05-07-2012, 10:27 AM
A low cost linear induction motor for laboratory experiments
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Abstract
In this paper we present a linear induction motor (LIM) prototype for education. LIMs allow easy
identification and study of the different concepts and parameters of the electromagnetic circuit that they have in
common with other types of electrical machine. Some experiments are presented that highlight the proposed approach.
Keywords linear electric machines; linear induction machines; modelling
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.
A laboratory experiment for undergraduate students is presented in order to
make them more familiar with the electromagnetic concepts, and to show how
to work and experiment with electric machines. The performed tests are based
on the conventional tests for rotary machines, and have some variations and
a broader perspective on electrical devices. These tests show the students how
International Journal of Electrical Engineering Education 38/2
118 J. Atencia, A. Garcı´a Rico and J. Flo´rez
to learn to identify the different electromagnetic parameters such as leakage
reactances, mutual reactance and their relation with the electromagnetic fluxes
and with the phenomenon of electromagnetic induction.
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.
$ 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.
$ 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.
$ 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.
$ 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.
Table 1 shows some chosen values of different parameters of the machine.
TABLE 1 Specifications of the prototype
Type of machine Three-phase asynchronous
Supply voltage 220 V
Maximum currents 5 A
Maximum travel 2 m
Maximum velocity 5 m/s
Maximum acceleration 10 m/s2
Natural cooling
International Journal of Electrical Engineering Education 38/2
Low cost LIM for laboratory experiments 119
Construction of the prototype
Figure 1 shows the prototype reported here. It corresponds to a three-phase
linear induction motor with aluminium sheet over the secondary iron. Figure 2
shows the linear motor, with its most important components.
The basic considerations for developing the prototype are the following.
Fig. 1 L IM prototype.
Fig. 2 Prototype structure.
International Journal of Electrical Engineering Education 38/2
120 J. Atencia, A. Garcı´a Rico and J. Flo´rez
Primary windings
There is a wide spectrum of types of winding for linear motors. For the design
of this prototype, a double-layer, full pitch winding has been chosen (Fig. 3).
This distribution of windings is simple but very effective, and has been used
widely for rotary machines. The winding has two layers, full pitch coils and
(2p+1) poles with half-filled end slots.
The prototype has been designed with four pairs of poles per phase, in order
to limit the phase-current asymmetries.7
Primary iron
Figure 4 shows the structure of the primary iron of the laboratory prototype.
It is highly recommended to use a laminated core in the primary in order to
reduce magnetic losses. A preliminary value of the physical magnitudes may
be obtained using the rotary machine expressions.8
The nominal velocity is a function of the nominal electrical frequency and
of the pole pitch. The value for the nominal velocity at 50 Hz is 5 m/s (Table 1).
Therefore the pole pitch must be
t=nsinc/(2f )=50 mm (1)
Fig. 3 T wo poles machine windings.
Fig. 4 A view of the primary iron.
International Journal of Electrical Engineering Education 38/2
Low cost LIM for laboratory experiments 121
Since it is a full pitch coil winding of a three-phase machine, the slot pitch is
ts=
t
3
=16.7 mm (2)
The primary part has open slots, because they are a great advantage from the
construction point of view, and do not present a great problem for holding the
windings.
The slot width bs and tooth width bt (Fig. 5) are, as in rotary machines,
approximately equal. However, for mechanical reasons, the width of the slots
has been set to bs=10 mm and bt=6.7 mm.
It is interesting to design the slots as short as possible to minimise the
magnetic saturation of the teeth. The dimensions of the slots are lower-limited
for thermo-mechanical reasons· the coils must fit inside them and the heat of
the windings must be withdrawn. Once the width of the slot has been set, the
maximum current density per active slot area determines the slot height.
With a maximum current density per active slot area of 3.5 A/mm for natural
cooling,9 the height of the slot is hs=35 mm. The factor hs/bs is 3.5, which is
a reasonable value for open slots.10
The height of the back iron, hc, can be estimated from the total flux that
goes through a pole pitch. This magnetic flux returns through the back iron,
so there is a direct relation between both measures.
hc#bg
bc
2t
p
(3)
For laboratory purposes the value of the airgap flux density bg=0.5 T is
reasonable. It is also possible working with 0.7 T as in rotary machines, but
then the attraction forces between primary and secondary will be too high. In
the back iron the flux density bc should not be higher than 1.7 T, in order to
avoid extra losses and hot points.7 Setting bc=1 T, then hc#15 mm.
With the special geometry of the linear motor, the total length of the machine
can be calculated as follows:
length=t(number of poles per phase+1)+bt (4)
Fig. 5 Open slot structure of the primary iron.
International Journal of Electrical Engineering Education 38/2
122 J. Atencia, A. Garcı´a Rico and J. Flo´rez
Therefore
length=50 mm(8+1)+7 mm=457 mm
The thrust that the linear machine is able to develop depends on the width of
the machine, and the number of poles.
Normally, the relation between the primary width and pole pitch is at least
about 2.47 in rotary machines, to keep the proportion of leakage flux of the
end coils low. In this case, the width of the prototype has the same value as
the pole pitch, to keep the total force low without having to reduce the
whole engine.
Figure 6 shows the main dimensions of the primary part of the prototype.
Secondary electrical circuit
The easiest way to build a secondary electrical circuit is using an aluminium
plate (Fig. 2). It is cheap, and easy to handle.
If the thickness of the aluminium is small the conducting plate will get hot
if it is too big, the airgap would be large and the efficiency of the machine low.
A good choice for the prototype is 1.5 mm.
The plate may be a little bit wider than the primary iron, to allow the current
closing its path outside the active area.
Secondary iron
The secondary iron length must be at least the primary length plus the length
of travel.
The secondary iron is as wide as the primary iron, to maximise the linkage
flux. If the same induction level in both irons (primary and secondary) is
targeted, the secondary iron must be as high as the back iron of the primary.
In the secondary part, a solid core can be tolerated and is preferred to a
laminated one, because of simplicity and price.