Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
1
DEGRADATION OF MAGNETIC PROPERTIES OF NON-ORIENTED
SILICON IRON SHEETS DUE TO DIFFERENT CUTTING TECHNOLOGIES
Veronica Manescu (Paltanea)
University Politehnica of Bucharest, (Romania)
E-mail: m1vera2@yahoo.com
Gheorghe Paltanea
University Politehnica of Bucharest, (Romania)
E-mail: paltanea03@yahoo.com
Dorina Popovici
University Politehnica of Bucharest, (Romania)
E-mail: karina.popovici@yahoo.com
Gabriel Jiga
University Politehnica of Bucharest, (Romania)
E-mail: gabijiga@yahoo.com
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
2
ABSTRACT
The magnetic properties of non-oriented silicon iron alloys are strongly influenced
by the cutting technology. Medium quality electrical steel M800-65A samples were
cut through mechanical punching, laser, water jet and electro erosion technologies
and were characterized with an industrial Single Strip Tester at the peak magnetic
polarizations Jp of 0.5, 1 and 1.5 T, in the frequency range starting from 10 Hz to
200 Hz. The influence of the cutting technology on the energy losses and magnetic
permeability was investigated.
KEYWORDS
Non-oriented electrical alloys, Cutting technology, Energy losses, Relative magnetic
permeability.
1. INTRODUCTION
Non-oriented electrical steels have a crystalline texture, which is characterized
through a very low magneto-crystalline anisotropy. They are intensively used in the
manufacture of the high efficiency electrical machines, in order to save energy and
to avoid the overheating phenomenon. It is well known that the electrical machines
have an energy consumption of 50% from the worldwide electricity consumption
and almost 6040 Mt of CO2 emissions are due to these devices. According to the
newest regulations, it is expected that in 2030, without proper environmental
decisions, the energy consumption of the electrical motors will increase until 13360
TWh/year and the emissions will be equal to 8570 Mt/year. Recently the energy
losses of the newly produced electrical machines have been decreased with an
amount of 20%, by comparing them with those, measured before 2016. The most
important machine producers made motors in classes of premium efficiency (IE3)
and above, i.g. Super Premium Efficiency (IE4) and Ultra Premium Efficiency (IE5)
[1, 2, 3]. Nowadays it is a great challenge to produce an efficient electric motor,
because thermal stresses, insulators’ aging and low energy losses have to be taken
into account. The most efficient motors are based on rare earth permanent magnets,
but copper rotor electrical machines and synchronous reluctance motors could have
comparable efficiency standards. Alternative current motors are designed to work
at a constant speed, and its electronic drives improve the electrical machine
flexibility. When a variable speed drive (VSD) is used, the inrush currents are
decreased, the power factor has a good value, the effects of torque variation and
speed drop are eliminated. In order to increase the efficiency of the electric motor,
good quality of non-oriented materials, with low energy losses should be used [4, 5].
Usually the magnetic cores of the electrical machines are prepared, by cutting the
non-oriented alloys through mechanical punching, because this method is very fast
and cheap, although it generates inside the material mechanical stresses that affect
the energy losses and the magnetic properties. A work hardening phenomenon is
expected to appear at the cut edge and deformation of the crystalline grains is
present. Usually during a recrystallization process, these deformed grains are
entirely transformed in new magnetic grains with better properties. The most
important parameters of the cutting procedure are the clearance and the cutting
angle, but also the hardness of the blade it is usually taken into consideration. When
it is chosen a proper value of the clearance the induced mechanical stresses are
minimal, and this fact contributes to the life increase of the cutting shears. A good
quality cut edge is obtained, based on empirical determination for each cutting
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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machine. When a strip is cut, only a small part is cut, and the rest is separated
through fracture phenomenon [5]. The samples involved, in the paper were
prepared, using a classical Computer Numerical Control (CNC) Turret punching
machine. This device has a 3 axis Siemens special CNC system and uses AC
servomotors. Its main components are actioned pneumatically, and it is equipped
with concentrated lubricating systems, which decrease the friction during the
cutting procedure.
