Electromagnetic Design of Electric Motors
1.1 Avoid Designing Motors That Are Too Long or Too Flat
Electromagnetic Design of Electric Motors aims to achieve optimal performance with the least amount of material and cost. Generally speaking, flat motors use less iron but more copper and structural materials. In contrast, long motors use more iron but less copper and structural materials, yet they often have poorer structural rigidity.
Therefore, there is an optimal ratio of diameter to length for electric motors, typically around 1:1 for the ratio of the core’s inner diameter to its length. Motor design should be optimized based on various performance requirements and the prices of effective and structural materials available in the market. Additionally, considerations should be made for standardization, parts interchangeability, structural manufacturability, and the cost of tools and molds. See the diagram below.
**1.2 The Current Density of Motor Coils Should Not Be Too High or Too Low**
Motor coils have a certain resistance, and when current flows through the coils, it causes energy loss, reducing motor efficiency and increasing winding temperature. In motor design, it’s desirable to reduce resistance to minimize losses, lower temperature rise, and improve efficiency. Reducing current density and increasing the cross-sectional area of the conductor can reduce resistance, but this leads to an increase in the amount of coil material used. Enlarging the slot area increases the magnetic flux density in the core, raising the magnetizing current and iron losses of the motor. Therefore, the choice of current density should comprehensively consider the motor’s performance. Typically, a current density of 3 to 7 A/mm² is used. For large motors and enclosed motors, the lower value is used, while the higher value is applied to small and open motors. See the diagram below.
**1.3 The Magnetic Flux Density in the Motor Core Should Not Be Too High or Too Low**
When the core material, frequency, and thickness of the silicon steel sheet are constant, the iron loss is determined by the magnetic flux density. High magnetic flux density increases iron loss, reduces motor efficiency, and raises the temperature due to the core heating up. It also increases the magnetizing ampere-turns, thereby lowering the motor’s power factor. Thus, the magnetic flux density of the core should not be too high; it’s best to avoid operating in the saturated section of the magnetization curve. For small motors, the magnetic flux density generally should not exceed 1.55 T. Too low magnetic flux density increases material usage, raising production costs.
**1.4 The Slot Fill Factor of the Motor Should Not Be Too High or Too Low**
The slot fill factor refers to the ratio of the conductor area within the slot to the effective slot area, i.e., N²d. See the diagram below.
Where N is the number of turns of the conductor, d is the conductor’s external diameter after insulation, and S is the effective slot area (slot area minus the area occupied by slot insulation). A high slot fill factor indicates tight filling within the slot, whereas a low slot fill factor indicates loose filling. To make full use of the motor materials and ensure good operational performance, the slot fill factor is best when it’s moderate. However, too high a fill factor makes winding difficult, increases labor and time, and easily damages insulation. A low fill factor results in conductor movement within the slot during motor operation, which can damage insulation. Moreover, too much air gap in the slot hinders heat dissipation from the coils, increasing motor temperature rise. Generally, the slot fill factor should be 75% to 78% and not exceed 80%. See the diagram below.
**1.5 Whenever Possible, Choose Parallel Teeth Trapezoidal Slots for Motor Slot Design**
Silicon steel sheets work in the saturated section of the magnetization curve, where the ampere-turns consumed per unit length of magnetizing increase significantly with higher magnetic flux density. To reasonably utilize the internal space of the motor, the silicon steel sheets are typically saturated in motor design. If trapezoidal slots are used, the narrow part of the teeth has a high magnetic flux density, significantly increasing the magnetizing ampere-turns and lowering the motor’s power factor. In contrast, using parallel teeth results in uniform magnetic flux density along the length of the teeth, greatly reducing the magnetizing ampere-turns consumption. See the diagram below.
**1.6 Avoid Sharp Corners on Slot Edges**
The design of the slots should consider ease of punching die manufacturing. During quenching of the punching die, stress concentration at the sharp corners of the groove often causes cracks. Rounded corners help extend the life of the punching die. The edges of slot designs should adopt rounded corners where possible, with a minimum radius of 1 mm. See the diagram below.
**1.7 Prefer Round Bottom Slots Over Flat Bottom Slots**
Advantages of round bottom slots:
A. Round bottom slots improve the filling condition of the conductor and make slot insulation less prone to damage. Winding is easier in round bottom slots than in flat bottom slots when the slot fill factor is the same.
B. During aluminum die-casting, round bottom slots allow better aluminum filling than flat bottom slots.
C. Round bottom slots are easier to manufacture in molds compared to flat bottom slots. See the diagram below.
**1.8 Avoid Excessively Wide Slot Openings in Motor Cores**
If the slot opening of the motor is too small, winding is difficult. If the slot opening is too wide, it causes uneven distribution of the air gap magnetic flux, increases tooth harmonics, and adds extra loss. The width of the semi-closed slot opening is generally 2 to 3 times the diameter of the conductor, approximately 3.5 mm. Low-voltage formed coils use a semi-open slot structure with four elements in the slot, reducing the slot opening width to half of the slot width. See the diagram below.
**1.9 Avoid Having Too Many or Too Few Stator Slots**
In asynchronous motors, having more stator slots results in better magnetic potential and EMF waveform, lower additional losses, and higher motor efficiency. More slots also increase the contact area between the coil and core, improving coil heat dissipation and reducing temperature rise. However, too many slots make the core teeth too narrow, causing significant deformation during punching and resulting in poor manufacturability. More slots also increase mold manufacturing costs and add time to coil manufacturing and winding. Generally speaking, more stator slots improve motor performance but increase costs.
For asynchronous motors, the number of slots per pole per phase, q, should be at least 2. See the diagram below.
**1.10 Avoid Using Excessively Large or Small Air Gaps in Asynchronous Motors**
The air gap refers to the space between the stator and rotor of the motor. The size of the air gap significantly impacts motor performance and manufacturing processes. A large air gap increases magnetic reluctance, magnetizing ampere-turns, and magnetizing current, reducing the motor’s power factor. However, a large air gap weakens the harmonic magnetic field, reducing additional losses in the motor. A larger air gap also lowers the requirements for coaxiality and assembly precision of motor components. If the air gap is too small, it may cause rotor and stator contact, increasing additional losses and reducing motor efficiency. See the diagram below.
**Note: Extracted from “500 Cases of Electrical Design Taboos.”**
