Sandro Calligaro

Sensorless Control of IPM Motors in the Low-Speed Range and at Standstill by HF Injection and DFT Processing

In this paper, a sensorless controller for an interior permanent-magnet synchronous motor is presented based on well-known high-frequency signal injection techniques. The issue of the demodulation process is the key point of this paper. A novel approach based on discrete Fourier transform and nonconventional reference frame transformation is presented, allowing a simple and robust noncoherent demodulation, i.e., in which no information about the carrier phase is needed. In the classically adopted coherent approaches, in fact, uncertainty about carrier phase reflects in uncertainty in the demodulated signal amplitude, affecting observer gains and signal-to-noise ratio and definitively providing a degradation of the performance of the estimator. Analytical development of the sensorless algorithm, including the demodulation technique, is provided. A complete investigation by simulation is carried out aiming at showing the performance of the proposed method. Finally, experimental results are presented based on a prototype motor drive for city scooters.

Design Issues and Estimation Errors Analysis of Back-EMF-Based Position and Speed Observer for SPM Synchronous Motors

A back-electromotive force (back-EMF) based sensorless technique for surface-mounted permanent magnet synchronous motor (SPMSM) drive systems is considered in this paper. The model of the observer is developed in the Laplace domain and represents an original approach with respect to state-of-art proposals, normally employing a state-state representation. This allows a more intuitive but equivalent design of the observers gains, based on the standard frequency response, as compared to eigenvalues analysis. Moreover steady-state errors are obtained from a theoretical point-of-view, including the effects of the most common non-idealities affecting the drive system (e.g. offsets) and parameters sensitivity. Full simulation and experimental characterization of the sensor-less drive is provided with reference to a general purpose industrial drive, i.e. both in transient and steady-state conditions and in the whole speed/torque operating range.

Adaptive Flux-Weakening Controller for Interior Permanent Magnet Synchronous Motor Drives

Voltage feedback flux-weakening control scheme for vector-controlled interior permanent magnet synchronous motor drive systems is considered in this paper. The voltage controller is based on the difference between the amplitude of the reference voltage space vector and a proper limit value, related to the feeding inverter limitations, and adopts the phase angle of reference current space vector as the control variable. A novel theoretical analysis of the dynamics of the voltage control loop is carried out by considering nonlinear effects and discrete-time implementation issues as well. The design of the controller can therefore be optimized for each operating condition by an adaptive approach, allowing to define stability properties and to maximize bandwidth of the voltage control loop. Maximization of the dynamical performances provides the main advantage of the proposal, that is, allows a lower voltage (control) margin to be considered with respect to standard approaches, leading to a higher torque and system efficiency and/or a reduced value of the dc bus capacitance. A motor drive system for home appliances is considered as a test bench to prove the effectiveness and importance of the proposal.

Sensorless control for IPMSM using PWM excitation: Analytical developments and implementation issues

Persistent, transient and, very recently, pulse width modulated voltage excitation are employed to track saliency of IPM machines, aiming at estimating rotor position. In this paper current transient response introduced by standard (or slightly modified) PWM excitation is considered and analytical relationship between phase current derivatives, inductance and rotor position is derived. A complete mathematical model is developed in the case of IPM machine, by taking also into account the dependence of the rotor position estimation error on the mutual inductance, which is neglected in the past literature adopting the same sensorless approach. Estimation is performed within a single PWM period, differently from previous approaches. Measurement of current derivatives is obtained from dedicated Rogowski coils, oversampling and real-time processing of the measured values. Acquisition issues due to short application times of voltage vector (e.g. during zero and low speed operations) have been overcome by means of a proper edge-shifting technique on the PWM signals. A motor drive system for fractional power high speed IPM motor is considered as a test bench to prove the effectiveness of the proposal.

Self-Commissioning of Inverter Dead-Time Compensation by Multiple Linear Regression Based on a Physical Model

Dead-time and switch voltage drops represent the most important sources of distortion of the (average) output voltage in pulsewidth modulation inverters. Their effect is a function of the parameters of the drive system and of the operating conditions and is often intolerable in many drives applications, thus requiring a proper compensation strategy. Many techniques are implemented in industrial drives and reported in the literature, even very recently. Differently from standard approaches, the proposed methodology is based on a detailed physical model of the power converter (including output capacitance), described by a small set of parameters. A novel self-commissioning identification procedure is introduced, adopting multiple linear regression. The technique is tested on a commercial drive in comparison with state-of-the-art techniques. In addition, back electromotive force estimation improvements in a permanent-magnet synchronous motor sensorless drive system are shown to provide additional validation of the method.

