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issue: September 2003 APPLIANCE Magazine

Engineering Motors
New Stator Core Laminations for Brushless Motors


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by Spolaor Franco, Polifibra Group and Fabio Arpino, Bitron Group

Until 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.

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.

Figure 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).

This 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.

Another important 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.

The permanent 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.

A disadvantage, 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.

These reasons 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

The inability 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.

Figure 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 economical way.

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).

An advantage 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 magnetic flow.

In addition, 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 systems.

A Motor Application

One 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 electrical motor.


Figure 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.

Having achieved 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 time.

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.


About the Authors

Spolaor Franco is director of New Projects at Polifibra Group, where he has worked for approximately 25 years.

Arpino Fabio is the R&D director and project manager at Bitron Group.

 

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