Vibration Technique for Processing and Monitoring Electrical and Mechanical Defects in Electrical Drives Using 2-D Mathematical Model

The radial flux density in the air-gap of rotating machines sets up a force of attraction between the stator and the rotor surfaces. In a symmetrical machine, the radial stresses distributions are balanced resulting in zero net force on the rotor. However, if the rotor of a rotating machine is supported eccentrically with respect to the stator or if rotor short circuits occur, a one-sided magnetic force will be developed which generally tends to increase the eccentricity and increases considerably the critical speed of the machine. The resultant force created by the unbalanced forces of attraction is called unbalanced magnetic pull (ump). Under certain conditions these forces may cause the individual parts of the machine to vibrate and thus develop a noise. The vibrating parts are more stressed and are frequent sources of troubles, they also cause a rapid ageing of the machine. Furthermore, the machine vibrations are transferred to the bases and may, with large machines, cause a vibration of the entire surroundings of the machine. In the following paper a brief outline of the mathematical analysis associated with a technique for monitoring defects in rotating machine whilst the machine is running in normal service is described. This technique is based upon the use of sensors in the air-gap, so arranged that the symmetrical air-gap is eliminated and only the lack of symmetry due predominantly to the missing flux associated with electrical and mechanical failures (eccentricity, increased vibrations, bending of the rotor shaft etc.), are displayed. A small four-pole machine with a modified field winding and bearings is used to examine experimentally both electrical and mechanical anomalies of various magnitude and position.