Mitigation of vibration in large electrical machines

In this study two new technologies are investigated with a view to improving the efficiency and reducing the vibrations of large electrical machines. These machines are used for high powered industrial applications. Individually controlled conductors are part of an active stator project that Converteam Ltd are developing. This involves individual conductors located in each stator slot that can be controlled separately. These replace traditional polyphase windings in order to provide a high level of control over the operation of the machine. A linear magneto-mechanical finite element model has been used here to show that this control method can enable a 44% reduction in the copper loss from the machine compared to a sinusoidal supply whilst maintaining the same operational torque. This method introduces extra Fourier harmonics into the excitation pattern supplied to the machine that are not present in the supply current to traditional polyphase windings. These extra harmonics utilize saliency advantageously to produce torque. However, they also increase the vibration of the stator and may increase hysteresis loss in the iron. The bimorph concept is an idea that is unique to this thesis. This concept involves individually controlled conductors positioned through the root of each stator tooth. On application of a relatively small power input to these conductors a magnetic field is created in the stator which distorts the existing magnetic field. Under certain conditions, the magnetostriction phenomenon causes the teeth to act as a vibration absorber. The effect of this method on the Maxwell forces in the air gap is small. This enables the cancellation of components of vibration of the stator whilst the machine torque is maintained. This is a vibration control method suited to high frequency vibrations where the deformed shape of the stator includes a rocking motion of the teeth and where the resultant stator vibrations lead to tonal noise emission from the machine. This thesis investigates the two technologies mentioned above with a magnetomechanical finite element model and two experimental investigations. The thesis also contains background information relevant to this study including an introduction to electrical machines and power electronics, noise radiation and sources of noise in electrical machines, finite element modelling, vibrations of electrical machines and vibration absorbers