07-08-2012, 04:46 PM
A low cost linear induction motor for laboratory experiments
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Introduction
There is a wide bibliography for the different applications of linear electrical
machines in industry. Normally they are used in special applications or where
the task requires a dynamic performance that rotary machines are unable
to give.1–4
A linear motor can be obtained by cutting a rotary motor along its radius
from the centre axis of the shaft to the external surface of the stator core and
rolling it out flat. This particular geometry makes them suitable for special
industrial applications that can be found in transportation systems, manufacturing
processes, pumping of liquid metals, etc.5 Progress in power electronics and
a.c. variable speed drives has had a strong impact on the development of linear
induction drives. Linear electric machines are direct drives, they allow accelerations,
velocity and position-accuracy far better than their rotary counterparts;
however, they are usually more expensive.6
As well as the industrial benefits, this paper observes some advantages of
linear motors in the field of education. This type of electrical machine allows
undergraduate students of electrical engineering courses to identify easily and
to understand the different concepts and parameters of the electromagnetic
circuit that can be found in common with any other types of electrical machine.
In this paper, a low cost way to develop a laboratory linear induction
machine prototype is presented. The main objective of the design is to build
up a low cost prototype that is easy to handle, manipulate and test. The
purpose of this prototype is not centred on achieving a great dynamic performance
of the machine but on highlighting the electromagnetic effects that are
involved. The main components of the linear motor are described, and some
possibilities of the design are discussed.
Design of a LIM
Design considerations
The purpose of this prototype is for use and testing in a laboratory. The
construction and subsequent tests will provide valuable information on this
type of electrical machine, and the results, once extrapolated, could serve to
design industrial prototypes. This requires some special characteristics that
must be taken into account before calculating the different main parameters of
the device.
1 A low performance motor is targeted. High dynamic performance is not
needed in order to study the characteristics of the linear motor in relation
to its geometry. Actually, a high performance motor for laboratory testing
has some disadvantages, such as the extra protection needed, higher cost,
and greater difficulties of working with it.
2 As a first approach, an oscillating movement gives enough information to
study the motor. The basic electronics should at least generate this type of
movement. However, it would also be interesting to supply the motor from
a commercial regulator to generate more complex types of movements.
Therefore, the primary voltage and demanded currents must be compatible
with the supply characteristics of a standard regulator.
3 The guides, travel and main dimensions of the prototype must be designed
to fit in a laboratory and must have an open structure. This last need is
important for testing; for instance in order to introduce probes or sensors
inside the machine.
4 A common sense need is that everything must be as simple as possible, in
order to achieve a low cost construction and to avoid problems of mounting
and of manipulating the geometry of the linear machine.