Kazunobu Itomi

Small induction motor noise calculation

There has been an increasing demand for a quiet induction motor. Electromagnetic noise is jarring to the ear. Most offensive electromagnetic noise is generated when the natural frequencies of the stator coincide with or are close to the frequencies of the magnetomotive forces. Electromagnetic noise is calculated by BEM program using the electromagnetic forces of Maxwell stress. The natural frequency and behavior of the stator are calculated by mechanical FEM, considering the contact between stator core and frame. Spring elements are introduced between the stator frame and the pressed core. Vibration is calculated by mechanical FEM. Calculated and experimental noise level were within 3 dB(A) at 1,200 Hz and 4,080 Hz.

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 Youngs 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.

Natural Frequencies of Stator Core with Frame in Induction Motor.

The construction and the frame with a fin in a totally enclosed induction motor have almost the same design worldwide. A method for estimating natural frequencies of a stator core pressed into the frame of this type of motor was found by a finite-element method (FEM) analysis. First, the number of circumferential contact points between the inner surface of the frame and the outer surface of the core was determined by experiments and FEM analysis. It was confirmed that the number of circumferential contact points was three. In these analysis, spring elements with radial and circumferential spring constants were set at these contact points. Good agreement in natural frequencies obtained by measurement and by FEM analysis was attained when the number of spring constants in the radial direction was infinite and that in the circumferential direction was finite. FEM analysis with spring elements at contact points is useful in practical applications to obtain the natural frequencics of a stator core with frame.

Experimental Study on Heat Transfer from Ball Bearing to Lubricating Oil. In the Case of Oil Bath Lubrication.

Until today, size reduction of the bearing housing in induction motors has been made based mainly on past experiences. However, size reduction based on these experiences sometimes has induced excessive temperature in the bearing. Therefore, in order to better design the housing, it is necessary to analyze heat flow and temperature distribution around the bearing. One of the factors which largely affects the accuracy of the results of these analyses is the heat transfer coefficient from bearing to lubricant. However, practical papers with respect to its heat transfer have yet to be published. In this paper, the following experimental equation of its heat transfer in the case of oil bath lubrication was obtained through experimental study intended for practical use. When the Prandtl number and Reynolds number are expressed by Pr and Re, the Nusselt number Nu can be expressed as follows : Nu=0.27 Pr1/3Re0.53.