Samuli Kallio

Decoupled Vector Control Scheme for Dual Three-Phase Permanent Magnet Synchronous Machines

Dual three-phase electric machines have many advantages compared with conventional three-phase machines. However, the properties of such machines present challenges for current control that make it difficult to produce sufficient performance for the electric drive. This paper introduces an improved vector control scheme for dual three-phase permanent magnet synchronous machines. The study includes detailed solutions for key parts of the control such as reference frame transformations, decoupling of the current control loops, and modulation. The performance of the suggested control scheme is evaluated using finite-element analyses and experimental results. The results show that the scheme can produce desired dynamics for the current control and guarantees balanced current sharing between the winding sets. In addition, the proposed solution is capable of reducing current harmonics produced by the internal structure of the machine. This problem is, however, only partly solved since complete elimination of harmonic components is not achieved. Nevertheless, the suggested control scheme overcomes many of the disadvantages found with other control solutions. Improved control performance allows the full benefits of dual three-phase drives to be utilized even in demanding applications.

Decoupled d-q Model of Double-Star Interior-Permanent-Magnet Synchronous Machines

This paper describes a d-q transformation to be applied to double-star permanent-magnet synchronous machines with interior magnets. The d-q model consists of two d-q reference frames decoupled with respect to each other and applies to a generic angular displacement between the winding sets. The decoupled d-q model simplifies the analysis of the machine behavior and the drive control implementation. Machine parameters are determined by a sequence of finite-element (FE) analyses. The effectiveness of the proposed modeling method is verified by a comparison with the results related to measurements on a test machine and FE simulations. It was assumed that the mutual inductances between the two winding sets can be represented with two coefficients. Although such an assumption inevitably leads to inaccuracy in the machine model, the simulation results with the proposed d-q model agree remarkably well with the experimental results and FE simulations.

Dual three-phase permanent magnet synchronous machine supplied by two independent voltage source inverters

This paper investigates a dual three-phase permanent magnet synchronous machine supplied by two independent three-phase voltage source inverters (VSIs). Dual three-phase machines have many important advantages compared with their conventional three-phase counterparts. Instead of six-phase converters and special vector controls, it would be a very interesting alternative to supply these machines by two conventional three-phase VSIs since they are readily commercially available. This paper shows that the proposed supply method can be used successfully although it suffers from a decrease in the dynamic performance and an error in the estimation of torque. On the other hand, two independent VSIs do not cause low-frequency current harmonics and guarantee balanced current sharing between the winding sets thus avoiding the two most common problems with dual three-phase machines. Experimental results are provided to verify the conclusions. The results suggest that the simple supply method of two conventional VSIs could be a feasible alternative for many industrial applications.

Online Estimation of Double-Star IPM Machine Parameters Using RLS Algorithm

Accurate and updated knowledge of the electric machine parameters provides benefits in many applications, such as model-based control methods. This paper presents an online parameter estimation method for double-star interior permanent-magnet (IPM) machines supplied with voltage-source inverters (VSI). The estimation method is based on a decoupled D-Q model of the machine and a recursive least-squares (RLS) algorithm. Initial machine parameters are estimated at a standstill. Then, in the rotating operating state, the inductances and the flux linkage produced by the PMs are updated. The estimation in the operating state is performed with a flux-linkage-based approach. The experimental results are compared with the corresponding values obtained from finite-element analyses. The results show an acceptable agreement and demonstrate the feasibility of the estimation scheme to estimate the parameters of double-star PM machines. The parameters are validated by comparing experimental and simulated open-loop responses and steady-state torques.

Current Harmonic Compensation in Dual Three-Phase PMSMs Using a Disturbance Observer

Dual three-phase electric machines are known to offer many important advantages compared with their conventional three-phase counterparts. However, these machines also suffer from a significant disadvantage of easily occurring large stator current harmonics. To solve this problem, this paper introduces a current harmonic elimination method based on a disturbance observer (DOB). The study provides a detailed analysis and design principles for the DOB. The performance of the proposed method is verified by experimental results. The results show that nearly complete elimination (> 99% reduction) of harmonic components is achieved. In addition, it is shown that the method is robust against uncertainties. The method provides a simple yet effective alternative for solving the issue of stator current harmonics in dual three-phase drives, and the results of this paper can be easily applied also to other multiphase machine types.

Finite element based phase-variable model in the analysis of double-star permanent magnet synchronous machines

This paper describes a finite element (FE) based phase-variable model for double-star permanent magnet synchronous machines. In a double-star configuration where two three-phase windings are spatially shifted by 30 electrical degrees, a high amount of the 5th and 7th current harmonics may be excited because of the winding layout. However, a conventional d-q axis model considers only fundamental components, and thus, the machine model may be oversimplified. Improvement in the accuracy can be obtained by introducing an FE-based phase-variable model that includes the higher-order harmonics occurring in the no-load flux linkages and in the self- and mutual inductances. Moreover, when compared with an FE analysis with controlled power electronics, significant reduction in the computation time can be achieved. The effects of the harmonics on the machine behavior are evaluated by simulations, and the accuracy of the proposed model is verified by measurements.

Boost operation of three-phase half-controlled rectifier in wind power system using permanent magnet generator

Different power electronic converters can be applied in a wind energy conversion system (WECS). In this paper, a half-controlled rectifier is evaluated for the purpose. A simple chopper type control is presented as the control algorithm where all the three controllable switches are controlled by the same signal. The half-controlled rectifier can only operate in two quadrants. The main advantages of this kind of a rectifier are its robustness and cost-effectiveness.

Partial Current Harmonic Compensation in Dual Three-Phase PMSMs Considering the Limited Available Voltage

Multiphase electric machines are known to have problems with stator current harmonics. The harmonics can be eliminated by various current harmonic compensation methods. However, using the active harmonic compensation can increase the required output voltage of the inverter supplying the machine. Because the maximum voltage is limited, complete elimination of the current harmonics may not always be possible. For such cases, this paper introduces a strategy for a partial compensation of the current harmonics. The aim of the proposed strategy is to provide a maximum reduction in the magnitude of the harmonics when the available voltage is limited. The strategy is verified by experimental results.

Transforming dynamic system models between two-axis reference frames rotating at different angular frequencies

This paper introduces a general method to transform dynamic system models between two-axis reference frames that are rotating at different angular frequencies. Such a transformation is needed in the analysis and implementation of a control system where the entire system is not presented in the same reference frame. Detailed derivation of the transformation for transfer function matrices is presented. Contrary to the previous solutions, the suggested transformation is not limited by the structure of the transfer function matrix of the system. Application examples illustrate the theory. Theoretical analysis indicates important design limitations especially for multiple frame control systems. Experimental results verify the analysis.

Emulation of PMSG/half-controlled converter combination with digital signal processor

Permanent magnet synchronous generators (PMSG) with full-power converters are becoming more and more popular a choice in wind power conversion systems. This paper presents a real-time emulator for a PMSG drive with a half-controlled bridge. One major advantage of a real-time emulator is that it can be safely used to test the system response in a malfunction of e.g. a speed sensor. The emulator is implemented on a digital signal processor (DSP). Obtained emulation results are verified against measurements with a real PMSG drive system. Results show a good agreement between emulated and measured values.