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"Electronic replacement motor" (EC motor) and "stepping motor" belongs to the permanent magnet synchronous motor, EC motor is usually designed for continuous drive task, stepping motor design to perform as little as possible deviation of the rotation Angle, and to keep the specified step position, they should have the largest possible to keep torque. This requires that the size of the magnetic circuit should be able to produce high groove torque (no current torque in phase) and the groove torque overlaps with the retention torque (current flows into the phase). Thus, torque fluctuations and torque mutations are expected, like in EC motors, where sinusoidal currents are not used for stepping motors and in microstepping where the phase currents can be opened and closed in small increments in order to be in both holding positions, often used.
Stepping motors do not require any permanent magnet, so they do not have any slot torque, and these motors ("switched resistance motors") use very low airgaps with a gap and a width of 50 μ m is typical, which leads to extremely high radial forces, resulting in severe deformation of the stator and high noise. In the electronic commutation motor, the current applied to each motor phase is disconnected and connected through the electronic circuit. According to the mode of these phase current over time, the radial force and tangential force are used for the stator tooth and the force for the conductor in the motor phase, the latter case is especially occurred on the slot motor with an air gap winding. When the rectangular current mode changes over time, local rectangular forces are also generated over time, thus causing a sudden oscillatory excitation at the stator, which stimulates the oscillations and their harmonics at the switching frequency (fundamental oscillation) of the current phase.
Under the ideal rectangular current and the ideal symmetric motor, the torque remains constant over time, and if there is geometric asymmetry, phase inductance and resistance difference and electronic circuit difference in reality, then the current time interval or overlap will lead to each phase, which will lead to torque fluctuation and stator vibration excitation. The resulting noise is often called "commutation noise", its importance can not be underestimated! Using an electronic control circuit that works with a sinusoidal current mode causes reduced motor efficiency, but it does eliminate or at least substantially reduce this type of noise.
The mechanical vibration of small motor sliding bearing is the result of intermittent mechanical contact between the shaft and the bearing surface. When the lubricating oil film carries the load around the shaft and the bearing, there is no such contact and any noise. However, at startup, when the radial force on the bearing is too large (belt drive, gear, air gap field), when the shaft and / or bearing sleeve are not round or bent, if there is no enough porosity on the sintered bearing surface, if the shaft running surface is too smooth, or if there is not enough lubricant in the bearing (mixed friction). The result is the vibration at the peak roughness of the bearing surface, which depends on the elasticity of the shaft or bearing, at many frequencies in the audible range (frequency bands in the spectrum). The frequency of the rotation and its multiple are particularly pronounced. The amplitude of high-frequency vibrations is often simply modulated, making frictional noise particularly a nuisance. As the lack of lubricating oil increases, mechanical contact increases, causing mechanical friction, and wear increases sharply, which happens