Sometimes the electrical machines’ producers want to obtain a free burr cut edge
and they choose the non-conventional cutting technologies as laser. Laser method
is a very flexible one, but unfortunately it induces important thermal stresses and
the method is expensive. In the case of CO2 laser, the single generated wavelength
is in Infra-Red spectrum. The beam has an 0.025 mm diameter, when it travels from
the laser resonator to the beam path. It is guided through a mirror or a special lens’
system and finally is focused on the material. The laser beam is accompanied by a
compressed gas, as Oxygen or Nitrogen. The cut edge is almost perfect, but the
high power density of the beam has an unwanted result, which consists of a rapid
heating, melting or partial vaporizing of the material. In the paper a Morn Laser
machine was used to cut the samples. This machine is a very performant one and
the laser cut has no cracks or supplementary deformation due to the thermal
stresses and it has a very high stability of the cutting tool [6, 7].
Other non-conventional cutting technologies are the electro-erosion and water jet
methods. The electro-erosion (EDM) produces any stresses, but the process is very
slow, and it can be used only for small dimension electrical machines. Today EDM
machines are very stable, starting with 1980 due to the introduction of the CNC in
the EDM technology and they could be used to cut complex shapes. The basic
phenomenon involved in the EDM cutting procedure is the energy transformation
from electrical into thermal energy, using a series of electrical discharges that
appear between an electrode and a workpiece, introduced in a dielectric fluid. This
fluid is a very special one, because it has to avoid the electrode electrolysis. The EDM
procedure is a modern technique, based on material erosion, when a spark appears
as a result of an applied voltage between the electrode and the material surface. The
dielectric fluid is a cooler medium and it helps the discharge energy to be
concentrated on a small area. As the erosion advances, the electrode is moved
through the dielectric fluid. Recently, servo systems are used, to assure a constant
gap voltage, between the material and the wire and to retract the electrode, when a
short circuit occurs [5, 6, 7, 8]. A Kingred Wire EDM, controlled by a CNC system,
which utilizes brass electrode wire and high frequency impulses, was used. This
machine is very suitable for cutting materials with high precision [9].
The water-jet leads also to a very good quality of the cut edge, but special expensive
equipment is needed, and the cutting speed is relatively slow. Abrasive particles as
Garnets are used and the price of the cutting procedure is very high. The abrasive
particles do the material cut through a sawing action and it leaves a precision cut
surface. This method is suitable to cut almost any type of steels and it has a narrow
kerf width. Physical properties such as melting point, thermal and electric
conductivity, density have a limited importance, although the hardness of the
material could reduce the cutting speed. This process induces no heat affected zone
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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and no hard oxidation layers on the cut edges, which could lead to the microcracks
apparition. This technology damages minimal the magnetic properties of the
material due to the plastic deformation. An Omax Waterjet machine was used. This
device performs a very high accurate edge cut and it uses Garnet as abrasive
particles.
The non-conventional methods are adequate in the prototyping production.
2. MATERIALS AND METHODS
A medium quality commercial non-oriented steel M800-65A was investigated. The
material properties and the geometrical parameters are shown in Table 1.
Table 1. Properties and geometrical parameters of the M800-65A samples.
Material
Cut
direction
Density
[g/cm
3
]
Electrical
resistivity
[Ω/m]
Mass
[g]
Width
[mm]
Thickness
[mm]
M800-65A
Rolling
direction
7.80
2510
-8
44.73
30
0.65
The influence of the cutting procedure on the energy losses was analyzed using the
energy loss separation method. According to this theory, the total energy losses are
divided into hysteresis, classical (Foucault) and excess energy losses.
The hysteresis losses are due to the pinning points and impurities that are present
in the medium quality non-oriented steels and they are usually analyzed, by taking
into account the coercivity mechanisms. They can be computed, by extrapolating in
zero the measured total energy losses.
