Are you confused about the difference between slot fill factor vs copper fill factor in motors? During motor winding design and manufacturing, these two parameters play a critical role. They directly affect the motor’s efficiency, thermal management, cost, and performance. This article provides a detailed analysis of slot fill factor and copper fill factor.
What Is Slot Fill Factor?
Slot fill factor, also called slot filling ratio, refers to how much of the stator slot is occupied by windings. It is the ratio of the total cross-sectional area of the conductor to the total cross-sectional area of the stator slot.
Formula:
Slot Fill Factor = (Total cross-sectional area of conductors) / (Stator slot cross-sectional area)
Factors That Affect Slot Fill Factor
Core Aspect Ratio
Aspect ratio refers to the ratio of the motor core length to its diameter. When length increases while diameter stays the same or decreases, the achievable slot fill factor decreases (see table), and the manufacturing difficulty increases. Longer cores reduce the leverage of insertion tools, making the wire pressing process more difficult.
| Core Aspect Ratio | 10 | 3 | 1 |
| Max Slot Fill Factor | ≥60% | ≥70% | ≥80% |
Wire Diameter
Choosing the right diameter for enameled wire is critical in motor design. A larger effective conductor area is always preferred. One way to increase the conductor area is to use thicker wires, which results in more copper and relatively less insulation.
However, as the wire diameter increases, it becomes stiffer and harder to handle. Hard wires are difficult to conform to slot shapes and fit poorly with other wires.
Using smaller wire gauges improves handling but increases the ratio of insulation to conductor. It may also require more turns or parallel paths, raising the risk of wire damage during insertion.
Stator Slot Shape
The goal of slot shape design is to maximize copper and other elements while minimizing the effect on flux or manufacturability. The two most common slot types are flat-bottom slots with square or rounded corners, and pear-shaped slots with a curved bottom.
Flat-bottom slots appear to have more area but waste corner space. Rounded bottom slots fit better with insulation and allow more wire to occupy the slot edges. Compared with flat-bottom slots, round slots can increase the slot fill factor by 5-10%.
Stator Slot Dimensions
Since coils are inserted through the stator slot opening on the inner diameter, a wider opening makes insertion easier. However, it also disrupts the magnetic path. If the slot opening is too narrow, insertion becomes harder and slot fill factor decreases due to limited tool space.
In general, the opening width should be at least 2 times the wire diameter. For larger wire bundles, it should be 3-4 times the diameter.
Slot Insulation Material
Insulation thickness can range from 0.18 mm to 1.2 mm depending on the type. Thicker insulation means less space for actual winding.
Coil Gap Space
In practical slot usage, you must keep enough clearance between coils, called the “aisle.” Without it, later coils may collide with earlier ones, causing damage or even short circuits. Ideally, the aisle width should be 1.6 times the finished diameter of the enameled wire, and never less than the wire’s full diameter.
For example, using enameled wire with a finished diameter of 1 mm and a bare conductor of 0.95 mm, if 50 turns are inserted in one slot, the difference between the total space of finished wire and effective conductor is 9.7%. Thus, the actual slot fill factor is only 42.68%.
How to Improve Slot Fill Factor
Use the thinnest possible insulation layer on electrical steel laminations.
Use the thinnest possible enamel coating on the wire.
Reduce aisle spacing with uses finer wire.
Reduce the number of turns to improve the proportion of copper.
What Is Copper Fill Factor?
Copper fill factor is the ratio of the actual copper wire area inside the stator slot to the effective winding area.
Formula:
Copper Fill Factor = (Actual copper wire cross-sectional area) / (Effective winding area)
Factors That Affect Copper Fill Factor
Copper Wire Type
Choosing a suitable wire diameter and using high-precision manufacturing reduces gaps between wires and increases copper fill factor.
Filling Method
Using tight-filling techniques, such as advanced automatic winding machines, improves copper utilization.
Insulation Technology
High-performance insulation reduces the space wasted around copper wires, increasing copper fill factor.
What’s the Relationship Between Slot Fill Factor and Copper Fill Factor?
In actual design, slot fill factor and copper fill factor are closely related. Increasing slot fill factor usually increases the number and density of wires in the winding, which boosts copper fill factor.
However, excessive slot fill factor may cause heat dissipation issues. Fewer gaps between wires reduce thermal performance and reliability.
Both slot fill factor and copper fill factor are key parameters in motor design. They significantly affect motor performance, efficiency, and reliability. Designers must consider all influencing factors and balance both parameters to achieve optimal performance and cost-effectiveness.
High Slot Fill Factor Motor Stator Winding – Choose Lamnow
Lamnow is a professional electric motor stator and rotor manufacturer offering diverse higher slot fill factor stator winding services. Whether you need manual winding, automated winding, round wire winding, flat wire winding, concentrated or distributed winding, star or delta connection—we can meet your needs.
For BLDC motor stator windings, PMSM stator windings, or single and three phase asynchronous motor stator windings, feel free to contact us anytime.
