Markku Niemela

Modeling of axial flux permanent-magnet machines

In modeling axial field machines, three-dimensional (3-D) finite-element method (FEM) models are required in accurate computations. However, 3-D FEM analysis is generally too time consuming in industrial use. In order to evaluate the performance of the axial flux machine rapidly, an analytical design program that uses quasi-3-D computation is developed. In this paper the main features of the developed program are illustrated. Results given by the program are verified with two-dimensional and 3-D finite element computations and measurements. According to the results, it is possible to evaluate the performance of the surface-mounted axial flux PM machine with reasonable accuracy via an analytical model using quasi-3-D computation.

Performance comparison between low-speed axial-flux and radial-flux permanent-magnet machines including mechanical constraints

Performance comparison between different machine topologies is not a straightforward task since many variables exist if electromagnetic, thermal and mechanical aspects are taken into account. In this paper some methods to take into account relevant mechanical constraints in the performance comparison are proposed. A comparison study between low-speed axial-flux permanent-magnet machines and radial-flux permanent-magnet machines is provided with introduced mechanical constraints, respectively

Lumped-Parameter Thermal Model for Axial Flux Permanent Magnet Machines

A lumped-parameter thermal model is presented for axial flux permanent magnet (AFPM) machines. The model provides the steady-state thermal solution to derive the temperatures at different parts of the machine, including the temperatures in the stator windings and the temperature of the magnets. Temperature-dependent thermal properties of the materials used in the machine construction as well as the stator winding resistance and consequently copper losses require temperature update in accurate design of the machine. Therefore, an iterative coupled electromagnetic and thermal design program is proposed in this study. A 5-kW AFPM generator is designed using the proposed program with regarding the thermal behavior of the machine. Tests performed on the designed machine verify that the defined thermal resistance network has a high ability to predict the nodal temperatures accurately.

Design Principles of Permanent Magnet Synchronous Machines for Parallel Hybrid or Traction Applications

Hybrid and full electric technologies are fast emerging in vehicles and mobile working machines, where electric machines and internal combustion engines are used together to power the systems. Permanent magnet (PM) technology plays an important role here despite the high magnet prices. This paper theoretically and empirically studies the design principles of PM synchronous machines (PMSMs) for hybrid applications, where a high starting torque and a wide field weakening range are needed. Several embedded-magnet PMSM magnetic circuit topologies are considered as possible candidates. A 10-kW PMSM prototype was built and tested. Experimental results verify the theoretical considerations well.

Estimation of the flux linkage in a direct-torque-controlled drive

An improved integration method is presented for the estimation of the stator flux linkage for speed- and position-sensorless direct-torque-controlled AC machine drives. The method is based on monitoring the scalar product of the estimated stator flux linkage and the measured stator current. The AC part of the scalar product is extracted using filtering and the correction for the estimated stator flux linkage is formed from that part. Adequate performance is obtained by using simple low-pass filtering. By using adaptive filtering in the extraction, the performance of the drive is excellent. Both simulation and laboratory test results are presented, which show that the presented method works well both in steady state and in transients.

Measurements and Simulations of DTC Voltage Source Converter and Induction Motor Losses

Energy efficient pulse-width modulation inverters are widely used to control electrical machines accurately for process needs. The pulse-width modulation, however, has also adverse effects and produces additional losses in the motor. These losses increase the motor temperature and result in derating of the machine power in converter use. A reliable and reasonably accurate loss model of an induction motor drive system is important for the performance prediction of a variable-speed drive. A two-level frequency converter main circuit model is coupled to a finite-element method motor model. The drive model is controlled by closed-loop direct torque control. The frequency converter losses are calculated analytically, and the finite-element method motor model provides an analysis of the motor losses. The simulation results are compared with measurement results.

Limitation of the load angle in a direct-torque-controlled synchronous machine drive

The basic direct torque control (DTC) principle is to rotate the flux linkage forward, if the torque must be increased, and reverse, if the torque must be decreased. The torque is, however, increased only up to a maximum torque of the machine, which corresponds to a load angle of about 90/spl deg/ in a synchronous machine. This paper presents a method to overcome the possible loss of synchronism when using DTC. This requires either that the rotor angle is measured or estimated. Both of these cases are considered. Simulation and laboratory results are presented to show the effectiveness of the presented method.

Identification of grid impedance for purposes of voltage feedback active filtering

A voltage feedback active filter is vulnerable to unknown grid impedance. To overcome this problem we propose an identification method, which uses the control system of a frequency selective active filter to measure the grid impedance at selected frequencies. The usefulness of the method is experimentally demonstrated with a 19 kVA active rectifier with a voltage feedback active filtering function. The voltage feedback active filtering is performed in a case in which the active filter control is not stable before the impedance is identified with the method proposed. It is shown that the use of the measured grid impedance in the control system greatly enhances the dynamic stability of the system. Also, the grid impedance measurements are provided in two cases.

FEM calculation of rotor losses in a medium speed direct torque controlled PM synchronous motor at different load conditions

Summary form only given. In direct torque control (DTC), the switching frequency of the converter is not constant but it varies according to optimal switching condition. For example, when driving a permanent magnet synchronous motor at no load operation, the bandwidth of the switching frequency can be several kHz which cause high distortion of stator currents and, therefore, increase in rotor losses. In this work rotor losses of a medium speed permanent magnet synchronous motor driven by a direct torque controlled electrical drive were calculated using 2D time-stepping finite element method coupled with the motion equation. Circuit equations were used in the model in order to take the effects of the distorted phase currents into account. Calculations were performed at different load conditions.

Guidelines for Designing Concentrated Winding Fractional Slot Permanent Magnet Machines

The paper discusses the guidelines for designing concentrated winding fractional slot permanent magnet machines. The winding factor and different slot and pole combinations are studied. Various low-speed high-torque permanent magnet synchronous motors with concentrated windings and with different slot and pole combinations are calculated analytically and by applying the finite element method (Flux2D¿ by Cedrat). All the motors calculated in this study have the same frame size, air-gap diameter, air-gap length and the same amount of permanent magnet material.