Olli Pyrhönen

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.

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.

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.

Verification of frequency converter with small DC-link capacitor

Traditionally, large DC-link capacitors have been used in voltage source inverters. By using MPPF-type capacitor the line current harmonic content is improved and the unreliable electrolytic capacitor can be omitted. In this paper, the dynamic behavior of an inverter with small DC-link capacitor has been studied by simulations and laboratory tests. A new modulating principle, named differential space vector pulse width modulation (DSVPWM), can deal with heavily fluctuating DC-link voltage and produce correct voltage vector with high accuracy. The performance of the DSVPWM has been verified with simulations

Magnetic Bearing Spindle Tool Tracking Through

A method is presented for tooltip tracking in active magnetic bearing (AMB) spindle applications. The proposed tool tracking approach uses control of the AMB air gap to achieve the desired tool position. A μ-synthesis-based controller is designed for the AMBs with the goal of robustly minimizing the difference between the tool reference and estimated tool position. In such a way, the model-based control approach concurrently addresses the tracking problem and the inability to directly measure real-time tool position in the presence of machining disturbances. To ensure the tractability of the control problem, a model of the desired tracking dynamics is included in the plant. The method is demonstrated on a high-speed AMB boring spindle. To confirm the tool tracking capability, characteristic part geometries are traced including stepped, tapered, and convex profiles. Tool tracking is demonstrated for the rotating AMB spindle in the range of 90 μm. Also, static and dynamic external loading is applied to the spindle tool location to confirm the disturbance rejection ability of the closed-loop system.

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Overmodulation (OM) is used to increase the voltage output of the PWM controlled frequency converter. Full inverter voltage utilization is important because of the cost and output power improvement perspectives. The voltage source inverter (VSI) with high overmodulation performance is less sensitive to inverter DC-link voltage disturbances. DC-link voltage drop may result in unintentional entrance to the Overmodulation region. DC-link drop is often due to line voltage sag or fault conditions. When small DC-link capacitor is used, voltage drops due to full-wave rectifier bridge are present all the time. A high performance overmodulation method improves the drive performance under such conditions

-Synthesis Robust Control

Many control schemes rely on an analytical model of the servomechanism to be controlled, and hence, accurate knowledge about the position- and time-dependent parameter variability becomes crucial in many contexts, such as robust control methods. Although a properly designed robust controller can cope with a large parameter variation, real-time identification of the system parameter behavior could lead to several advantages by means of monitoring the varying dynamics, e.g., the predetermined uncertainty region around the nominal value. This paper addresses issues in online parameter estimation of a linear tooth belt drive with a limited stroke. Particular attention is paid to detecting the position-dependent changes in the system dynamics by using recursive least squares algorithm and exciting the system in different cart positions in order to identify the varying dynamics. The algorithm used is based on an indirect output-error identification scheme. The experimentally estimated parameters are compared with the corresponding two-mass model parameters. The results show an acceptable agreement and demonstrate the feasibility of the estimation method to estimate the parameters of a closed-loop controlled servomechanism with a limited stroke and time-varying parameters.

Overmodulation in Voltage Source Inverter with Small DC-link Capacitor

Stray inductances cause various problems in a power inverter and should be appraised and minimized at an early design stage to avoid later required countermeasures and redesign after the device prototype has been built. The stray inductances of the commutation loops of an Active Neutral Point Clamped (ANPC) inverter are estimated in this paper. Detailed model of the Insulated Gate Bipolar Transistor (IGBT) module is created to improve the accuracy of the stray inductance calculation. The influence of the coupling between the IGBT modules and between the IGBT modules and the busbars on the commutation loop inductance is considered. The partial inductances of the components of the commutation loops are estimated by AnSYS Q3D parasitic extractor to calculate the total stray inductance. It is found that the commutation loop inductances of the studied ANPC inverter are overestimated by neglecting the couplings between the IGBT modules and between the IGBT modules and the busbars.

Online Estimation of Linear Tooth Belt Drive System Parameters

Synchronous motor drives are well suited for rolling mill applications, where good performance is required both at zero and high speed in field weakening. Direct torque control (DTC) sets high dynamic requirement for the excitation control too. DTC controls only the stator flux linkage and stator current is determined according stator flux linkage. The stator current and drive stability can be, however, controlled by the excitation current. For that reason, excitation current control has an important role in DTC controlled synchronous motor drives, especially at high speeds and high loads, when the drive is working near its stability and current limits.