Today I'd like to share some rules regarding motor design.
1. The magnetic flux density of the motor core should not be too high or too low. When the core material, frequency, and silicon steel sheet thickness are constant, the iron loss is determined by the magnitude of the magnetic flux density Ø. Excessive magnetic flux density increases iron loss, reduces motor efficiency, and causes the core to overheat, increasing the motor temperature. Furthermore, the increased excitation ampere-turns lower the motor 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. Insufficient magnetic flux density increases the amount of motor material used, raising costs. Narrower core teeth and higher magnetic flux density mean larger slot openings, i.e., larger core slots, and vice versa.
2. The slot fill factor of the motor should not be too high or too low. Generally, it should be 75-85%. If the slot fill factor is too low, the wires will become loose in the slots when the motor is running, which can easily damage the insulation. In addition, there are many gaps in the slots, and due to the poor thermal conductivity of air, the heat dissipation of the coil is affected, which increases the temperature rise of the motor.
3. The motor slot design should preferably use parallel tooth trapezoidal slots with no sharp corners on the slot edges. Round-bottomed slots are preferable because they are easier to fill during aluminum casting and easier to mold. Stator chip embedding is also easier.
4. The coil current density should not be too high or too low: Motor coils have a certain resistance. When current passes through the coil, losses are generated. As the winding temperature rises, motor design aims to reduce resistance to decrease losses and improve efficiency. Increasing the wire diameter and lowering the current density can reduce resistance, but this increases the amount of coil material used. The increased slot area leads to an increase in the core magnetic flux density, which in turn increases the motor's excitation current and iron losses. Typically, the current density (J) for induction motors is 3~7 A/mm² (a larger slot area means a larger core slot in the chip design).
5. The width of the motor slot should not be too large. If the slot is too large, the air gap magnetic flux distribution will be uneven, the tooth harmonics will increase, and the additional loss will increase. It is usually about 3.5mm. If it is too small, it will be difficult to insert the wire.
6. The number of stator slots should not be too many or too few. A higher number of stator slots in an asynchronous motor results in better magnetomotive force and electromotive force waveforms, lower additional losses, and higher motor efficiency. More slots also increase the contact area between the coil and the core, leading to better coil heat dissipation and lower temperature rise. While this results in better performance, it also makes manufacturing more difficult and increases costs.
7. A large air gap in an asynchronous motor results in high magnetic reluctance and a high number of excitation ampere-turns, which increases the motor's excitation current and reduces its power factor. However, a large air gap weakens the harmonic magnetic field and reduces additional losses. A small air gap can easily cause stator and rotor rubbing and reduce motor efficiency due to increased additional losses.
8. Rotor skew in asynchronous motors can weaken the phase difference of harmonic potentials along the axial direction of the rotor bars, thereby reducing additional synchronous torque and additional asynchronous torque, reducing additional motor losses, improving efficiency, and reducing noise and vibration.
9. Single-layer windings should not be used for motors with large capacity, and double-layer windings should not be used for motors with small capacity.
10. Large motors are not suitable for using aluminum alloy brackets due to their lower rigidity and strength.
11. Avoid using keyways (i.e., slots) of different widths on the shaft as much as possible. The secondary position of the shaft core should be rounded.
12. When both ends of the motor use rolling bearings, avoid axial jamming. Because the heat dissipation of the shaft is worse than that of the stator support when the motor is running, the temperature rises slightly, and the thermal expansion of the shaft is greater than that of the stator components, support, and base. The shaft cannot expand freely, so corrugated bushings and bearing covers are generally added.
13. The length of the mating part between the shaft and the rotor chip should not be too long. (This can be solved by reducing the size of the middle part of the shaft core.)
14. For motors with rolling bearings, avoid axial movement of the rotor. (Therefore, add a corrugated insert inside the bearing.) For sliding bearings, add a bent insert at the front end of the rotor. Handheld vacuum cleaners without bent inserts have an E-type insert structure at the front end, with very small play (0.1~0.4mm).
15. The stator and rotor cores of the motor should not be misaligned. Misalignment is detrimental because: a) Misalignment reduces the effective area of the air gap, increases the excitation current, lowers the power factor, and also increases stator current, stator copper losses, and efficiency, leading to higher temperatures. b) The rotor experiences an axial force, accelerating bearing wear and increasing motor noise and vibration. (However, some finished products run in one direction, often wearing down one end of the bearing; in this case, misalignment is intentional.) c) It affects the normal ventilation of the motor.
16. Pay attention to the quality of the aluminum casting on the asynchronous motor rotor (this can be checked by dissolving the chip with sulfuric acid).
17. The starting current of a motor is generally about 6 times the rated current, so avoid frequent starting. The stall current is usually 2.5 to 3 times the rated current. The stall current requires 200% of the rated current to trip.
18. The lead wire should not be too thin. If the lead wire is too thin, the insulation will easily age. The conductor current density is usually 4~6A/mm². Use a larger value for small motors and a smaller value for large motors.
19. Friction noise from the commutator and carbon brushes is caused by: a) insufficient surface machining precision and poor roughness of the commutator; b) excessive gap between the carbon brush and brush holder; c) failure of the carbon brush spring; and additionally, debris on the commutator surface can easily generate sparks.
20. The critical speed of the rotor should be greater than 1.2 times or less than 0.8 times the rated speed to avoid resonance.
21. When drying the motor, avoid a rapid increase in temperature. For excessively damp windings, avoid drying by applying current.
22. The stator slot wedge should not be higher than the inner circle of the iron core.
23. When disassembling a bearing, force should be applied to the inner ring of the bearing, and no force should be applied to the outer ring or the ball bearing.
24. In dusty environments, avoid using protective motors; instead, use enclosed motors. (However, enclosed motors do not dissipate heat easily.)
25. Do not operate the motor at excessively low power supply frequencies. With a constant power supply voltage, a decrease in frequency increases magnetic flux. Motor design typically positions the silicon steel sheets in the saturation region of the magnetization curve. Increased magnetic flux leads to a significant increase in excitation current, a decrease in the power factor, increased motor current, increased copper losses, and decreased efficiency. A decrease in frequency also reduces motor speed, reduces airflow, hinders heat dissipation, and increases motor temperature. A decrease in power supply frequency not only reduces motor output power but also degrades various performance characteristics. The rated frequency should not exceed 1%.
26. Do not allow the power supply voltage of the asynchronous motor to be too high or too low. If the voltage is too low, the motor magnetic flux will decrease, and the motor torque will decrease proportionally to the square of the voltage. This will cause difficulty in starting the motor, and when the motor load remains unchanged, the motor current will increase, resulting in greater losses and higher temperature rise. Prolonged low-voltage operation may cause the motor to burn out.
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