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21th International Symposium INFOTEH-JAHORINA, 16-18 March 2022
Design and Verification of 22kW / 220Vdc
Electromagnet for Separation of Steel Parts
from Coal on Conveyor Belts for Delivery
Željko V. Despotović
Institute Mihajlo Pupin, University of Belgrade,
Belgrade, Serbia
zeljko.despotovic@pupin.rs
Borko Čupić
IMP Projekt Inženjering
Belgrade, Serbia
borko.cupic@impprojekt.com
Đorđe Lekić
Faculty of Electrical Engineering, University of Banja Luka
Banja Luka, Republika Srpska, Bosnia and Herzegovina
djordje.lekic@etf.unibl.org
Abstract— The paper presents dimensioning and verification of
the design of a powerful 22 kW electromagnet powered from a
220 V DC source. The application of the designed electromagnet
refers to systems for electromagnetic separation of medium and
large steel parts that may be found in the coal that is transported
on the conveyor belts for the delivery of coal to thermal power
plants. The application of the electromagnetic separator is direct-
ly related to the system for detecting the presence of metal steel
parts (metal detector). In the case of inefficient operation of the
system for separation of steel parts from coal, severe accidents
can occur in the system for grinding and crushing coal at thermal
power plants. The design of the electromagnet presented in this
paper was verified in the software package FEMM 4.2. The mod-
el is defined as a 2D planar magnetostatic model. At the end of
the paper, the final simulation results of this project are present-
ed.
Keywords-electromagnet; electromagnetic separation; FEMM;
coal delivery; thermal power plants
I. INTRODUCTION
Electromagnetic separation is most often used to separate
iron parts from materials used in thermal power plants (TPP)
(most often material is coal), then from earthen clay used in
brickyard technology, in lime production systems, in crushing
and stone processing systems, in cement industry, etc. [1]–[4].
The use of this technology is indispensable in the mentioned
systems, where it should be noted that iron pieces, torn from
tool parts and/or machines during operation, must not come to
the mills for processing, crushing or transport of materials.
Using powerful electromagnets and relatively strong magnetic
fields, the separated pieces or parts are collected at the poles of
the electromagnet [1].
For these reasons, it is necessary to occasionally turn off
the power supply of the electromagnet and remove the accumu-
lated unwanted iron parts from the surface, or use an adequate
conveyor together with the electromagnet, which will constant-
ly remove the accumulated steel or iron material [3], [4]. Based
on the above, systems for the separation of iron parts can be
based on the so-called "Manual cleaning"-Manual Clean (MC)
systems or the so-called "Self-cleaning" - Self Clean (SC) sys-
tems. MC units must be periodically turned off in order to dis-
charge iron accumulated on the face of the magnet. They are
suitable for applications where only occasional tramp iron is
expected. These magnets are usually suspended from a travel-
ling trolley so that they can be swung clear of the conveyor
before the iron is released [3].
SC suspended electromagnets provide continuous, automat-
ic removal of tramp iron and feature a heavy-duty belt, a chan-
nel frame for supporting the pulleys, adjustable belt take-up
and drive. They are recommended where a large amount of
tramp iron is expected or where there may be limited access to
the magnet for cleaning purposes [3]. The electromagnetic sep-
arator is mounted transversely above the conveyor belt in a
horizontal or oblique position. The place and position of instal-
lation depends on the type and looseness of the transported
material. Since the magnet forms a strong magnetic field
around itself, there must be no steel or iron objects in the im-
mediate vicinity that form secondary fields around it. Methods
of assembling electromagnets (EMs) for several specific indus-
trial applications are given in Fig. 1 [3].
Figure 1. EM mounting methods for several industrial applications of EM
separators; (a) MC on the outpouring location, (b) SC on the outporing
location, (c) MC above the conveyor, (d) SC above the conveyor
The research was supported by the projects of the Ministry of Education,
Science and Technological Development of Republic Serbia (time period
2022-2023)
In the first case for MC and SC systems, the electromagnet
is suspended over the path of the material discharged from the
belt conveyor, i.e. at the outpouring location, as shown in Fig-
ures 1 (a) and 1 (b). This is the preferred option because it is
the most efficient use of the magnetic separator, i.e. when the
burden is "opened up" in flight and is moving directly towards
the magnet face. The iron’s momentum towards the magnet can
assist in its separation. When the magnet is in this position, it is
essential that the conveyor head pulley is made of non-
magnetic material. In the second case for MC and SC systems,
the electromagnet is located over the moving bed of material
and at right angles to the conveyor – see Figs 1(c) and 1(d).
