chapter 7 performance improvement using cobalt iron...
TRANSCRIPT
103
CHAPTER 7
PERFORMANCE IMPROVEMENT USING COBALT IRON
ALLOY MAGNETIC CORE
7.1 INTRODUCTION
M19, 29 gage silicon steel lamination sheet stack was used for
magnetic core of the stator assembly in the integral slot (48 slots)
configuration, fractional slot (60 slots) configuration and improved 60 slots
configuration discussed in the previous chapter. The commonly used
lamination core material for commercial motors is AISI M45 or M36 with
0.5mm thickness. These laminations are readily available in the market.
However, M19 silicon steel material with 0.35mm is considered for the
magnetic core in order to reduce the core loss component of the motor.
Sourcing and procuring small quantity of M19, 29 gage material for the
developmental work was very difficult and the cost per kg of M19, 29 gage is
also high compared to M45 grade with 0.5mm thickness.
The loss characteristics for different flux density levels for M19, 29
gage silicon steel material is shown in Figure 7.1. The power loss per weight
is lower in M19 compared to M45 and M36 electrical sheet. The saturation
flux density of the M19, 29 gage material is 1.9 Tesla. The main electrical
property is listed below.
Temperature : 20° C
Frequency : 60 Hz
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Mass Density : 7600 kg/m³3
Curie temperature : 1350 ° F or 732 ° C
Specific gravity : 7.65
Silicon content : 2.85 to 3.25 %
Electric resistivity at 20°C : 0.47 e-6 m
Saturation : 1.9 Tesla
Figure 7.1 Loss characteristics of M19, 29 gage silicon steel
The improved fractional slot (60 slots 16 poles) quadruplex
winding redundancy brushless dc motor is tested for its static torque
performance using the torque pickup coupled to testing fixture. Figure 7.2
and 7.3 shows the linearity test profile and stall torque output respectively
with M19 29 gage lamination core for the improved 60 slots stator. The
excitation is given to the two phase coils and the maximum torque rotor
position is obtained. Fixing the rotor position, the line to line current is
increased by 2 Ampere up to full load current of 13 Ampere. The torque
exerted by the rotor for corresponding current is measured and plotted as
shown in Figure 7.2. For stall torque measurement, the full load excitation is
given to the two phase coils. The permanent magnet rotor is rotated in steps
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and the corresponding stall torque is measured and plotted as shown in
Figure 7.3.
LINEARITY TESTTwo phase excitation, maximum torque rotor posiiton
0
1.5
3
4.5
6
7.5
9
0 2 4 6 8 10 12 14 16
Current in Amps
Figure 7.2 Linearity test: Improved 60 slots stator with M19, 29 gage steel
STATIC TORQUE PROFILETwo phase exci tation, Current=13 A
-9-7.5
-6-4.5
-3-1.5
01.5
34.5
67.5
9
0 5 10 15 20 25 30 35 40 45 50 55 60
Rotor position in mech deg
Figure 7.3 Static torque: Improved 60 slots stator with M19, 29 gage steel
The peak torque output is 7.5 Nm for 13 Ampere line to line
current. The torque output characteristic is not linear during the higher
winding currents. This improved fractional slot configuration motor is said to
be optimal design for the given volume constraint because of the optimal
magnetic loading and electrical loading. The magnetic loading is limited by
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the saturation at 1.9 Tesla for M19, 29 gage silicon steel. The electrical
loading is such that the slot occupies maximum number of turns feasible for
winding keeping the overhang thickness requirement. The torque performance
of the motor can be increased further if high permeability magnetic material
with saturation flux density over 2.0 Tesla is used for the stator magnetic
core. This is accomplished by replacing M19 29 gage silicon steel lamination
with high permeability Cobalt Iron alloy (Hiperco 50A) lamination for stator
magnetic core. The dc magnetization curve and loss characteristics curve for
Cobalt Iron alloy (Hiperco 50A) is shown in Figure 7.4 and 7.5 respectively.
The saturation flux density is 2.4 Tesla for this high permeable magnetic
material. Hence the performance comparison is carried out between the M19
29 gage silicon steel lamination material and Cobalt Iron alloy lamination
material for the armature magnetic core without changing the geometry and
loading product of improved 60 slots configuration.
Figure 7.4 Saturation curve for Cobalt Iron alloy material
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Figure 7.5 Loss characteristics curve for Cobalt Iron alloy material
7.2 COBALT IRON MAGNETIC MATERIAL FOR STATOR
CORE
The improvement in motor torque constant of the developed motor
is studied by replacing Cobalt Iron alloy lamination for stator core instead of
conventional silicon steel lamination material. The torque output of the motor
for both Cobalt Iron alloy (Hiperco 50A 0.014) lamination and silicon steel
(M19, 29 gage) lamination is simulated to show the improved performance.
