19-09-2012, 02:42 PM
Design, Development and Testing of an Electromagnet for magnetic levitation system
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ABSTRACT
This paper presents details of the design and development of an electromagnet for use in magnetic levitation control
system experiment. The method is purely based on numerical approach. Cast steel was used as the material, because
of its high permeability and fairly good coercivity. The genesis for the design is based on the phenomenon of lifting
power of a magnet. The design was implemented using a local available component; the electromagnet was able to
lift various weights from distances below the pole of magnet, by varying the current through the winding of the
magnet. In the final analysis, the electromagnet was designed and built with a cost saving of about 70% of its market
value. The electromagnet suspends different weights at different distances below the pole by taking a current. The
force constant of the magnet was determined.
INTRODUCTION
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of an electric current. The
magnetic field disappears when the current ceases. Electromagnet uses electricity to produce magnet force [1]. The
main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated
over a wide range by controlling the amount of electric current. However, a continuous supply of electrical energy is
required to maintain the field.[1]
The lifting power of an electromagnet is the ability of the magnet to lift a ferromagnetic material from a given
distance. Different magnets have different magnetic strength. [2]
Computing the force on ferromagnetic materials is, in general, quite complex. This is due to fringing field lines and
complex geometries. It can be simulated using finite element analysis. However, it is possible to estimate the
maximum force under specific conditions. If the magnetic field is confined within a high permeability material, such
as certain steel alloys, the maximum force is given by [1], [2].
DESIGN FEATURES
The main consideration in the design of an electromagnet is its lifting power of the magnet [1]. Cast steel was
selected for this design because it has a narrow loop area which gives it a high permeability and fairly good
coercivity, hence making it suitable for core of electromagnet [1]. The core of the electromagnet is first specified,
the core area (shape) , diameter, and the required length of the winding are then selected by estimating or calculating
amount of current expected to pass through when lifting the required load.
LIFTING POWER OF THE MAGNET
Computing the magnetic field and force exerted by ferromagnetic materials is difficult, because the magnetic field
and force are non-linear function of the current, depending on the nonlinear relation between B and H for the
particular core material used [7]. However, in designing a DC electromagnet, in which the current is either on or off,
the relations, can be simplified [7]. The main feature of ferromagnetic materials is that the B field saturated at a
certain value, which is around 1.6T for most high permeability core steels. The B field increases quickly with
increasing current up to that value, but above that value the field levels off, regardless of how much current is sent
through the windings [7]. So it is not possible to obtain a much stronger magnetic field from electromagnet than
1.6T. The maximum force (holding force) exerted by an electromagnet is given by equation (1.0). So saturation set a
limit on the maximum force per unit core area, or pressure, an electromagnet can exert [7].
Given a core geometry, the B field needed for a given force can be calculated from equation (1.0); if it comes out to
much more than 1.6T, a larger core must be used [7]. Once the B field needed is known, the magnetomotive force
the product of current and the number of turns in the winding can be calculated [7].
The lifting power of an electromagnet is the ability of the magnet to lift a ferromagnetic material from a given
distance [2]. Equation (1.0) is the lifting power or force for I-shape core of the electromagnet. For U-shape the force
formula is divided by two, while for E-shape it’s divided by three, because the U-shape has 2 poles and E-shape has
3 poles respectively which will participate in lifting of the load.
Discussion and Conclusion
The electromagnet was designed and constructed and it was found out that it can fully suspend the steel ball at
different positions when the current through it was varied. The more the current through the electromagnet the
smaller the distance between the pole and the steel ball of the same weight. The electromagnet can suspend any
ferromagnetic material irrespective of its shape and size. The current through the magnetic coil limits the maximum
load it cans lift. Figure 1 shows a relationship between the current and the voltage across the magnet the inverse of
the slope gives the resistance of the magnet. The measured resistance of the magnet is almost the same as the value
obtained from the graph. Figure 2 shows both the current and the voltage on the same base. Figure 3 shows the
relationship between the current and the weight picked by the magnet and it can approximate to linear when the rest
of the variables are constant. Table 1 gives the values of some selected weights the current taken by the weight, the
distance between the weight and the magnetic pole and the resultant force constant of the magnet. The force constant
of the magnet is approximately the same irrespective of the weight and distance. There is a cost saving when
compared with modern electromagnet.