This article provides an in-depth analysis of the actual slot fill factor in motor products. It is important to note that there exists a reduced slot fill factor due to manufacturing constraints, primarily seen in motors using needle-type winding machines, as these require extra space for the needle operation.
A higher slot fill factor is naturally pursued in design since it directly affects motor efficiency. However, electric motors also require insulation treatment to ensure performance and safety, which occupies additional slot space. The thickness of insulation materials can range from 0.18 mm to 1.2 mm, depending on the type, with thicker insulation reducing the available winding space.
In winding practice, the insulating varnish layer on the enameled wire must also be considered, as it affects the final outer diameter of the coil. For example:
A 1 mm bare copper wire may become 1.1 mm after insulation, resulting in a ~10% increase in diameter.
Therefore, winding plans should be based on the finished outer diameter of the enameled wire, not just the conductor’s effective diameter.
In practical slot applications, a certain clearance must be maintained between coils, referred to as the "passage."
Without this space, later windings may interfere with earlier ones, potentially causing coil damage or even short circuits.
Ideally, the passage width should be 1.6 times the finished wire diameter, with a minimum of at least one full wire diameter.
The real slot fill factor must account for insulation thickness and passage space. For example:
Insulation occupies 28.5% of the slot space.
Passage clearance takes up another 7.6%, totaling 36.1% reduction.
Additionally, the insulation layer on the enameled wire further reduces the effective conductor area.
Enameled wire: Finished diameter = 1 mm, conductor diameter = 0.95 mm.
If 50 turns fit in a slot, the effective conductor area loss due to insulation is 9.7%.
Thus, the true slot fill factor (relative to the conductor’s effective space) is only 42.68%.
In reality, practical slot fill factors typically range between 30% and 50%, often lower than design expectations.
To effectively increase the slot fill factor, the following conditions must be met:
1. Thinner insulation on silicon steel laminations (minimal possible).
2. Thinner enamel insulation on the winding wire.
3. Finer wire diameter (reduces required passage space).
4. Fewer coil turns (increases the effective conductor ratio).
However:
Points 1 & 2 are constrained by safety regulations and cannot be arbitrarily modified.
Point 4 depends on operational requirements—typically, after determining the number of turns, the wire diameter is maximized to improve efficiency.
Therefore, the most viable optimization is Point 3: reducing or eliminating passage clearance. This leads to the concept of "Dragon Bone Motors," which use an innovative assembly method to achieve the highest possible slot fill factor.
In overall motor design, insulation treatment is a critical step. Only after completing a comprehensive insulation plan can a truly optimized motor product be achieved.
