Effect of Coils on Natural Frequencies of Stator Cores in Small Induction Motors

The electromagnetic noise of a motor is offensive to the ear. Most electromagnetic acoustic noises are generated when natural frequencies of a stator core coincide with or closely parallel frequencies of the magnetomotive forces. Therefore, to minimize such noise, an accurate estimate on natural frequencies of the stator is necessary. In this paper, the stator of a small induction motor is studied as to various factors such as the stator core shape, including a circumferential cut, and stiffness of varnished random winding in stator slots. Furthermore, the effect of coil ends on natural frequencies of the stator core is newly clarified. As a result, in the 2.2kW motor, an equivalent Young's modulus as stiffness of windings in the slots is obtained as being about 1/100th that of copper. Also, study clarifies that coupling vibration arises from the coil end and the stator core. It is found that this coupling vibration can be estimated briefly from a two free degree system.