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Cobham Technical ServicesVector Fields Software

Modelling permanent magnet (de-)magnetisation and soft iron hysteresis within an FEA environment

November 13, 2009COMMERCIAL IN COFIDENCE1 Cobham Technical Services – Vector Fields Software

Contents

• Introduction to Vector Fields Software

• Magnetisation & de-magnetisation in permanent magnets

– Permanent magnet DC machines

– De-magnetisation under fault conditions in a PM machine

• Ferromagnetic hysteresis

– Hysteresis brake

– Electric Steering motors

– Hysteresis motor


Vector Fields Software provides

• SOFTWARE

• SUPPORT

• TRAINING

• CONSULTANCY

• PARTNERSHIP FOR R&D

for High Frequency Engineering

for Low frequency Engineering

• Founded at Oxford England in 1984 by former employees of the Rutherford Appleton Laboratory

• Acquired by Cobham plc in 2005

• Name changed in May 2009


CONCERTO suite

• High Frequency Electromagnetics

– FDTD Methods

• 3d Microwave analysis

• Antennas, waveguides, filters

• Allows solution to larger problems

• Thermal analysis also available

– Method of Moments

• Discretise material surfaces

• RCS, antenna placement

– Finite Element Analysis Solver


OPERA suite

• OPERA-2d– Statics and time varying fields– Stress and thermal analysis– Rotary & linear motion analysis– (de-)magnetisation & hysteresis– Lossy-dielectrics – Space charge– Optimisation– Electrical Machines environment

• OPERA-3d – TOSCA : static analysis– ELEKTRA : time varying fields– CARMEN : rotating and linear machines and actuators– SCALA : space charge analysis– TEMPO: thermal analysis– DEMAG: (de-)magnetisation solver– QUENCH: quench in Superconducting magnets– SOPRANO: wave propagation problems– Optimisation


The (De-)magnetisation Solver

• Transient MagnetisationAnalysis– Developed specifically to model

the magnetisation process in hard magnetic materials

– Uses ‘virgin’ BH curve for magnetisation and ‘demag’curves as field decreases

– Inclusion of temperature effects either as a• simple, single temperature model

OR• distributed temperature model in coupled electromagnetic/thermal simulations


Typical Appliance motor

• Magnet Inserts in Steel Ring

• Magnetize in a solenoid using a magnetic core

• Transient solver, incl. eddy current effects

• Use of the true magnet characteristic in the ‘application device’(e.g. electric motor)

• Computation of de-magnetisation effects arising during operation due to eg. reverse armature currents


Magnet ring during magnetisation pulse

Results courtesy of Magnequench

Note initial flux exclusion from conducting cylinder


Complete machine - no armature reaction

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

0 45 90 135 180 225 270 315 360

Degrees

Gauss

Measurement Model

Radial Field in the Airgap

Results courtesy of Magnequench


Application in magnetizing of PMDC magnets

Steel magnetizing fixture in uniform field

Applied flux density as a function of time


Remanent field strength after magnetizing


Behaviour during operation

Air-gap flux density during operation

Minimum values of flux density in magnets


Modelling Magnetising Fixtures in OPERA-3D

Magnetising fixtures may exhibit some important

three-dimensional effects, due to end windings


Demagnetisation ‘in service’

• Polynomial ‘demag curve’ representation for hard rare-earth magnets

• Look-up table more appropriate for alnico and ferrite materials, where finding coefficients for the quadratic becomes difficult.

• Recoil line slope user-specified


Demagnetisation ‘in service’

• PM Generator– Run on Open Circuit

– Impose a sudden short circuit and observe demagnetisation

– Clear fault


Demagnetisation in ‘Service’

• Minimum Flux density level reached in magnet during open circuit and short circuit event


Short Circuit Currents & Voltages

– Phase Voltages and Currents before, during and after 3-phase short circuit

– Observe reduced post-fault open circuit voltage, due to PM demagnetisation


Treatment of hysteresis in soft magnetic materials


Treatment of hysteresis in soft magnetic materials

• The magnetic behaviour is considered as a trajectory B(H)

• The trajectory is based on a measured major symmetric loop that is supplied by the user.

• The data may be obtained from in-house measurements or published data-sheets, and are imported as standard input tables.

– From the data, the method uses the turning points of the B(H) trajectory to predict the behaviour of arbitrary minor hysteresis loops.

The method is practical because it:

• Makes only realistic demands on the user for material data

• Provides a good approximation to the true physical behaviour

• Does not require large computational resources


Flux Distribution in Hysteresis Brake


Eddy Current Loss (Brake at 600 rpm)


Instantaneous Hysteresis Loss (brake at 600rpm)


Hysteresis Brake Figures

• T (ave) = 2.7 Nm/m

• Speed = 600 rpm

• => Power=169 Watts

OR

• Hyst Loss = 151 W

• Eddy Current Loss= 14 W

=> Consistent!


Drag Torque

• Electric Steering motors are required to meet extremely demanding specifications

• One of these is that it should take very little torque to rotatethe motors' rotors when the coils are not energized (this torque is referred to as 'drag-torque‘)

• The Hysteresis solver is ideally placed to investigate these effects

• Collaboration between Vector Fields and TRW Conekt (Partners in TSB funded ‘Advanced Electrical Machines via Materials’) focuses on accurate evaluation of Drag Torque in PM machines for different materials, the properties of which are supplied byTRW.


Instantaneous Hysteresis loss after 40 milliseconds


Instantaneous Hysteresis loss after 60 milliseconds


Typical Drag Torque Patterns

Typical cogging – No Hysteresis

Drag Torque for 2 Silicon Steel materials and 1 hard material


Hysteresis motor

• Simple 50 Hz, 4-pole 3-phase hysteresis motor– 12 slot conventional stator

– Annular rotor made from hysteretic material


Hysteresis motor

• Torque produced due to hysteresis with a consequent rise in rotational speed

• Ripple of the torque arises because the applied field from the stator is rotating at 1500 RPM but the rotor is almost stationary in comparison


Conclusions

• De-magnetisation and hysteresis can be modelled

• Available as full transient solvers (Elektra TR) & in Rotating and Linear machines solvers (Carmen RM & LM)

• Available in 2D and 3D

Email : [email protected]

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