F. De Belie

A Sensorless Drive by Applying Test Pulses Without Affecting the Average-Current Samples

The rotor position of a salient-pole permanent-magnet synchronous machine (PMSM) at standstill or rotating at low speed is often estimated by measuring the responses on high-frequency test signals. In some drives, the rotor position is computed by measuring important current ripples that are generated by supplying the PMSM periodically with high-frequency voltage test pulses. Besides these ripples, undesired distortions in the average-current samples have been measured. Simulation results have revealed that these distortions are caused by a test signal, as it produces a nonzero voltage deviation from the steady-state stator voltage. In this paper, a low-speed sensorless strategy is discussed where a strong reduction of the aforementioned distortions is obtained by adapting the test signals to the steady-state stator voltage. The main assumption is that an accurate estimation of the steady-state voltage is made by using the controller output. The computation of the adaptive test signals is done by taking into account the voltage restriction of the dc-bus voltage. Simulation results, as well as experimental measurements, indicate the effectiveness of the adaptive test signals in a sensorless controlled interior PMSM.

Generation of Multisinusoidal Test Signals for the Identification of Synchronous-Machine Parameters by Using a Voltage-Source Inverter

With the standstill frequency-response (SSFR) test, accurate electrical-machine models can be identified. However, it can be a time-consuming method, particularly in case the machine has to be identified at low frequencies. To shorten the required time for identification, in this paper, the response on a broadband signal is measured, resulting in a multisine SSFR test. To generate the broadband signal, a high-power linear amplifier can be applied as a waveform generator. As this signal generator is not commonly available in the field, the application of a voltage-source inverter (VSI) is discussed. The multisine SSFR test with a VSI allows swift evaluation of the influence of frequency, saturation, and cross saturation on the q-and d-axis parameters with a signal generator that is often already available to control the machine. Extensive measurements are performed on several permanent-magnet synchronous machines and the method can be extended to synchronous machines with rotor-field winding. By applying a switching converter instead of a linear amplifier, it can be expected that the identification results are affected by the switching actions. Therefore, multisine SSFR tests with either a VSI or a high-power linear amplifier as well as conventional tests as described in the IEEE standard are performed, and the results are compared.

Effect of Rotor Geometry and Magnetic Saturation in Sensorless Control of PM Synchronous Machines

Sensorless control of permanent-magnet synchronous machines (PMSMs) at low speed and standstill is often based on a difference between q- and d-axis inductances. By determining the inductances, i.e., by evaluating current responses that result from the supply of voltage test vectors, an estimation of the rotor position is obtained. These inductances are dependent on the stator current-because of (cross-)saturation-and on the geometry of the PMSM. Changing inductances strongly affect the accuracy of the rotor position estimation. This paper investigates the influence of geometrical parameters of the rotor on the inductances and on the position estimation. First, for several angles, widths, heights, and radial positions of the buried magnets in the rotor, finite element models (FEMs) calculate the inductances and the torque as a function of the stator current. Second, to study the effect of the variable q- and d-inductances on the position estimator, time-domain simulations are carried out in combination with FEM evaluations. The simulated control is validated on an experimental interior PMSM. The FEM is not needed by the controller in the experiments.

Rotor Geometry Design of Interior PMSMs With and Without Flux Barriers for More Accurate Sensorless Control

At low speed, the rotor-position estimation in sensorless control is often carried out based on the evaluation of the phase-current ripples resulting from the supply of high-frequency voltage test signals. However, the rotor-position estimation is affected by cross-saturation in the machine, resulting in less accurate position estimations at higher loads. As the importance of sensorless control of interior permanent-magnet synchronous machines (IPMSMs) increases, it is useful to design IPMSMs in such a way that they are optimized for accurate sensorless control. The goal of this paper is to determine design aspects in the rotor geometry of an IPMSM to minimize the position estimation error due to cross-saturation. Simulations of a sensorless drive are usually based on a state-space model with constant q- and d-axis inductances and no mutual inductances. In this paper, this technique is improved by calculating the inductance matrix from several finite-element models, which allows the study of the effect of variable q- and d-axis inductances and cross-saturation on the performance of the sensorless control. The rotor design is discussed, for both IPMSMs with and without flux barriers, in order to reduce the estimation error caused by cross-saturation.

High-Frequency Issues Using Rotating Voltage Injections Intended For Position Self-Sensing

The rotor position is required in many control schemes in electrical drives. Replacing position sensors by machine self-sensing estimators increases reliability and reduces cost. Solutions based on tracking magnetic anisotropies through the monitoring of the incremental inductance variations are efficient at low-speed and standstill operations. This inductance can be estimated by measuring the response to the injection of high-frequency signals. In general, however, the selection of the optimal frequency is not addressed thoroughly. In this paper, we propose discrete-time operations based on a rotating voltage injection at frequencies up to one-third of the sampling frequency used by the digital controller. The impact on the rotation drive, the computational requirement, the robustness, and the effect of the resistance on the position estimation are analyzed regarding the signal frequency.

