In motor design, there is a seemingly small but crucial indicator—slot fill rate. It directly affects the motor's performance, cost, and reliability. Some say that the art of motor design is, to some extent, the art of "manipulating" slot fill rate.
Slot fill ratio, as the name suggests, is the ratio of the total cross-sectional area of the conductors in the slots of a motor stator or rotor core to the net area of the slot, usually expressed as a percentage.
The calculation formula is:
Slot Fill Ratio Sf = (N * Sd * n) / As * 100%
N: Total number of conductors in the slot
Sd: Cross-sectional area of a single conductor (including insulation varnish)
n: Number of conductors wound in parallel (if present)
As: Net usable area of the slot (after deducting the area occupied by slot insulation, slot wedges, spacers, etc.)
Improving slot fill rate requires understanding the factors that constrain it, which can be mainly divided into three categories:
1. Design and Material Factors
Slot Shape Design: Different slot shapes, such as circular, pear-shaped, and rectangular slots, have vastly different space utilization rates. An optimized slot shape is the foundation for efficient space utilization.
Conductor Type: Round enameled wire or flat copper wire? Round wires have natural gaps between them, while flat copper wires can be spliced almost seamlessly, resulting in extremely high space utilization.
Insulation Material Thickness: The thickness of slot insulation paper, phase-to-phase insulation, slot wedges, etc. Thinner but reliable insulation materials can directly increase the net slot area (As).
Conductor Insulation Coating Thickness: Under the premise of meeting the withstand voltage rating, using a thinner coating (such as UEW/AIW grade enameled wire) can reduce Sd, thereby filling more copper in the same space.
2. Manufacturing Process Factors
Winding Method: Manual, semi-automatic, or fully automatic winding? Precision fully automatic winding machines can arrange wires more neatly and tightly, effectively improving slot fill rate.
Wire Insertion Process: Skilled inserters or advanced automatic inserting equipment can insert the wires into the slots in an orderly manner, reducing gaps caused by messy crossings.
3. Electromagnetic Performance Factors
Slot Size: While a slot with a too-small opening may result in a higher slot fill factor, it makes wire embedding extremely difficult, even impossible to manufacture. This needs to be considered during the design phase.
How can this key indicator be improved safely and effectively? The following are the core methods:
1. Use Flat Copper Wire (Hair-pin / Pin-wave Technology)
This is currently the mainstream technology for driving high-performance EV motors. Replacing round wire with flat copper wire of a rectangular cross-section can achieve nearly 100% space filling, easily increasing the slot fill factor to over 70%, or even exceeding 80%, far surpassing the 40%-60% of round wire motors.
2. Optimize Slot Shape and Opening Design
Through electromagnetic field simulation software, the slot shape is parametrically optimized to achieve the highest space utilization while meeting requirements for magnetic flux density and yoke strength. Simultaneously, the slot opening is rationally designed to strike a balance between easy wire insertion and reduced cogging torque.
3. Apply Thinner/Superior Insulation Materials
Use composite insulation materials such as DMD and NMN: These materials can be thinner than traditional PVC-coated paper while maintaining the same or even better insulation strength.
Select Thin-Film Enameled Wire: Within the limits of temperature and voltage resistance, select enamel-coated wire with a first- or second-level enamel film thickness.
4. Upgrade Manufacturing Processes and Equipment
Employ Automated Winding and Insertion Equipment: Machines ensure the order and consistency of winding and insertion, resulting in tight and uniform wire arrangement, maximizing the reduction of gaps—an advantage unmatched by manual methods.
Introduce a "Wire Twisting" Process: Before insertion, the equipment "twist" multiple parallel wires of a single turn into a tighter whole, reducing gaps between wires. Adjusting wire diameter and number of parallel windings: While maintaining the same total cross-sectional area, replacing thicker wires with thinner ones, or optimizing the number of parallel windings, can sometimes better fill the corners of the slots, improving space utilization.
Increasing slot fill factor is not without its drawbacks; it's a classic double-edged sword.
Positive Impacts:
Increased Power/Torque Density: Increased copper content in the slots reduces resistance, lowering copper losses (I²R) at the same current, thus improving efficiency. Alternatively, with the same losses, greater torque and power can be output, enabling motor miniaturization and weight reduction.
Improved Heat Dissipation: Copper's thermal conductivity is far superior to air. A high slot fill factor means less air gap within the slots, allowing heat to be conducted more effectively to the core and housing through the copper wires, reducing hot spot temperatures.
Enhanced Mechanical Stability: Tightly packed coils are less prone to movement within the slots, improving the motor's mechanical reliability and vibration resistance, while also reducing electromagnetic noise.
Potential Risks:
Dramatically Increased Wire Insertion Difficulty: Excessively high slot fill rate makes wire insertion exceptionally difficult, easily scratching the conductor insulation film during operation, leading to quality defects such as inter-turn short circuits. This places extremely high demands on the manufacturing process.
Increased Production Costs: Requires higher-quality raw materials (such as flat wire and thin insulation), more sophisticated automated equipment, and higher process control costs.
Dramatically Increased Wire Insertion Difficulty: Excessively high slot fill rate makes wire insertion exceptionally difficult, easily scratching the conductor insulation film during operation, leading to quality defects such as inter-turn short circuits. This places extremely high demands on the manufacturing process.
Increased Production Costs: Requires higher-quality raw materials (such as flat wire and thin insulation), more sophisticated automated equipment, and higher process control costs.