The classical energy losses are generated by the eddy currents and the material is
treated as a homogenous medium [10]. They can be computed with the following
equation:
2 2 2
,
6
p
cl
Jd
Wf

(1)
where d is the sample thickness, σ is the electrical conductivity, ρ is the non-oriented
steel density and f is the experimental frequency.
The excess losses are due to the micro eddy currents, which are formed in the
vicinity of the domain walls. They are computed, subtracting the classical and
hysteresis losses from the total energy losses.
The magnetic measurements were done, by using an industrial Brockhaus Single
Strip tester, with a double C yoke, which is a standardized device and it controls at
each step of the measurement the form of the secondary voltage, in order to be a
sinusoidal one according to DIN 50 462 standard. This device permits the
measurement of the energy losses and of the relative magnetic permeability with its
components: real and imaginary parts. An external magnetic field is applied, and a
magnetic flux is generated into the tested sample. The current, in the magnetizing
coil, is determined with the help of a shunt resistor. The magnetic polarization is
computed, by integrating the experimentally induced voltage on the measuring coil
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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with a 16-bit processor. The accuracy of the measurements is very high, and the
device provide a 0.2% repeatability of the results. The maximum current is equal to
5 A and the maximum voltage is set at 32 V. The magnetic path length, between the
polar pieces is 240 mm. Along this path are placed the measuring (723 windings)
and the magnetizing (704 windings) coils. A sample of minimum 280 mm length
and maximum 30 mm width could be investigated.
3. RESULTS AND DISCUSSIONS
Samples of medium quality commercial electrical steel of M800-65A grade were cut
through punching, laser, water-jet and electro-erosion technologies.
The normal magnetization curve, defined as the geometrical place of the
symmetrical hysteresis cycle peak points, which extends from the demagnetized
state to the saturation is presented in Figure 1. The demagnetized state could be
obtained, by increasing the sample temperature to a value, higher than the Curie
temperature, followed by a normal cooling, in the absence of a magnetic field.
Another technique consists of applying an alternative magnetic field, whose
amplitude is progressively decreased through zero, starting from a higher reference
value, which implies the technical saturation point. After the material
demagnetization, if a monotone magnetic field is applied, the sample behavior
follows the virgin magnetization curve. If this procedure is done after the cyclic
demagnetization of the material, the normal magnetization curve is obtained. In the
soft magnetic material case there are some minor differences between these two
curves and also the magnetic polarization J is considered to be equal to the magnetic
flux density B [1].
In order to experimentally determine the normal magnetization curves of the
samples, measurements were done at the industrial frequency of 50 Hz. In the case
of each sample, symmetrical hysteresis loops were determined at a peak magnetic
polarization Jp of 5 mT, 10 mT, 20 mT, 50 mT, 100 mT, 200 mT, 500 mT, 750 mT,
900 mT, 1000 mT, 1100 mT, 1200 mT, 1300 mT, 1400 mT, 1500 mT, 1600 mT. It
can be noticed from Figure 1 that the electro-erosion and water jet technologies
determines an easier magnetization of the material, because the cutting procedure
induces any thermal or mechanical stresses. The punching and the laser procedures
leads to a more difficult magnetization process due to the generated stresses,
although the M800-65A grade is an alloy, which contains a relative high percent of
non-magnetic impurities that acts as pinning points for the magnetic domain wall
movement. All the normal magnetization curves meet at the saturation point of the
material, for a magnetic field strength of 2000 A/m. The principal magnetization
process in this type of steel is the reversible domain wall movement and near the
saturation zone rotating of the spin magnetic moments occur.
In Figure 2 is presented the variation of the total energy losses as a function of the
frequency, for three values of the peak magnetic polarization Jp of 500 mT, 1000
mT and 1500 mT. At 500 mT the water-jet technology leads to the lowest value of
the total energy losses, followed by the punching method and the highest energy
losses are measured in the case of laser. These observations are valid in the case of
1000 mT, but for 1500 mT the lowest value of the energy losses is determined for
the electro-erosion technology. It can be noticed that with increase of the peak
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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magnetic polarization, the influence of the cutting technology on the energy losses
is reduced.
