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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.