What is the cause for motor electromagnetic noise?
Effect of performance consistency. Inconsistent residual magnetism: Even though the magnetic properties of individual products are particularly high, it is not acceptable since the inconsistent magnetic flux of each unidirectional magnetic field section will lead to torque asymmetry and vibration. Inconsistent coercive force: Especially if the coercive force of individual products is too low, it is easy to generate reverse demagnetization, causing inconsistent magnetic flux and vibration. This impact is more significant for brushless motors.
What is the correlation between the surface magnetic field of permanent magnets and the residual magnetism?
The surface magnetic field is a product characteristic, while residual magnetism is a material characteristic. They are correlated to each other, just like specific gravity and weight.
The surface magnetic field is related to residual magnetism, shape, and size. The thinner the magnet, the lower the surface magnetic field (can be low to zero); the thicker the magnet, the higher the surface magnetic field.
Demagnetization of permanent magnet motor
For a permanent magnet that operates in an open circuit, its magnetic induction strength is not on point Br in closed-circuit state under the action of demagnetization field, but on a certain point on the demagnetization curve lower than Br, which is known as operating point of permanent magnet. Obviously, the operating point is related to the shape of the demagnetization curve and the size of the demagnetization field of the working magnet. BD and HD at the operating point shall meet the following two formulas:
BD= μ0(HD +MD)
Where, MD is the magnetization intensity of the permanent magnet under the HD action, and N is the demagnetization factor. Substituting HD=-NMD into BD=μ0(HD +MD), we get BD= μ0HD (1-1/N), indicating that the straight slope of the joint working point D and original point O is μ0(1-1/N). This line, i.e. the OD line is known as load line. Its slope is related to the demagnetization factor of the permanent magnet. The slope of the load line is also referred to as magnetic guide, represented by PC: Pc = BD/HD=μ0(1-1/N)
About magnetic saturation.
The capacity of silicon steel sheet to convert electric energy to magnetic energy and then to kinetic energy is limited. When the motor current exceeds a specified range, the electric energy cannot be converted to more magnetic energy, but can only be converted to thermal energy, resulting in rapid temperature rise of the motor, which is a consequence of magnetic density saturation. Like a popular metaphor: Take the silicon steel sheet as a bucket. There is a pipe at the bottom for water outlet and another pipe at the top for water inlet. When the water inlet rate is equal to outlet rate, it reaches flux density saturation. If the the water inlet rate is higher than that of outlet and lasts for a period of time, water will overflow, which is the reason for rapid temperature rise of the motor.