K.R. Geldhof

Rotor-Position Estimation of Switched Reluctance Motors Based on Damped Voltage Resonance

This paper proposes a method to obtain the rotor position of switched reluctance motors (SRMs) by means of voltage measurements. It is shown that the combination of a motor and a power-electronic converter defines a resonant circuit, comprising the motor phase inductances and the parasitic capacitance of converter switches, power cables, and motor phase windings. For salient machines, in general, the associated resonance frequency of the circuit depends on the rotor position. In the position-estimation method, an initial voltage distribution is imposed over the impedances of the resonant circuit after which the circuit is let to oscillate freely. During this phase of free oscillation, the induced voltage over a phase winding exhibits a damped oscillatory behavior, from which position information can be retrieved. An overview is given of the different possibilities to trigger the voltage resonance. It is shown that the proposed position-estimation method has favorable characteristics such as measurement of large-amplitude voltages, robustness against temperature deviations of motor and power semiconductors, very high update rates for the estimated position, and absence of sound and disturbance torque. Experimental results are given for a sensorless commutation scheme of an SRM under small load.

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

A GENERAL DESCRIPTION OF HIGH-FREQUENCY POSITION ESTIMATORS FOR INTERIOR PERMANENT-MAGNET SYNCHRONOUS MOTORS

This paper discusses fundamental equations used in high-frequency signal based interior permanent-magnet synchronous motor (IPMSM) position estimators. For this purpose, an IPMSM model is presented that takes into account the nonlinear magnetic condition, the magnetic interaction between the two orthogonal magnetic axes and the multiple saliencies. Using the novel equations, some recently proposed motion-state estimators are described. Simulation results reveal the position estimation error caused by estimators that neglect the presence of multiple saliencies or that consider the magnetizing current in the d-axis only.

Influence of flux penetration on inductance and rotor position estimation accuracy of switched reluctance machines

In one class of position-sensorless control strategies for switched reluctance motors, the position estimation relies on the measured inductance of an active or idle motor phase. The inductance is calculated from the response of the current to a voltage excitation, e.g. pulse-width modulation in an active phase or injection of high-frequency voltage pulses in an idle phase. With the calculated inductance or flux linkage, the rotor position is retrieved by means of a look-up table or an analytical formula. It is shown in this paper that the measured inductance can significantly deviate from the static inductance when flux penetration in the magnetic core becomes important. This is especially the case when the switched reluctance motor is in a position near alignment and when high-frequency voltage pulses are applied. An analytical wide-frequency model for the motor phase impedance in the aligned position is proposed and calculated results are compared to measured data. The influence of excitation frequency and duty ratio, magnetic core properties and motor geometry on the accuracy of phase inductance and rotor position estimation is discussed.

A discrete‐time model including cross‐saturation for surface permanent‐magnet synchronous machines

Purpose – To provide a discrete‐time nonlinear model for surface permanent‐magnet synchronous machines (SPMSMs) in order to discuss the stability of such machines.Design/methodology/approach – Through differencing the co‐energy, obtained from a finite element method, the main flux path can be described by a complex reluctance. Furthermore, for a SPMSM, an equivalent circuit is presented that includes the eddy‐current losses and the voltage drops across stator resistance and leakage inductance. The model is transformed to a discrete‐time state‐space model by using a forward rectangular rule. By using a root locus technique, the stability of the new model is discussed.Findings – From the calculated root locus it is concluded that the stability of a SPMSM is only guaranteed for certain values of the open loop gain. Moreover, by using the forward rectangular rule, it is concluded that a well‐considered time step has to be chosen.Research limitations/implications – The model considers the fundamental space har...

A Space Vector Strategy for Smooth Torque Control of Switched Reluctance Machines

A new space vector torque control strategy is developed for switched reluctance machines. The method is based on an existing space vector control strategy, which yields an explicit expression for the phase currents realizing a desired torque at a given rotor position. With the improved strategy, a higher torque-current ratio is achieved. A new approach based on space vector trajectories is given to determine the commutation between phases. For a constant reference torque, the improved method guarantees continuous reference currents. Therefore, smooth torque control is possible for a wide speed range and servo-grade performance can be achieved.

A Simulink state-space model of induction machines including magnetizing-flux saturation

Nowadays numerous techniques to model the behaviour of electrical drives are discussed in the literature. In many cases a linear magnetic network is proposed. This paper proposes a large-signal simulation model of an induction machine that takes into account the magnetizing-flux saturation. The model is given as a state-space model with nonlinear feedback and is implemented in Matlab/Simulinkreg. The advantages of the proposed model include versatility and ease of use. Simulation results from both this model and a linear flux model show the differences between the models. Advantages and disadvantages of the proposed simulation model are discussed and the area of application is defined. The usefulness of the model to study large transients and the dynamic behaviour of induction machines in control loops is discussed. For this purpose the proposed model is used to simulate induction machines in drives, such as direct torque control and indirect field orientation

Modelling air-gap flux harmonic components to describe motion-state estimators for PMSMs including magnetic saturation and multiple-pole spatial saliencies

In this paper a nonlinear model for AC machines is proposed that takes into account the magnetic saturation and magnetic interaction between both orthogonal magnetic axes. Furthermore, the model includes the presence of air-gap flux harmonic components caused by magnetic saturation and multiple-pole spatial saliencies. Such a model is used to describe in a uniform way motion-state estimators for AC drives that are recently discussed in the literature. Emphasis is given to salient-pole synchronous machines with a rotor field winding or a permanent-magnet excitation (PMSM)

Mitigation of cross-saturation effects in resonance-based sensorless switched reluctance drives

The stator and rotor yoke in a switched reluctance motor form magnetic circuit parts that are typically shared by different phases. If these parts saturate due to the excitation of one phase, this will lead to a change of the magnetic characteristics of all other phases sharing these parts. In several position-sensorless methods, cross-saturation leads to a load-dependent position estimation error. In this paper, the influence of cross-saturation on a resonance-based position estimation method is studied. The method extracts position information from electrical resonances triggered in an idle motor phase. A cross-saturation mitigation scheme is presented in order to reduce the commutation position error. The scheme uses only one additional parameter per phase which can be measured automatically during commissioning of the drive. Experimental results at low and medium speed show that the position estimation error remains smaller dan 2 mechanical degrees over the rated load range.

Influence of electrical eigenfrequencies on damped voltage resonance based sensorless control of switched reluctance drives

In switched reluctance motor drives, the combination of power-electronic converter and a motor phase defines a resonant circuit, comprised by the motor phase inductance and the parasitic capacitance of converter switches, power cables and motor phase winding. If a motor phase is excited by applying very short voltage pulses, the resonance frequency of the circuit can be observed through the subsequent damped oscillation of the induced voltage in the phase. As the phase inductance and associated resonance frequency depend on the rotor position, the method provides a means for estimating the rotor position. This paper discusses the influence of the magnetic inductive coupling between motor phases on the observed damped voltage resonance. It is shown that the motor-converter combination can be modelled as a system comprising different resonant circuits, each associated with one phase of the machine, which are mutually coupled due to the inductive coupling between the motor phases. An eigenvalue analysis reveals the different eigenfrequencies and modes of oscillation for this system. It follows from the analysis that damped voltage resonances occur in all phases of the machine due to the mutual coupling. The model is used to determine the influence of voltage pulses, applied to a single phase or simultaneously applied to different phases, on the observed damped voltage oscillations, and thus on the rotor position estimation.