The flux density in the designed airgap, flux density distribution in the stator
core, generated back-EMF voltage at specified speed are simulated for the
two stator lamination material in the finite element based electromagnetic
software for performance comparison.
The 60 slots motor configuration is modelled and the material
properties are assigned to the stator, airgap and rotor parts of the two models.
The transient 2D with motion solver is used to solve the model. The flux
density distribution with full load winding excitation and for random rotor
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position is shown in Figure 7.6 for M19, 29 gage silicon steel and Figure 7.7
for Hiperco 50A 0.014 Cobalt Iron alloy. The higher tooth flux density in
Cobalt Iron alloy material implies that it carries larger flux in the magnetic
circuit.
Figure 7.6 Flux density in motor model (M19, 29 gage silicon steel)
Figure 7.7 Flux density in motor model (Hiperco 50 0.014)
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The airgap flux density due to permanent magnet flux is plotted forthe motor with M19 29 gage steel and Hiperco 50A 0.014 material for thearmature magnetic core. The stator assembly and rotor assembly for the twoconfigurations are same. Figure 7.8 shows the airgap flux density profile forboth the magnetic core materials. The values are same for both the materialsat different rotor position over one pole pitch.
FLUX DENSITY PLOT
0
0.2
0.4
0.6
0.8
1
1.2
3.833 6.333 8.833 11.33 13.83 16.33 18.83 21.33 23.83
Position in mechanical degrees
M19 29 gage Si Steel Hiperco 50A 0.014
Figure 7.8 Airgap flux density for one pole pitch
Figure 7.9 and 7.10 shows the tooth flux density plot for M19 29gage silicon steel magnetic core and Hiperco 50 0.014 magnetic corerespectively. The maximum flux density in the tooth for cobalt iron alloy is2.2 Tesla whereas 1.9 Tesla in Silicon steel magnetic core. This ensuresmaximum flux carrying capacity of the high permeability Cobalt Iron material.
Figure 7.9 Tooth flux density in M19 29 gage armature stack
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Figure 7.10 Tooth flux density in Hiperco 50 0.014 armature stack
The back-EMF is simulated for the calculated number of turns,
designed flux density and given surface velocity for the improved 60 slots
stator with two armature magnetic cores. The phase to phase back-EMF
voltage at 1000rpm rotation is shown in Figure 7.11. The profile shows the
peak value of 78V for 1000 rpm rotor speed and same for both magnetic core
types. The back-EMF constant is 0.74 V/(rad/sec).
LINE AB BACKEMF VOLTAGE
-100-80-60-40-20
020406080
100
0 89.59 178.58 268.08 357.08
Rotor Position in electrical degrees
M19 29 Gage Si Steel Hiperco 50A 0.014
Figure 7.11 Back-EMF voltage at 1000 rpm
The torque performance output is simulated for both the core types
with six step commutation drive. The characteristics curve is shown in
Figure 7.12.
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TORQUE PROFILESix sequence commutaion drive, 13A, 1000 rpm
0123456789
10
0 30 60 90 120 150 180 210 240 270 300 330 360
Position in electrical degrees
M19 29 gage Si Steel Hiperco 50A 0.014
Figure 7.12 Torque profile: six step commutation drive
The motor with M19 silicon steel magnetic core produces torque in
the range of 7 Nm and 8.1 Nm whereas the motor with Cobalt Iron magnetic
core generates 8 Nm minimum and 9.1 Nm maximum with the same full load
current. The stator with Cobalt Iron alloy yields 1Nm higher torque compared
to silicon steel lamination core. The higher permeable Hiperco material has
peak torque of 9.2 Nm compared to 8.2 Nm of silicon steel lamination core in
design simulation. The effect of saturation limits the output torque magnitude
for the given full load current in silicon steel electrical sheet lamination
magnetic core. The high permeability Cobalt Iron alloy stator core yields
higher torque output compared to silicon steel stator lamination for the same
stator assembly and rotor assembly design configurations.
7.3 SUMMARY
The improved 60 slots quadruplex redundancy permanent magnet
brushless dc motor is analysed for the torque performance with two different
stator core materials keeping all electrical loading, magnetic loading and the
design configuration same. As per the simulation the Cobalt Iron (Hiperco
50A, 0.35mm thick lamination) magnetic core produces approximately 12%
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higher torque output compared to (M19, 0.35mm thick lamination) silicon
steel magnetic core for the same load current. The Cobalt Iron lamination
stator core has better magnetization characteristics compared to silicon steel
stator core and it produces peak torque of 9 Nm for 13 Ampere current. The
Cobalt Iron alloy lamination core carries larger flux compared to silicon steel
lamination core for the same teeth and back iron dimensions and has higher
saturation flux density yielding higher torque output for the same rotor
position and current.