Modulation techniques for three-phase three-level NPC inverters: A review and a novel solution for switching losses reduction and optimal neutral-point balancing in photovoltaic applications

A number of modulation strategies have been proposed in literature in the particular case of three-phase three-level NPC inverter, each one focusing on the optimization of specific aspects and performance of the converter. Nevertheless a comprehensive analysis of state-of-art techniques and their specific features suitable for a photovoltaic (PV) applications is still missing, such a study being highly desirable. Both carrier-based and space vector modulation techniques are analyzed in this paper, highlighting specific features and limitations, especially related to PV applications. Basic issues are considered and compared among modulation strategies, namely: switching losses, low-frequency oscillations of the neutral-point (NP) voltage, total harmonic distortion (THD) and weighted total harmonic distortion (WTHD) of the phase currents and line voltages, dynamic response of the neutral-point voltage control loop under imbalance conditions and modulation algorithm complexity. A hybrid modulation strategy assisted by an optimal neutral-point voltage controller is then proposed aiming at both the reduction of the switching losses with a limited deterioration of the output voltage/current quality and fast dynamics control of the neutral point voltage. Those features allow the development of reduced dc bus capacitance PV inverters with optimized efficiency and quality of the output waveforms. Simulation and experimental results based on a TMS320F28069 DSP controller and a PV inverter are presented to confirm the effectiveness of the proposal.

Sensorless control of IPM motors in the low-speed range and at stand-still by HF-injection and DFT processing

In this paper a sensorless controller for an IPM synchronous motor is presented, based on well-known high-frequency signal injection techniques. The issue of the demodulation process is the key-point of the paper. A novel approach based on Discrete Fourier Transform (DFT) and non conventional reference frame transformation is presented allowing a simple and robust non-coherent demodulation, i.e. in which no information about the carrier phase is needed. In the classically adopted coherent approaches, in fact, uncertainty about carrier phase reflects in uncertainty in the demodulated signal amplitude, affecting observer gains and signal-to-noise ratio, definitively providing a degradation of the performance of the estimator. Analytical development of the sensorless algorithm including the demodulation technique is provided. A complete investigation by simulation is carried out aiming at showing the performance of the proposed method. Finally, experimental results are presented based on a prototype motor drive for city-scooters.

Stand-Still Self-Identification of Flux Characteristics for Synchronous Reluctance Machines Using Novel Saturation Approximating Function and Multiple Linear Regression

Motor characterization has a fundamental role in dynamics, torque accuracy, and efficiency of vector controlled Synchronous Reluctance Machine (SynRM) drives. Control performances and robustness in the whole speed/torque range, including the flux-weakening region, and in sensorless operation strongly rely on the knowledge of machine flux versus current characteristics. A convenient flux saturation approximating function is proposed in this paper, together with an efficient parameters self-identification procedure. The adopted strategy is very simple and can be performed at stand-still by injecting a proper voltage stimulus (current control is not involved), and does not require any additional hardware (motor can be either connected or disconnected from mechanical load). Nevertheless, an excellent fitting for the flux curves on both axes is obtained, using reasonable memory and computational resources. These features make the technique very suitable to motor self-identification in industrial drives. Experimental results based on a commercial drive and two SynRMs are reported to demonstrate the effectiveness of the proposal. Extensions of the method to the evaluation of the whole flux map (including cross-saturation effects) or to interior permanent-magnet machines is also investigated and verified.

Accurate modeling, compensation and self-commissioning of inverter voltage distortion for high-performance motor drives

Dead-times, power devices voltage drops and total output capacitance represent the most important sources of distortion of the average output voltage in voltage-fed PWM inverters. Their effect is a function of actual parameters of the drive system and of the operating conditions, and is often intolerable in many drives applications, thus requiring a proper compensation strategy. Many techniques are implemented in industrial drives and reported in literature, even very recently. Necessary dead time between the two switches of each leg for sure represents the main cause of distortion and is therefore the most investigated topic in technical literature. Less often the compensation strategy is based on an actual model of the converter due to the increased complexity of the modeling task and consequently due to the difficulties in the compensation approach. The methodology considered in this paper belongs to this last class and is based on a detailed physical model of the power converter. A novel approach is presented to obtain a properly simplified model, thus allowing the derivation of a simple but effective compensation strategy. A novel self-commissioning identification of the compensation model parameters is proposed and its validity and effectiveness is verified on a commercial general purpose drive. Comparison with respect to standard compensation techniques is reported and demonstrates the superior characteristics of the proposal. Finally, dramatic performance improvements of a sensorless drive system are also shown to gain an additional validation of the method.

Design issues and estimation errors analysis of back-EMF based position and speed observer for SPM synchronous motors

A back-electromotive force-based sensorless technique for surface-mounted permanent magnet synchronous motor drives is considered in this paper. The model of the observer is developed in the Laplace domain and represents an original approach with respect to state-of-art proposals, normally employing a state-space representation. This allows a more intuitive but equivalent design of the observers gains, based on the standard frequency response, as compared with eigenvalues analysis. Moreover steady-state errors are obtained from a theoretical point-of-view, including the effects of the most common nonidealities affecting the drive system and parameters sensitivity. Full simulation and experimental characterization of the sensorless drive is provided with reference to a general purpose industrial drive, i.e., both in transient and steady-state and in the most meaningful speed/torque operating conditions.