CHAPTER 7 PERFORMANCE IMPROVEMENT USING COBALT IRON ALLOY MAGNETIC CORE

Transcription

1 103 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 Frequency : 20 C : 60 Hz

2 104 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

3 105 and the corresponding stall torque is measured and plotted as shown in Figure 7.3. LINEARITY TEST Two phase excitation, maximum torque rotor posiiton Current in Amps Figure 7.2 Linearity test: Improved 60 slots stator with M19, 29 gage steel STATIC TORQUE PROFILE Two phase excitation, Current=13 A 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

4 106 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

5 107 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

6 108 position is shown in Figure 7.6 for M19, 29 gage silicon steel and Figure 7.7 for Hiperco 50A 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 )

7 109 The airgap flux density due to permanent magnet flux is plotted for the motor with M19 29 gage steel and Hiperco 50A material for the armature magnetic core. The stator assembly and rotor assembly for the two configurations are same. Figure 7.8 shows the airgap flux density profile for both the magnetic core materials. The values are same for both the materials at different rotor position over one pole pitch. FLUX DENSITY PLOT Position in mechanical degrees M19 29 gage Si Steel Hiperco 50A Figure 7.8 Airgap flux density for one pole pitch Figure 7.9 and 7.10 shows the tooth flux density plot for M19 29 gage silicon steel magnetic core and Hiperco magnetic core respectively. The maximum flux density in the tooth for cobalt iron alloy is 2.2 Tesla whereas 1.9 Tesla in Silicon steel magnetic core. This ensures maximum flux carrying capacity of the high permeability Cobalt Iron material. Figure 7.9 Tooth flux density in M19 29 gage armature stack

8 110 Figure 7.10 Tooth flux density in Hiperco 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 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 Rotor Position in electrical degrees M19 29 Gage Si Steel Hiperco 50A 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.

9 111 TORQUE PROFILE Six sequence commutaion drive, 13A, 1000 rpm Position in electrical degrees M19 29 gage Si Steel Hiperco 50A 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%

10 112 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.