Figure 1. Normal magnetization curve for M800-65A samples, cut through punching, laser, water-
jet and electro-erosion.
The hysteresis energy losses are invariable with the frequency and they are a direct
consequence of the magnetization processes, which are due to the magnetic domain
wall movements.
It can be observed from Figure 3 that the most important variation of the hysteresis
energy losses is noticed for the high magnetic polarization domain, especially in the
case of punching and laser. The lowest values of the hysteresis energy losses are
determined for the electro-erosion and water-jet cutting technologies.
The classical energy losses are generated by the eddy currents and they are directly
proportional with the peak magnetic polarization and the frequency.
It can be noticed that from a specific value of the frequency these losses become
predominant with higher values than in the case of excess and hysteresis energy
losses.
In Figure 4 is presented the variation of the classical energy losses with the
frequency.
a)
b)
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
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c)
Figure 2. Total energy losses versus frequency at three peak magnetic polarizations of 500 mT
(a), 1000 mT (b) and 1500 mT (c).
The excess energy losses are influenced by the cutting procedures in the case of all
the peak magnetic polarization values. The water-jet technology leads to the lowest
value, followed by the laser and the electro-erosion procedures.
The mechanical punching has a strong influence on the magnetic domain structure
and is directly linked to the existence of higher values of the excess energy losses.
Figure 3. Hysteresis energy losses versus peak
magnetic polarization, in the case of different
cutting technologies.
Figure 4. Classical energy losses versus
frequency at three peak magnetic polarizations of
500 mT, 1000 mT and 1500 mT.
a)
b)
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
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c)
Figure 5. Excess energy losses versus frequency at three peak magnetic polarizations of 500
mT (a), 1000 mT (b) and 1500 mT (c).
The relative magnetic permeability µ
r
is a physical quantity, which describes the
material property to concentrate the magnetic field lines, when an external
magnetic field is applied. A magnetic material is more adequate to be used in
industrial applications in the case of high values of magnetic permeability. In Figure
6. is presented the variation of the relative magnetic permeability for two peak
magnetic polarization of 500 mT and 1000 mT. It can be noticed that the magnetic
permeability presents an inversely proportional variation with the frequency. The
influence of the cutting procedure is more pronounced for frequency values lower
than 100 Hz. The highest value of the magnetic permeability is obtained for the
water-jet and electro-erosion technologies.
Figure 6. Magnetic permeability versus frequency at two peak magnetic polarizations of 500 mT and 1000
mT, in the case of different cutting technologies.
4. CONCLUSIONS
The cutting procedure damages the magnetic material microstructure, more
pronounced in the case of laser, followed by punching, electro-erosion and water-
jet. To reclaim the initial magnetic properties of the alloy some thermal
recrystallization treatments are required, but this step is not taken into
consideration by the electric motor manufacturers, because it damages the insulator
layer that covers the magnetic core sheets. The use of water-jet or electro-erosion
technologies that have a reduced impact on the energy losses and the relative
permeability is taken into consideration only in the prototyping and special cases,
because of their slow cutting speed. As a compromise between cutting speed and the
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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induced damage on the magnetic properties the electrical motor manufacturers still
prefer the classical mechanical punching.
5. ACKNOWLEDGEMENTS
The work of Veronica Mănescu (Păltânea) has been funded by University
Politehnica of Bucharest, through “Excellence Research Grants” Program, UPB-
GEX 2017. Identifier: UPB-GEX2017, Ctr. No. 04/25.09.2017 (OPTIM-IE4). The
work of Gheorghe Paltanea has been funded by University Politehnica of Bucharest,
through “Excellence Research Grants” Program, UPB-GEX 2017. Identifier: UPB-
GEX2017, Ctr. No. 02/25.09.2017 (ANIZ-GO).