A nonlinear model for synchronous machines to describe high-frequency signal based position estimators

This paper discusses fundamental equations which can be used in high-frequency signal based position estimators for synchronous machines. For this purpose, a small signal dynamic flux model is presented that takes into account the nonlinear magnetic condition and the magnetic interaction between the two orthogonal magnetic axes. The derivation of this model is based on the relationship between flux and coenergy. The model is given in a complex notation and is used to discuss most high-frequency signal based position estimators that have appeared in the literature. By using the finite element method, the coenergy of a given salient-pole synchronous machine is calculated and from it an estimation is made of the parameters in the proposed model. Through experiments it is shown that, by using the relationship between magnetizing current and flux as modelled in this paper, the nonlinear behaviour of the synchronous machine is quite accurately estimated. Furthermore, the new model is compared with a classical model that neglects mutual saturation effects between quadrature and direct axis windings

Losses in Sensorless Controlled Permanent-Magnet Synchronous Machines

The aim of the research is to investigate how much the losses in a permanent-magnet synchronous machine (PMSM) increase by the test pulses that are injected for sensorless control, i.e., to obtain rotor position information without using a position sensor. The current responses to the voltage test pulses in the machine are used as input of a 2-D transient finite element model (FEM) of the PMSM, to compute the field distribution. The magnet losses are calculated by a 3-D FEM of one magnet and a part of the rotor yoke only. To find the iron losses in the SiFe part of the magnetic circuit, quasi-static and dynamic BH-loops were measured at various amplitudes by using an excitation winding and a measurement winding around the stator yoke of the PMSM. For field waveforms recorded in the 2-D FEM, the corresponding induction is calculated by a static Preisach model. The dynamic field is added based on the theory of loss separation. The test pulses cause minor hysteresis loops. Measurements and simulations show that losses due to the test pulses are usually small and have the same order of magnitude in the permanent magnets and the yoke.

Sensorless commutation and speed control of Brushless DC-machine drives based on the back-EMF symmetric threshold-tracking

The operation of Brushless DC permanent-magnet machines requires information of the rotor position to steer the semiconductor switches of the power-supply module which is commonly referred to as Brushless Commutation. Different sensorless techniques have been proposed to estimate the rotor position using current and voltage measurements of the machine. Detection of the back-electromotive force (EMF) zero-crossing moments is one of the methods most used to achieve sensorless control by predicting the commutation moments. Most of the techniques based on this phenomenon have the inherit disadvantage of an indirect detection of commutation moments. This is the result of the commutation moment occurring 30 electrical degrees after the zero-crossing of the induced back-emf in the unexcited phase. Often, the time difference between the zero crossing of the back-emf and the optimal current commutation is assumed constant. This assumption can be valid for steady-state operation, however a varying time difference should be taken into account during transient operation of the BLDC machine. This uncertainty degrades the performance of the drive during transients and higher speed. In this paper a new method is proposed to overcome this problem which improves the performance while keeping the simplicity of the back-emf zero-crossing detection method. The proposed sensorless method operates parameterless in a way it uses none of the brushless dc-machine parameters.

Reducing steady-state current distortions in sensorless control strategies by using adaptive test pulses

The motion states of a salient-pole permanent- magnet synchronous machine (PMSM) at standstill or rotating at low speed are often estimated by measuring the responses on high-frequency test signals. In some drives, the rotor position is computed by measuring the current ripples which are generated by supplying the PMSM periodically with high-frequency voltage test pulses. Besides these ripples, undesired distortions in the steady-state current have been measured. Simulation results have revealed that these distortions are caused by deviations of the steady-state voltage during the test periods. In this paper, a sensorless strategy is discussed where a strong reduction of the aforementioned steady-state distortions is obtained by adapting the test pulses to the steady-state voltage vector. As voltage measurements are often avoided, an estimation of the steady- state voltage is used which is given by the current controller output. The computation of the adaptive test pulses is done taking into account the voltage restriction of the dc-bus voltage. Simulation results as well as experimental measurements indicate the effectiveness of the enhanced test pulses in a sensorless controlled interior PMSM.

Identification of PM synchronous machines in the frequency domain by broadband excitation

Modelling and identification of synchronous machines has attracted much attention during the past decades. Although the standstill frequency response (SSFR) method has become standardized (IEEE Std. 115) it has some disadvantages (e.g. it is a time consuming method). To overcome these, the use of broadband excitation has been proposed in the literature. In this paper the use of broadband excitation for the frequency domain identification of permanent magnet synchronous machines is investigated. The measurement set-up and method are discussed, with special attention to the selection of the applied excitation signals. Three broadband signals suitable for frequency domain parameter identification are discussed and compared in measurements. The obtained frequency domain data can be used in different applications. Examples are given in this paper. Furthermore the extraction of a parametric model from the nonparametric frequency domain data is briefly discussed. The identified parameters can be used to study the influence of saturation or can be applied in the simulation and control of permanent magnet synchronous machines. Measurement results demonstrate that broadband frequency domain identification can be applied to extract the parameters (inductances) of permanent magnet synchronous machines. Moreover, the reproducibility of the method is shown to be good as the variation in the parameters obtained in different measurement sessions is low. Future work on the subject will include a closer investigation of the variations in the obtained parameters, the use of higher order models and the application of more complex estimation algorithms.