This position requires a stronger magnet and is not recom-
mended for excessive belt speeds or deep material burdens
where the removal of smaller tramp iron is necessary.
When dimensioning such systems, it is very important to
model the electromagnetic field of the separator [5] and then
calculate the electromagnetic force of the separator. Reference
[2] provides a comprehensive calculation of the electromagnet
used to separate steel and iron parts from slag. Reference [6]
gives a detailed calculation of the attractive force of a strong
electromagnet. This article explains the manner in which the
force of a solenoid varies with coil diameter, coil length, wire
gauge, supply voltage, packing density, and the number of
turns. Particular attention is given to explaining how force var-
ies with the number of turns, as the author has found that sole-
noid behavior in this regard is often non-intuitive and surpris-
ing for engineers who have only been exposed to basic magnet-
ic field expressions for coils [6].
In this paper, when sizing electromagnets and systems for
separation of steel parts from the conveyor belts of surface
mine PK "Drmno", several factors were taken into account: (1)
material size, (2) material type / density / condition, (3) type
and minimum size of tramp iron to be removed, (4) maximum
lump size, (4) amount of tramp iron material, (5) capacity in t/h
or m³/h, (6) conveyor belt width, (7) conveyor belt speed, (8)
conveyor belt incline, (9) head pulley diameter, (10) head pul-
ley material, (11) angle of troughing idlers, (12) ambient tem-
perature, (13) machinery to be protected, (14) available AC or
DC power supply.
In the practice of electromagnet sizing, even in this case,
FEMM 1.2 and FEMM 4.2 [7], [8] software packages for mod-
eling and calculation of electromagnets are most often used.
The calculation and simulation of the separator electromagnet
in this case was performed in the software package FEMM 4.2.
The next chapter describes in more detail the application of the
designed electromagnet on a specific conveyor on the delivery
of coal from surface mine "Drmno" to the TPP "Kostolac".
II. APPLICATION IN THE ELECTROMAGNETIC SEPARATION
SYSTEM ON SURFACE MINE “DRMNO”
The technological line for production, transport and pro-
cessing of coal in the system of excavators, conveyors and
crushers, is a unique production unit of the surface mine "Drm-
no" intended for continuous operation (24 hours / 7 days, all
year round, except in the cases of planned and unplanned
downtime). This technological line represents one large whole
of mining machines and facilities in the series, whose individu-
al operation directly depends on the others in the technological
series. In other words, stopping one machine/object, stops all
machines/objects in the sequence after it. Due to the frequent
appearance of steel pieces and iron waste in the conveyor belt,
which are caused by maintenance work on machines in the
mine, by installing only metal detectors on a certain conveyor
belt, it often stops, but also all belts behind it in the technologi-
cal line. This leads to a frequent start of the conveyor, which is
relatively long in time (high inertia requires a longer time of
starting of moving masses), so that the losses in the time utili-
zation of the system on an annual basis are also large.
Based on the above, there is a need to install separators of
iron parts on the location as close as possible to coal mining
machines: excavators and floor conveyors, where most of the
metal waste is generated. The chosen location for the installa-
tion of the extractor is the overflow point between the conveyor
U-Z-3 (receiving or transmitting part), which is one of the clos-
est lanes to the exit from the mine.
The project envisages the installation of a metal detector in
front of the location for the installation of the plant for the sep-
aration of steel parts and "tramp iron", in the direction of the
conveyor belt. The role of the metal detector is to detect pieces
of steel on the conveyor belt and to send signals for the reac-
tion of a powerful electromagnet within the electromagnetic
separation plant. The envisaged metal detector is of the tunnel
type with a construction that allows mounting without cutting
the conveyor belt.
Technical requirements set for the design of electromagnet
within the plant for the separation of steel parts are:
Electrical characteristics: 18 kW, 220 Vdc, ED 100%,
54 kW, 400 Vdc, ED 10%.
Electromagnet winding insulation class: C (> 220 ° C).
Magnetic flux density: 500 G = 50 mT at a distance of
700 mm from the electromagnet, at power 18 kW.
Conveying belt width 2000 mm and speed 3.1 m/s.
Conveying belt drive: 9.5 kW, IP 55.
Dimensions of the space for installation of electromag-
net: maximum length 5500 mm, maximum width
2150 mm, maximum height 1100 mm.
Weight: maximum 13200 kg.
Operating temperature (ambient ambient conditions):
–20 °C to +55 °C.
Electromagnet monitoring and measuring points:
1 × PT100 temperature sensor, 2 × thermal emergency
switch, 1 × Hall magnetic field sensor.
Built-in permanent magnetic plate 950×1400×250 mm
to prevent the circulation of metal around the separator.