6. REFERENCES
[1] G. Bertotti, Hysteresis in Magnetism, San Diego, CA: Academic Press, (1998).
[2] V. Mănescu (Păltânea), G. Paltanea and H. Gavrilă, Physica B, 486, (2016).
[3] A.T. De Almeida, F. Ferreira, J. Fong, B. Conrad, Electric Motor Ecodesign and
Global Market Transformation, Proceedings of IEEE Industrial &
Commercial Power Systems Conf., (2008), May, Florida, USA.
[4] M Enokizono, IEEE Trans. Magn., 48, 11, (2012).
[5] V. Manescu (Paltanea), G. Paltanea, H. Gavrila, G. Scutaru, Rev. Roum. Sci.
Techn.-Electrotechn. Et Energ., 60, 1, (2015).
[6] V. Manescu (Paltanea), G. Paltanea, H. Gavrila, Rev. Roum. Sci. Techn.-
Electrotechn. Et Energ., 59, 4, (2014).
[7] O.S. Bursi, M. D’Incau, G. Zanon, S. Raso, P. Scardi, JCSR, 133 (2017).
[8] B. Boswell, M.N. Islam, I.J. Davies, Int. J. Adv. Manuf. Technol., (2017).
[9] V. Manescu (Paltanea), G. Paltanea, H. Gavrila, A. Nicolaide, Rev. Roum. Sci.
Techn.-Electrotechn. Et Energ., 60, 2, (2015).
[10] M. Stanculescu, O. Drosu, M. Maricaru, Reduction of winding losses for
trapezoidal periodic currents, Proceedings of IEEE 8
th
International
Symposium on Advanced Topics in Electrical Engineering, (2013), May,
Bucharest, Romania.
[11] G. Paltanea, V. Manescu (Paltanea), , H. Gavrila, D. Popovici, Magnetic
property analysis in non-oriented silicon iron steels cut through water jet
technology, 2016 ISFEE.
[12] D. Popovici, F. Constantinescu, M. Maricaru, Modeling and Simulation of
Piezoelectric Devices, June 2008, book: Modelling and Simulation, Vienna,
Austria, ISBN 973-8067-96-0.
Degradation of magnetic properties of non-oriented silicon iron sheets due to different cutting technologies
DOI: http://dx.doi.org/10.17993/3ctecno.2019.specialissue.01
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AUTHORS
Veronica Mănescu (Păltânea)
Veronica Mănescu (Păltânea) was born in Bucharest, Romania,
on June 5, 1978. She received the B.E. degree in electrical
engineering from the Politehnica University of Bucharest in
2002, and the M.S. and Ph.D. degrees in electrical engineering
from the Politehnica University of Bucharest, Romania, in 2004
and 2008, respectively. She is actually an Associate Professor at
U.P.B.
Gheorghe Păltânea
Gheorghe Păltânea was born in Bucharest, Romania, on
November 3, 1978. He received the B.E. degree in electrical
engineering from the Politehnica University of Bucharest, in
2002, and the M.S. and Ph.D. degrees in electrical engineering
from the Politehnica University of Bucharest, Romania, in 2004
and 2008, respectively. He is actually an Associate Professor at
U.P.B.
Dorina Popovici
Dorina Popovici received in 1989 the Ph.D. degrees in electrical
engineering from the Politehnica University of Bucharest,
Romania. She is the author of over 100 scientific articles from
which over 80 international journals, conferences, symposiums
and workshops published 15 courses and applications books and
participated as project manager in over 23 national and
international research projects.
Gabriel Jiga
Gabriel Jiga received in 1996 the Ph.D. degree in civil
engineering at the Technical Military Academy, Romania with a
subject in structural analysis of composite structures. He is the
author of over 120 scientific papers, more than 80 being
presented at international conferences and symposia or published
in prestigious international journals. Nowadays he is full
professor at University Politehnica of Bucharest and teach