A new generation
of steel stator core laminations, produced by pouring a spout
of liquid metal melted in a rapidly rotating drum (to cause
the rapid cooling of the metal), allow the efficiency of the
motor to be improved. This also reduces the losses by 33 percent
in comparison with the equipment produced with the traditional
stator core laminations.
issue: September 2003 APPLIANCE Magazine
New Stator Core Laminations for Brushless Motors
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by Spolaor Franco, Polifibra Group and Fabio Arpino, Bitron Group
a few years ago, the basic technology of the fractional electrical
motor had not changed much compared to the technology used
at the beginning of the last century. Recently, however, there
have been several developments in this area.
1. Pictured is a Mylar HS tube on the rotor (left) before
the heat shrinking process, as well as the permanent
fixing of the permanent magnet on the rotor after a
heat shrinkage process (right).
new-generation material of stator core laminations achieves
the same (or better) magnetic properties
when compared with those produced with traditional stator
core laminations made of silicon.
innovation is represented by the permanent magnets that are
produced with alloys that contain some rare earth elements
such as neodymium and iron boron. The magnets produced with
the elements have a better available flow and allow for the
production of small and light motors that, in spite of the
small dimensions, are able to give a 50-percent higher torque
when compared with the traditional motor of the same dimensions.
magnets with rare earth elements are used to produce brushless
motors that function independently from the mains frequency.
Moreover, compared with traditional motors, it is believed
that brushless motors achieve higher speeds, require less
maintenance, disperse heat better, and have less down time
(which, in turn, means better dynamic reaction), can be overloaded,
and are more reliable.
however, is their higher cost, which is due to the complexity
of the system used to keep the permanent magnet on the rotor.
This problem stems from the fact that the material with which
they are produced (the permanent magnet) cannot be welded
to the rotor. Another reason for the higher cost is that the
complexity of fixing the magnet means that greater automation
of the production system is not possible.
have - until now - limited the use of the brushless motor
to, for example, industrial control units.
New System for Fixing Permanent
Magnets on the Rotor
to weld the permanent magnets on the rotor has pushed producers
of brushless motors to develop new solutions to fix them on
the rotor, but these solutions have been expensive, couldn't
be used in industrial production, and most of all, were not
reliable in the long term.
2. The shrinkage strength of the Mylar HS tube
in relation to temperature. CLICK
to see the full-size image.
To solve all these
problems, the Polifibra group has patented a new system to
produce a spiral-wound tube of heat shrinkable polyester to
fix the permanent magnet on the rotor in an efficacious and
The tube, used
with an appropriate diameter and thickness (to guarantee the
mechanical seal), is applied to the rotor while the magnets
are in the correct position; the second step is to use a heat
shrinkage process to obtain a strong adhesion and cohesion
even with high rotation speed (see Figure 1).
of this tube is that it can withstand the sudden change of
temperature of the motor even in a long production run, and
it maintains its technical properties in a range of temperatures
that can be between -70°C and +155 °C (see Figure 2). In addition,
the low and constant thickness of the tube (0.12 / 0.20 mm)
does not interfere with the air gap in the motor or with the
due to the short thermal cycle of shrinkage/thermo molding
(less than 10 sec when the correct temperature is applied),
the solution is also available for use with automated positioning
A Motor Application
of this material was in a new generation of high-efficiency
brushless motors developed by the Bitron Group (see Figure
3). The motor manufacturer has developed and produced a new
brushless motor, called STAR 4, which includes permanent magnets
and has been specifically devised for refrigerator fans. High
efficiency was the company's main goal in designing the new
3. The Star 4 brushless motor from Bitron Group comprises
a rotor with the new mylar HS tube.
The family of
STAR 4 motors includes motors with varying performances from
approximately 1 W of consumption to 5 to 6 W, with several
operating speeds, but with efficiencies of approximately 50
percent. The mains power supply voltage and the completely
integrated electronic control mean that it is perfectly interchangeable
with a traditional motor. The smaller dimensions allow for
improved installation inside the refrigerator and optimized
the volumes and the thickness of the thermal insulation.
its efficiency goal, the second step was to find a motor suited
to mass production that could also be cost effective. To keep
the costs under control, the motor has been designed with
the aim of reducing the number of components and the setup
The result has
been a motor with a small number of components (22 total)
that are not only multi-functional, but do not require specific
fixing parts (i.e., screws). Additionally, the motor can be
assembled using press fit or snap fit (these operations are
easy to automate).
In this respect,
the construction of one of the two types of rotors, which
is produced with four segments of ferrite magnet assembled
on a laminated core, has benefited from the use of a heat
shrinkable sleeve. The material is used to completely cover
the rotor, keeping and fixing the magnets in the right position
and, in turn, permits a simple, quick, and reliable set up.
A possible alternative
method of fixing such as glue would have brought with it several
reliability problems because it wouldn't have been possible
to properly test the efficiency of the assembly in terms of
cost. Another advantage with this system is that it avoids
the problems connected with cleaning the surfaces and with
applying the correct amount of glue.
At the moment,
this motor is produced on a production line that is almost
completely automated with a production time of 7 sec.
Spolaor Franco is director of New Projects
at Polifibra Group, where he has worked for approximately
Arpino Fabio is the R&D director and
project manager at Bitron Group.