Design, Modelling and Control of Electrical Machines

Abstract – This thesis consists of two parts, the first dealing with the design of iron-powder synchronous machines, and the second with the analysis and prediction of the acoustic noise in electrical machines.

In Part I, a 1.6 kW electrically magnetized claw-pole machine with magnetically conducting end-plates has been analyzed and a prototype tested. The machine is built from soft magnetic composite material (SMC), also known as iron-powder. The magnetic isotropy of SMC gives enormous flexibility in electrical machine design, enabling new topologies exploiting three dimensional flux paths. This is the main advantage compared to conventional machines using laminations, where the flux is constrained into two dimensions. The novelty of the machine presented lies in that the slip-rings in the rotor are no longer needed, since the field coils are removed from the rotor and placed in magnetically conducting end-plates attached to both sides of the stator. This also improves the cooling capability of the copper losses from the field winding, allowing an increased electric loading. The rotor is of the claw-pole type, and the end-plates close the magnetic circuit between the stator and the rotor. The machine has been optimized using a magnetic equivalent circuit model allowing rotation, where non-linearities have been included using an iterative approach based on the linearisation of the BH curve. The traditional leakage paths in claw-pole machines are modified because of the magnetically conducting end-plates, and alternatives are proposed to reduce them. The machine has also been compared to two alternative topologies with electrical magnetization and another with permanent magnets. The comparison has been carried out for a similar temperature rise in the windings, and thermal models have been developed for every machine to determine their maximum electric loading. The rotational and alternating components of the iron losses are calculated using the finite element method (FEM). The results from the measurements indicate that the average torque is 14% lower than predicted. This is probably due to a leakage path between the end-plates through the shaft, which carries the homopolar flux, and that was not considered in the predictions. The concept of series magnetization has also been tested. This consists of feeding the field winding directly from the rectified three phase armature currents at the neutral point, therefore eliminating the need for the d.c. power source.

Electrical machines are finding application in new environments where lower noise levels are demanded. The main focus of Part II is the measurement and prediction of the noise emissions from induction motors using the vector control technique as well as the analysis of some structural changes to reduce these emissions. A digital drive system has been developed for a 2.2 kW induction motor, and its dynamic capabilities demonstrated for a wide range of the frequency spectrum. This tool has been used for the experimental evaluation of the noise emissions when the flux and/or the torque are modulated with high frequency noise signals. The results showed that the noise emissions were higher when the flux was modulated compared to the torque, although the differences were considerably reduced when the machine was loaded. It was also observed that the noise emissions were decreased importantly at load. Sound pressure and sound intensity measurements have been conducted with the rotor stationary and rotating at low speed, showing that the most proper way to quantify the noise emissions from electric machinery is to measure the sound power. A method for the prediction of the noise emissions has been proposed, based on the interactive use of commercial packages for mechanical, electromagnetic and acoustic analyses based in the finite and boundary element methods. The results show that the accuracy of the noise prediction depends on the proper calculation of the modes of vibration in the structural analysis, as well as a suitable selection of the material damping. Skewing also needs to be modelled in order to account for the rotor harmonics. A study has been conducted to assess the effectiveness of introducing peripheral air gap layers around the stator core to reduce the noise emissions. It was observed that the acoustic behaviour was not improved, since the reduction of the stiffness in the outer part of the core actually increased its sensitivity to the vibrations.

The full paper is available at IEA within Lund University – this will open in a new window.