Motors used to drive EVs and HEVs require small size, light weight, and low price. One of the important basic technologies is the "coil winding method". Wrap the flat wire longitudinally relative to the inner core; high-density winding without splitting the inner core; twist 16 bare wires together and avoid crossing. These winding measures will directly affect the performance of the motor, even the performance of the EV.
Motor technology supports the development of EVs (electric vehicles) and HEVs (hybrid vehicles). Although the field is not able to double the performance as the battery, but negligence will also lose the competition. Because this technology still has room for improvement.
The main component of the motor-the coil is currently developing and progressing with the duty cycle as the axis. The duty ratio is the proportion of the actual wire cross-sectional area in the cross-sectional area when the coil is cut in a plane perpendicular to the current. If the duty cycle is increased, a small, light and powerful motor can be realized.
Sooner or later will use the flat vertical winding method
Figure 1: Flat winding coil insulated with PEEK tape
You can see the tape winding diagonally. The corner was severely deformed due to processing, but the tape showed good followability
"Able to use 'Edge Wise' coil" is a motor technician's dream. The flat-wound coil is a coil that closes the edge of the wire to the inner core (iron core) and winds the wire longitudinally relative to the inner core (Figure 1). Because of its excellent heat dissipation, it can increase the output power of the motor.
Technology that is one step closer to this dream has already emerged. That is insulating tape made of polyetheretherketone (PEEK). PEEK resin distributor Victrex Japan, Towatec and Urayasu Engineering jointly developed a processing technology for high-density flat winding of flat wires using this tape.
As far as small-scale production is concerned, the flat vertical winding method is a technology that began in the Taisho era. In terms of EV, it has been adopted by small products such as reactors. But it has not been used for motors, let alone large motors for automobiles.
The wire of the coil is generally a circular wire with a circular cross section [Fig. 2 (a)]. Later, it developed into square and rectangular square lines. In rectangular conductors, flat lines with relatively large vertical and horizontal dimensions appeared. The flat-wound coil wound with a flat wire (Figure 2 (b)) has been popularized. The dream of motor technicians is to further develop into a flat vertical winding method based on this [Figure 2 (c)].
Figure 2: Differences due to different coil wires
From (a) round wire to (b) flat wound normal flat wire. (C) The next goal is to wind the flat wire vertically. The temperature of the red part will rise.
The alias of flat winding is also called reverse winding. This is a method of winding the flat surface of the flat wire against the inner core. The impression is that the ribbon thread is tightly wound on the inner core. In addition, although it is "close to the inner core", it is sometimes inserted into the inner core after processing in other processes.
The flat winding method differs from the flat winding method in that the wire winding method differs by 90 degrees. The curved part stretches greatly on the outside, while the inside shrinks greatly. The impression is changed from ribbon winding to plastic processing. The difficulty has improved a lot at once.
If the coil is wound with a round wire, it cannot be expected how high the duty ratio will be. Since circular objects are placed side by side in a square space, there will be gaps everywhere. The "standard duty cycle" in which the cross-sectional area of ​​the insulating film is not included in the molecule of the calculation formula is only about 35 to 55%. If the square line is used, the gap can be eliminated and the duty cycle can be increased to 62-85%.
Whether it is a flat winding method or a flat vertical winding method, the duty ratio will not make much difference. The difference is the heat dissipation. When the drive motor rotates, the power equivalent to the copper loss becomes heat in the coil. To prevent excessive current from damaging the insulating material, the upper limit of the current is fixed, so the output power is also fixed. To increase the output power, continuous cooling is required.
The heat generated by the coil will be radiated from the inner core to the ring outside the stator, and then released to the heat sink and cooling water flow channel on the surface of the motor. When the coil is wound with a round wire, heat will circulate repeatedly along the route of wire → insulation material → slot → insulation material → adjacent wire. Of course, due to the large thermal resistance to the inner core, the temperature of the wire will increase.
If it is a square wire or a flat wire, as long as it is carefully wound, the gap can be reduced, and the path of heat will be shortened to wire → insulation material → adjacent wire, which will improve the heat dissipation. Even so, the heat of the wire away from the inner core will eventually reach the inner core after passing through the insulating material several times. Because the wire is a copper wire, the thermal conductivity is high, while the thermal conductivity of the insulating material is low.
When the flat vertical winding method is adopted, the edges of all the wires will contact the inner core. Therefore, the heat will not be transferred to "adjacent wires". The path can be simplified to wire → core. The thermal resistance will become extremely small.
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