MOTORS & MOTOR CONTROL: Asymmetric Lamination
Engineers at Johnson Electric are studying numerous possibilities to achieve these objectives. A good example of that is an innovative patented design that provides the same or better performance from a motor that takes up less space and weighs less using a process called asymmetric lamination.
Direct-current (DC) brush electric motors typically use symmetrically wound wire on the armature, which lengthens the armature and takes up more space. In the new Johnson Electric approach, the winding wire is spread over a larger section of the surface asymmetrically by starting and ending at different radii. The result is a shorter armature that also promises to be lighter.
HistoryWhen motors were first designed, both the stator, the stationary part, and the armature, the revolving part, were made of soft iron or steel laminations, and were wound with wire around steel or iron stacks to create magnetic fields. The interaction between the stator and armature was such that the armature rotated, allowing its wound coils to be sequentially energized, through a commutator, to create a continuous rotating force or torque.
Winding wire around coils to create electromagnetic fields required that the wire "piled up" both in the slots of the stacks and around the axial ends of the stator and armature. This accumulation of wire increased the working length of the metal stacks by as much as 40 percent, which often determined the motor's total length.
Eventually, permanent magnets became a commercially viable option that replaced wound stators in DC motors. These permanent magnet DC (PMDC) motors initially did not have very high magnetic flux densities, and it was quite common to make the axial length of the magnet greater than the lamination stack of the armature with as much as a 30 percent overhang. This allowed the steel of the armature lamination to collect a little more flux, which improved performance ratios.
The wire overhang, due to the winding, was still not critical in determining the total length of the motor because the magnet overhang was also required. But magnets have improved greatly and their flux densities today are very high, eliminating the need for the magnets to substantially overhang the axial length of the lamination. That leaves the wire overhang as a stumbling block to shorter motor designs. The new design project establishes a way to reduce wire overhang and, consequently, shorten the motor length.
Design considerationsThe factors that affect the degree to which the winding wire overhangs the lamination stack include:
- The number of turns.
- The gauge of the wire.
- The pitch of the winding (that is, the number of consecutive slots that each coil spans).
- The relative length of the lamination stack itself.
With a short pitch, the overhang is reduced but performance is compromised and this is an important design consideration. For motors having short lamination axial lengths, the overhang of the winding wire becomes proportionally more significant, and it is these motors that require the greatest attention.
New lamination profileThe proposed alternative lamination has been designed to solve the problems of winding wire overhang that are found in standard DC brushed motors. The main characteristic of this lamination is the possibility of reducing the length of wire overhang and organizing the wire positioning to shorten the armature's total length.
A major objective of the new design is to provide a greater surface area at the end of the lamination to spread the end turns over a wider area and reduce the axial length of the end turns.
The profile illustrates five differing slot shapes. The dominant slot shape has a very narrow and deep shape, whereas the others are wider and closer to the peripheral surface of the lamination.
In the lamination images on the previous page, there are two groups, each comprising six dominant slots followed by four different slot shapes. The number of groups depends on the number of slots of the armature and its diameter.
Winding the coilsThe images shown in Fig. 1, Fig. 2, Fig. 3 and Fig.4 illustrate the winding of the new lamination for a four-pole motor in stages of progressive coil fill.
As the images show, the winding wire is spread over a larger section of the surface area of the lamination. Each coil is not wound as a chord on the circular lamination as it has been traditionally, but starts and ends at different radii allowing for a better layering of the wire as each coil is wound with a part overlying the previous coil.
When the coil winding is complete, a forming tool pressed over the end turns will press the wire over an even greater surface area, further reducing the axial length of the overhung winding.
Reduced overall lengthThe results of this novel lamination design and subsequent reduction in winding overhang can be illustrated by the drawings Fig. 5 and Fig. 6 that show an 18 percent reduction in overall motor length from 96 mm to 78mm.
Various factors can influence the extent of shortening, including: the number of poles, the number of lamination slots, the winding pitch, and more. Experiments conducted with variations in those aspects will lead to optimized arrangements.
Advantages derived from the new lamination process include:
1. Overall motor lengths are reduced, meeting the demand for smaller motors.
2. The closer proximity of wire to steel reduces any thermal gradient between lamination and copper.
3. The larger surface area of exposed copper improves dissipation of heat from windings.
4. The closer proximity of the coils to the lamination increases the magnetic flux density in the lamination teeth.
The alternative approach also exhibits some disadvantages, including:
1. The unusual slot shape requires that the lamination stack be coated with an insulation resin such as electrostatic coating.
2. Balancing of the armature is effected with negative, rather than positive, mass changes.
3. Individual coils do not have equal lengths, resistance or inductance.
4. Laminations must be designed specific to poles, teeth, diameter and pitch.