Power and signal cables for mining installation condi-
tions.
The proposed concept of operation, in addition to metal de-
tectors, envisages the use of a thyristor-controlled rectifier with
two levels of output power.
In the first stage, the power of the magnet would be
18 kW/ max 22 kW, without the operation of the belt drive, in
order to reduce its wear. Smaller pieces of metal that would
eventually pass without detection in this mode would be at-
tracted by the separator electromagnet, while their placement in
the bunker for separated pieces would be done by starting the
belt drive at equal intervals (every hour of the operating cycle).
In the second power level, which is generated after receiving a
signal from the metal detector that the piece has been detected,
the electromagnetic separator operates with a power of 54 kW,
in short-term overload (5-10 seconds) necessary for the separa-
tion of the steel piece. The intended method of cooling the
electromagnet is natural oil cooling.
The expected steel pieces on the conveyor belt intended for
separation are up to 600 mm in length, maximum weight up to
190 kg (the most critical case are pieces of three rollers - idlers
weighing about 60 kg connected in a chain), or pieces of mini-
mum dimensions 30×100 mm. The average frequency of oc-
currence of steel pieces for separation is up to 4 times per shift.
The mobility of the load-bearing structure is envisaged by us-
ing pontoons on both sides at a distance of 700 mm from the
conveyor belt, with the concept shown in Fig. 2.
Fig. 2 shows the disposition of the conveyor system and as-
sociated electromagnetic separator designed for the surface
mine "Drmno". An electromagnetic separator (2) is hung on the
supporting structure (1), which is placed above the main con-
veyor belt (3). This conveyor belt rests on the supporting struc-
ture (4). The receiving rubber (5) absorbs the mechanical
shocks of the separated pieces of iron, while the bumper plate
(6) directs them towards the sliding track (7). Along the sliding
track the separated pieces are transported to the receiving con-
tainer (8).
The project envisages the installation of a permanently ex-
cited plant for the separation of steel (or iron) parts, for the
maximum required rated power of 22 kW. This power of the
constantly excited electromagnetic separation plant was adopt-
ed after comprehensive analyzes and extensive calculations.
The verifications of the calculations were done through simula-
tions and analysis in software package FEMM 4.2, whose con-
cept is based on finite element methods. This part will be dis-
cussed in more detail below.
III. DEFINITION OF THE PROBLEM
In this Section the problem is defined and basic information
on the software and employed models is provided. The subject
of the analysis is the magnetic circuit of the electromagnetic
(EM) separator with dimensions defined in Fig 3. As shown in
Fig. 3, the winding of the electromagnet is sectioned (i.e. it
consists of 3 sections) and has a total of N = 1800 turns. It is
assumed that the winding of the electromagnet is supplied by a
constant DC current I = 100 A, which corresponds to a current
density J = 1.15 A/mm² in the conductors.
The task is to obtain the magnetic field distribution in the
vicinity of a lone electromagnet with current I = 100 A, as well
as the magnetic field distribution in the vicinity of the electro-
magnet in the presence of a 160 kg steel block at distance
d = 0.7 m from the electromagnet (See Fig. 9).
Figure 2. View of the designed mechanical construction of the
electromagnetic separator at the surface mine "Drmno"
Figure 3. Dimensions of the magnetic circuit of the EM separator
(in milimeters)
The case of a lone electromagnet with current I = 100 A is
analyzed in Section IV-A where following results are given:
Magnetic flux density field distribution in vicinity of
the electromagnet (See Fig. 6).
Amplitude of magnetic flux density vector as a func-
tion of distance from the electromagnet (See Fig. 8) for
distances up to 0.7 m.
Winding self-inductance.
The case of an electromagnet with current I = 100 A in the
presence of a 160 kg steel block at distance d = 0.7 m from the
electromagnet (See Fig. 9) is analyzed in Section IV-B where
following results are presented:
Magnetic flux density field distribution in vicinity of
the electromagnet whereas the steel object is placed at
distance 0.7 m from the electromagnet (See Fig. 10).
Amplitude of magnetic flux density vector as a func-
tion of distance from the electromagnet (See Figs 11
and 12) for distances up to 0.7 m, whereas the steel ob-
ject is placed at a distance of 0.7 m from the electro-
magnet.
Winding self-inductance as a function of distance of
the steel object from the electromagnet (See Fig. 14).
Amplitude of electromagnetic force vector acting on
the steel object as a function of the distance of the steel
object from the electromagnet (max 0.7 m, see
Fig. 13).
