A Aleksandar Borisavljevic

Overcoming limits of high-speed PM machines

For compact and efficient electromechanical systems often small-volume high-speed machines are desired. Permanent magnet machines are considered as the most suitable machine type for very high-speed applications due to their high power density and low loss in the rotor. The focus of this paper will be different aspects that lie behind speed limitations of permanent magnet machines. The limitations are correlated with basic machine parameters.

Modeling the influence of commutation in voltage source inverters on rotor losses of permanent magnet machines

Rotor losses in inverter-fed permanent magnet (PM) machines depend on stator current waveform and consequently on inverter output voltage. These losses can be very harmful due to the vulnerability of magnets to heat. In this study, the rotor losses in high-speed PM machines supplied by voltage-source PWM inverters were analytically correlated with the inverter switching parameters. An analytical model which shows the dependency of the rotor losses on a frequency modulation ratio was derived. The derived model was verified by a 2D transient finite element model.

Analysis of rotor eddy current losses in slotless high-speed permanent magnet machines

Rotor eddy current losses are one of the main reasons of permanent magnet demagnetization in high-speed permanent magnet machines. In this paper the rotor eddy current losses of high-speed permanent magnet machines with different slotless windings have been analysed. The analysis of the losses was performed using 2D and 3D analytical models. In the study, test machines with different windings and the same torque production capability have been analysed. Presented paper shows the dependency of rotor eddy current losses on sine- and square-wave PWM supply voltages and rotor sleeve properties. Several recommendations for reduction of rotor eddy current losses in high-speed permanent magnet machines are given.

Toroidally-wound permanent magnet machines in high-speed applications

The paper studies potentials and limits of permanent magnet machines with toroidal windings in high speed applications. Three designs in different applications are used as test cases. The analyses based on analytical and FE models illustrate both merits and weaknesses of toroidally-wound machines that have not been addressed in literature. Strong external leakage of the armature field leads to relatively high inductance for a slotless machine and, yet, to susceptibility to losses in the housing. Losses in copper are, arguably, the decisive factor for suitability of this type of machine for a particular application; in particular, the machine is hardly suitable to applications requiring high power density. The paper finally demonstrates the importance of having a flawless winding process in order to avoid excessive core losses.

Sensorless observation of a very-high-speed permanent magnet synchronous machine

For optimally efficient control of a high-speed PM machine, continuous accurate information of the rotor position is necessary. At very high speeds, however, measurement of the rotor angle is difficult and/or expensive. This paper uses a solution of a sensorless observer based on a linear quadratic estimator combined with a phase locked loop. The sensorless observer is thoroughly analyzed and implemented for the use in a vector controller of high-speed permanent magnet synchronous machine for a micro-CHP application. The analysis contains robustness and stability analysis. The influence of delays in the control loop is accounted for in the observer. Measurements performed on a test setup show very good performance of the observer in tracking rotor speed and position. The sensorless vector controller show better output performance than the same controller relying on a Hall-sensor signal.

The influence of the inverter switching frequency on rotor losses in high-speed permanent magnet machines: An experimental study

Harmonic content of the output voltage of pulse width modulated voltage source inverters (PWM VSI) is determined by the switching frequency. On the other hand, rotor losses in high-speed permanent magnet (PM) machines are caused, among other factors, by harmonics in stator currents. These harmonics are determined by the harmonics in the inverter output voltage, and therefore dependent on the switching frequency. In high-speed PM machines, due to the high fundamental frequency, harmonics in the stator currents caused by PWM are located at very high frequencies. Measurement of rotor losses caused by these harmonics in a structure with a conductive retaining sleeve on the rotor which is prone to eddy currents might be very challenging. This paper investigates issues related to this measurement and presents a measurement method which results are compared with results from a 2D analytical model that takes into account eddy currents in the rotor.

Inductance calculation of high-speed slotless permanent magnet machines

Purpose – The purpose of this paper is to give a simple, fast and universal inductance calculation approach of slotless-winding machines and comparison of inductances of toroidal, concentrated and helical-winding machines, since these winding types are widely used among low-power PM machines. Design/methodology/approach – Harmonic modeling approach is applied to model the magnetic field of the windings in order to calculate the synchronous inductances. The method is based on distinction between electromagnetic properties of different regions in the machine where each region is represented by its own governing equation describing the magnetic field. The governing equations are obtained from Maxwell’s equations by introducing vector potential in order to simplify the calculations. Findings – Results of the inductances of toroidal, concentrated and helical-winding slotless PM machines, which have the same torque and dimensions, obtained by the proposed analytical method are in good agreement with 3D FEM, where the relative difference is smaller than 15 percent. However, the calculation time of the analytical method is significantly less than in 3D FEM: seconds vs hours. Additionally, from the results it is concluded that the toroidal-winding machine has the highest inductance and DC resistance values among considered machines. Helical-winding machine has lowest inductance and DC resistance values. Inductance of concentrated-winding machine is between inductance of helical and toroidal windings; however, DC resistance of the concentrated windings is comparable with resistance toroidal windings. Originality/value – In this paper the inductance calculation based on harmonic modeling approach is extended for toroidal and helical-winding machines which makes the method applicable for most of the slotless machine types.

Force and torque calculation methods for airgap windings in permanent magnet machines

Methods for force and torque calculations in permanent magnet (PM) machines with airgap windings are studied. Both analytical and numerical methods are applied to calculate the Lorentz force. The presented methods are applicable to various winding types and allow evaluating the geometry quickly. For a comparison of the methods a rhombic winding has been selected as a test case.