Overview
An electric motor is a device that converts electrical energy into magnetic energy and then into mechanical energy. Therefore, its core function is electromagnetic conversion, which involves calculating the electrical and magnetic circuits. Drawings are merely the external representation; a true designer must be able to design and calculate the motor's electrical and magnetic circuits, including the dimensions of the stator and rotor, the ampere-turns of the coils, and crucial parameters such as the air gap flux.
1. Avoid designing motors that are too long or too thin.
Motor design aims to achieve optimal performance with minimal materials and cost. Generally speaking, flat motors use less iron and more copper, as well as more structural materials. Slender motors use more iron and less copper, as well as less structural materials, but have lower structural rigidity.
Therefore, there is an optimal ratio between the diameter and length of the motor. The ratio of the inner circle of the iron core to its length is approximately 1:1. Motor design requires optimization based on various performance requirements, available materials on the market, and the price of structural materials. Furthermore, it must consider issues such as standardization, component commonality, structural manufacturability, and the cost of molds and tools.
2. The current density of the coil should not be too high or too low.
Motor coils have a certain resistance, and losses occur when current flows through them, reducing motor efficiency and increasing winding temperature. Motor design aims to reduce resistance to minimize losses, lower temperature rise, and improve efficiency. Reducing current density and increasing conductor cross-sectional area can decrease resistance, but this increases the amount of coil material used. Increased slot area leads to increased core magnetic flux density, increasing excitation current and iron losses. Therefore, the selection of current density must comprehensively consider motor performance. A current density of 3–7 A/mm² is generally chosen. Lower values are used for large motors and enclosed motors, while higher values are used for small motors and open motors.
3. The magnetic flux density of the iron core should not be too high or too low.
When the core material, frequency, and silicon steel sheet thickness are constant, iron loss is determined by the magnetic flux density. Excessive magnetic flux density increases iron loss, reduces motor efficiency, and causes the core to overheat, leading to a higher motor temperature. Furthermore, the increased excitation ampere-turns lower the motor's power factor. Therefore, the magnetic flux density of the core should not be too high, and its use in the oversaturation section of the magnetization curve should be avoided as much as possible. Small motors generally do not exceed 155T. Insufficient magnetic flux density increases the amount of material used in the motor, raising costs.
4. The fill factor of the tank should not be too high or too low.
The slot fill factor refers to the ratio of the area of the conductor in the slot to the effective area of the slot. A high fill factor indicates tight filling, while a low fill factor indicates loose filling. From the perspective of fully utilizing motor materials and operational performance, a higher fill factor is better. However, an excessively high fill factor makes winding difficult, increases labor and time, and can easily damage the insulation. A low fill factor causes the conductor to move loosely within the slot during motor operation, easily damaging the insulation. Furthermore, the large gaps within the slot, due to poor air thermal conductivity, affect coil heat dissipation, leading to increased motor temperature rise. For manual winding, the slot fill factor is generally 75%–78%, not exceeding 80%. For automated winding, the slot fill factor is generally not higher than 75%.
5. Parallel tooth trapezoidal grooves should be selected whenever possible.
When silicon steel sheets operate in the saturation region of their magnetization curve, the ampere-turns consumed per unit length increase significantly with increasing magnetic flux density. To make efficient use of the motor's internal space, the silicon steel sheets are always designed to be relatively saturated. If trapezoidal teeth are used, the narrow section of the teeth experiences a high magnetic flux density, resulting in a substantial increase in excitation ampere-turns and a decrease in the motor's power factor. If parallel teeth are used, the magnetic flux density is uniform along the length of the teeth, greatly reducing the excitation ampere-turns consumed.
6. The edges of the groove should not have sharp corners.
The design of the groove shape should facilitate the manufacture of the die. During die quenching, cracks often occur at the sharp corners of the groove due to stress concentration. Rounded corners also help extend the die life. The edges of the groove shape should be rounded as much as possible, and the radius of the rounded corners should not be less than 1mm.
7. Use round-bottomed grooves instead of flat-bottomed grooves whenever possible.
Advantages of round bottom grooves:
A. Round-bottomed slots can improve the filling of wires, and the slot insulation is not easily damaged. Under the same slot fill rate, it is easier to insert wires into round-bottomed slots than into flat-bottomed slots.
B. When casting aluminum into the rotor, the round-bottomed slots are better filled with molten aluminum than the flat-bottomed slots.
C. Round-bottomed grooves are easier to mold than flat-bottomed grooves.
8. The width of the slot opening in the motor core should not be too large.
If the motor slot opening is too small, it will be difficult to run the wire. If the motor slot opening is too large, it will cause uneven distribution of air gap magnetic flux, increase tooth harmonics, and increase additional losses. The slot opening width of a semi-closed slot is generally the diameter of 2 to 3 wires, about 3.5 mm. Low-voltage forming coils adopt a semi-open slot structure with four elements on the sides of the slot, reducing its slot opening width to half the slot width.
9. The number of stator slots should not be too many or too few.
A higher number of stator slots in an electric motor results in better magnetomotive force and electromotive force waveforms, lower additional losses, and higher motor efficiency. However, a higher number of slots also leads to narrower core teeth, greater stamping deformation, and poorer manufacturability. A higher number of slots also increases mold manufacturing costs, motor design issues, coil manufacturing, and production time. Generally speaking, a higher number of stator slots results in better motor performance but higher costs. Typically, the slot dispersion per pole per phase in an asynchronous motor is q≥2.
10. The air gap should not be too large or too small.
The air gap refers to the space between the stator and rotor of a motor. The size of the air gap has a significant impact on motor performance and manufacturing processes. A large air gap results in high magnetic reluctance, a high number of excitation ampere-turns, which increases the motor's excitation current and lowers the power factor. However, a large air gap also weakens harmonic magnetic fields and reduces additional losses. A large air gap also allows for lower requirements on the coaxiality and assembly precision of motor components; conversely, a too-small air gap can easily cause stator-rotor rubbing and reduce motor efficiency due to increased additional losses.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964 ) . We apologize for any inconvenience.