IV. FINITE ELEMENT ANALYSIS
FEMM 4.2 software package [7], [8] is used for modeling
the magnetic circuit of the EM separator. The model is defined
as a 2D planar model with a depth of 1.6 m (corresponding to
length of magnetic circuit in Fig. 3) and the magnetic field dis-
tribution in cross section A-A of the magnetic circuit is ana-
lyzed. By adopting 2D rather than a 3D analysis approach, the
end-leakage magnetic field on both ends of the magnetic cir-
cuit, due to its finite length, is neglected. This approximation
does not significantly affect the accuracy of obtained magnetic
field distribution along the central cross section A-A. However,
neglecting the end-leakage magnetic field does affect the accu-
racy of calculated winding self-inductance, because the end-
leakage inductance is not taken into account by 2D analysis.
Based on experience, this error is usually less than 10 % and
can be neglected in the early design stage. However, for more
accurate results and precise quantitative analysis of the end-
leakage inductance, it is recommended to conduct an additional
3D finite element analysis, e.g. in Ansys Maxwell software
package.
Since a constant magnetic field originating from a constant
direct current is considered, a magnetostatic type of simulation
is adopted for the analysis. The magnetic field in the vicinity of
the electromagnet is simulated in a bounded cylindrical volume
filled with air, with base diameter 5 m and height 1.6 m. In
order to simulate unbounded free space for the first seven spa-
tial harmonics of the magnetic flux density, a boundary condi-
tion is imposed on the surface of the cylinder by invoking
command mi_makeABC(7,5,0,0,0) in FEMM 4.2 software. In
all simulations, a finite element mesh with an average of 16643
elements and 32924 nodes is used, as shown in Fig. 4.
In the simulation, commercially available materials are
adopted. Silicon steel M-19 is adopted for material of the elec-
tromagnet core (B-H curve for M-19 steel in Fig. 5). Standard
industrial aluminum is adopted for the electromagnet winding
material. For the material of the steel object in the vicinity of
the electromagnet, low-carbon steel 1006 is adopted (B-H
curve for 1006 steel in Fig. 5).
A. Analysis of a lone Electromagnet
In this Section the magnetic field of a lone electromagnet
(without any steel object nearby) is analyzed. Fig. 6 shows the
magnetic field distribution in the vicinity of a lone electromag-
net with current I = 100 A, without any steel object nearby.
Magnetic flux density as a function of Cartesian coordinates x
and y, according to coordinate system definition in Fig. 7, is
given in Fig. 8.
M-19 SteelM-19 Steel
Aluminum, 1100
[Coil+:600]
Aluminum, 1100
[Coil-:600]
Aluminum, 1100
[Coil+:600]
Aluminum, 1100
[Coil-:600]
Aluminum, 1100
[Coil+:600]
Aluminum, 1100
[Coil-:600]
Air
u1
u2
u3
u4
u5
u6
u7
Figure 4. A detail of the generated finite element mesh
in FEMM 4.2 software
Figure 5. B-H curves for M-19 and 1006 steel used in analysis
Density Plot: |B|, Tesla
1.425e+000 : >1.500e+000
1.350e+000 : 1.425e+000
1.275e+000 : 1.350e+000
1.200e+000 : 1.275e+000
1.125e+000 : 1.200e+000
1.050e+000 : 1.125e+000
9.750e-001 : 1.050e+000
9.000e-001 : 9.750e-001
8.250e-001 : 9.000e-001
7.500e-001 : 8.250e-001
6.750e-001 : 7.500e-001
6.000e-001 : 6.750e-001
5.250e-001 : 6.000e-001
4.500e-001 : 5.250e-001
3.750e-001 : 4.500e-001
3.000e-001 : 3.750e-001
2.250e-001 : 3.000e-001
1.500e-001 : 2.250e-001
7.500e-002 : 1.500e-001
<0.000e+000 : 7.500e-002
Figure 6. Magnetic field distribution in the vicinity of a lone electromagnet
with current I = 100 A, without any steel object nearby
Figure 7. Definition of Cartesian coordinate system with electromagnet
Figure 8. Magnetic flux density as a function of Cartesian coordinates for a
lone electromagnet with current I = 100 A, without steel object
Figure 9. Definition of Cartesian coordinate system with electromagnet
and steel block at distance d = 0.7 m
Self-inductance of the electromagnet with N = 1800 turns,
without any steel object nearby and at current I = 100 A in the
winding, is L = 10.87 H (end-leakage inductance is neglected).
