted that, there is a limit of power which a motor using a
particular size magnet will produce. As seen here, a
magnet with width of 20 mm is able to give 500 Watt of
power at lower armature current density and at
considerably good efficiency. It is seen that output power
of motor increases considerably but at the cost of some
compromise with efficiency. A parametric solution has
been run to check optimized value of conductors per slot
which can give almost 1000 Watt of power output at
considerable efficiency. It is seen that as width of magnet
increases, air gap flux density and motor output power
increases. Also a ripple in torque gets reduced as air gap
flux becomes more uniform.
REFERENCES
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Abstract
This article presents a new and powerful freeware software called MotorAnalysis-PM and discusses its application in electromagnetic design and analysis of permanent magnet (PM) motors for the electric vehicle (EV) industry. This new PM motor software utilizes both finite element (FE) and analytical methods to speed up the analysis and design process of PM motors significantly. The analysis and design methodology using MotorAnalysis-PM is presented and discussed for a 50 kW PM motor utilized in a commercial EV. To validate the accuracy of the software, the numerical results obtained from the PM motor design and analysis tool are compared with experimental results. The numerical and experimental results validate the flexibility of this software in achieving accurate motor design with short design times which is of great interest to EV and PM motor manufacturers.
1. Introduction
The development of high-performance permanent magnet (PM) motors in electric vehicles (EVs) presents engineers with the challenge of developing new designs to increase efficiency and power density while reducing the cost of the electromechanical devices [1]. The conventional design process for PM motors is based on utilizing analytical calculations given in generalized form which is labor-intensive and difficult to follow [2,3]. With the progress in computer and simulation technologies on the one hand and increasing demand for superior motor characteristics on the other hand, the advanced computer-based simulation tools become an essential part of the design process. Not only do they assist in better understanding of the behavior of the device under different operating conditions, but also allow performance optimization and cost reduction through better material utilization and minimizing the number of prototypes required during the development process [4,5]. The PM motor design methodology proposed in this paper is intended to transform the conventional textbook design process from a generalized form to a definite workflow centered on the use of a computer-based simulation tool.
In recent years, commercial and expensive electromagnetic tools have been employed for the design and analysis of PM motors for EVs with some interesting examples found in the literature [4,5,6,7,8]. For example, Huynh and Hsieh utilized the commercial finite element (FE) software called JMAG to analyze the electromagnetic and thermal performance of PM motors for EVs. Authors in [4] studied thin electrical steel laminations in the rotors with different configurations of flux barriers with neodymium–iron–boron (NdFeB) permanent magnets. Liu et al. analyzed the optimization process for PM synchronous motors utilizing ANSYS Maxwell software [5]. Cetin and Daldaban employed ANSYS Maxwell software and magnetic circuits to analyze axial flux PM motors with different PM geometry designs [6]. Eklund and Eriksson employed COMSOL Multiphysics software to study the effects of remanence and recoil permeability of a PM material on the performance of PM synchronous generators [7]. He and Wu employed ANSYS Maxwell software to analyze the design of interior PM brushless direct current (DC) motors for an electric impact wrench used for loading and unloading car bolts [8].
Nevertheless, the high cost of commercial analysis software is often unaffordable for many PM motor manufacturers with a limited budget. For this reason, it is important to develop new and free analysis and design tools for the PM motor industry. Some recent and interesting examples of free and non-commercial tools are found in the literature [9,10,11,12,13]. Meeker in [9] developed general-purpose and non-commercial FE software called “FEMM software” for two-dimensional (2-D) FE analysis. With FEMM software it is possible to perform magnetostatic and time-harmonic analysis, electrostatic analysis, heat-flow analysis, and current flow analysis. It is not possible to perform transient analysis using FEMM software. In addition, although FEMM software is freely available, the design of PM motors using this FE software is complex and laborious. Lu et al. [10] solved this problem by development of SyR-e tool based on Octave-FEMM and Matlab-FEMM scripting library and graphical user interface. SyR-e is a free tool but some calculations, such as steel core losses, PM losses and efficiency map, are still delegated to expensive commercial FE software. Lehikoinen et al. [11] developed an open-source Matlab 2-D finite-element method (FEM) library for electrical machines called SMEKlib. SMEKlib is a library in which more attention has been given to the features and basic analysis tools, rather than the user interface. Another popular tool for designing a PM motor which is available for free is JMAG-Express Public developed by the JSOL corporation [12]. JMAG-Express Public is an analytical tool developed for quick and rough estimation of motor characteristics. Since JMAG-Express Public is not FEM-based, it is suggested to be used only for basic design while a detailed design is achievable by commercial tools provided by the JSOL corporation. Finally, another free tool which is worth mentioning is Emetor [13]. Emetor is an online tool offering powerful magnetostatic FE analysis features with geometry templates, materials library and automated winding setup. Table 1 shows a comparison of different free PM-motor design tools. All these developments indicate that there is still an undeniable need for a freely available PM motor design/analysis tool that provides a dedicated user interface with accurate and flexible electromagnetic motor analysis capabilities.