B. Analysis of Electromagnet in Presence of Steel Object
In this Section the magnetic field of the electromagnet in
the presence of a steel block is analyzed. The dimensions of the
steel block are assumed to be 2000 mm × 1600 mm × 7 mm, its
mass is approximately 160 kg, while the distance of the block
to the electromagnet varies from 0 m to 0.7 m according to
Fig. 9. Fig. 10 shows the magnetic field distribution in the vi-
cinity of the electromagnet with current I = 100 A and with
steel block at distance d = 0.7 m. Magnetic flux density as a
function of coordinates x and y, according to coordinate system
definition in Fig. 9, is given in Figs 11 and 12 for the case
when the steel block is placed at distance d = 0.7 m. The same
results are given in Table I for five different values of coordi-
nates x and y.
TABLE I. TABULATED DATA POINTS FROM FIGS 11 AND 12
B (mT)
x = 0 m
x = 0.2 m
x = 0.4 m
x = 0.6 m
x = 0.8 m
y = -0.14 m
299.4
302.1
231.3
206.5
193.1
y = -0.28 m
212.8
202.2
169.4
142.9
120.7
y = -0.42 m
153.1
141.2
118.7
96.24
80.67
y = -0.56 m
120.6
107.5
84.31
66.97
53.76
y = -0.70 m
109.9
84.41
59.74
47.84
31.52
Density Plot: |B|, Tesla
1.425e+000 : >1.500e+000
1.350e+000 : 1.425e+000
1.275e+000 : 1.350e+000
1.200e+000 : 1.275e+000
1.125e+000 : 1.200e+000
1.050e+000 : 1.125e+000
9.750e-001 : 1.050e+000
9.000e-001 : 9.750e-001
8.250e-001 : 9.000e-001
7.500e-001 : 8.250e-001
6.750e-001 : 7.500e-001
6.000e-001 : 6.750e-001
5.250e-001 : 6.000e-001
4.500e-001 : 5.250e-001
3.750e-001 : 4.500e-001
3.000e-001 : 3.750e-001
2.250e-001 : 3.000e-001
1.500e-001 : 2.250e-001
7.500e-002 : 1.500e-001
<0.000e+000 : 7.500e-002
Figure 10. Magnetic field distribution in the vicinity of the electromagnet
with current I = 100 A and steel block at distance d = 0.7 m
Figure 11. Magnetic flux density as a function of Cartesian coordinates for
electromagnet with current I = 100 A and steel block at distance d = 0.7 m
Figure 12. Magnetic flux density as a function of Cartesian coordinates for
electromagnet with current I = 100 A and steel block at distance d = 0.7 m
Fig. 13 shows the amplitude of the electromagnetic force
vector acting on the steel block as a function of distance d of
the steel block from the electromagnet (see Fig. 9) for current
I = 100 A. The same dependence is presented in Table II for
five different distances of the steel block from electromagnet.
TABLE II. TABULATED DATA POINTS FROM FIG. 13
d = 0 m
d = 0.28 m
d = 0.42 m
d = 0.56 m
d = 0.7 m
F (kN)
17.12
10.54
6.78
4.38
2.85
Fig. 14 shows the self-inductance of the electromagnet
(with neglected end-leakage inductance) as a function of dis-
tance d of the steel block from the electromagnet for current
I = 100 A in the winding. The same dependence is presented in
Table III for five different distances of the steel block.
TABLE III. TABULATED DATA POINTS FROM FIG. 14
d = 0 m
d = 0.28 m
d = 0.42 m
d = 0.56 m
d = 0.7 m
L (H)
11.48
11.30
11.17
11.08
11.02
V. CONCLUSION
The formulation of the problem, technical requirements and
simulation results of calculations concerning the sizing of a
powerful electromagnet, are presented in this paper. Electro-
magnet for power 22 kW and voltage 220 Vdc is intended to be
used in the system for separation of steel parts and "tramp iron"
from conveyor belts on coal delivery from the surface mine
"Drmno" in Kostolac. Simulation results were obtained by the
finite element method in the software package FEMM 4.2. The
magnetic field distributions and electromagnetic force distribu-
tions in plane coordinates were obtained. The obtained results
showed that at a distance of 0.7 m from the surface of the elec-
tromagnet to the load on the conveyor belt, it is possible to
achieve an attractive force in the amount of about 2 kN. This
result fully satisfies the set technical requirements for the elec-
tromagnetic separation plant. The results obtained by these
calculations are the input data for the design of the SCR rectifi-
er that will be used for continuous regulation of the electro-
magnetic force.
Figure 13. Electromagnetic force acting on the steel block as a function of
distance of the steel block from the electromagnet with current I = 100 A
Figure 14. Electromagnet self-inductance as a function of distance
of the steel block from the electromagnet with current I = 100 A
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