Joachim Holtz

Pulsewidth modulation for electronic power conversion

The efficient and fast control of electric power forms part of the key technologies of modern automated production. It is performed using electronic power converters. The converters transfer energy from a source to a controlled process in a quantized fashion, using semiconductor switches which are turned on and off at fast repetition rates. The algorithms which generate the switching functions-pulsewidth-modulation techniques-are manifold. They range from simple averaging schemes to involved methods of real-time optimization. This paper gives an overview. >

Sensorless control of induction motor drives

Controlled induction motor drives without mechanical speed sensors at the motor shaft have the attractions of low cost and high reliability. To replace the sensor the information on the rotor speed is extracted from measured stator voltages and currents at the motor terminals. Vector-controlled drives require estimating the magnitude and spatial orientation of the fundamental magnetic flux waves in the stator or in the rotor. Open-loop estimators or closed-loop observers are used for this purpose. They differ with respect to accuracy, robustness, and sensitivity against model parameter variations. Dynamic performance and steady-state speed accuracy in the low-speed range can be achieved by exploiting parasitic effects of the machine. The overview in this paper uses signal flow graphs of complex space vector quantities to provide an insightful description of the systems used in sensorless control of induction motors.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Sensorless Control of Induction Machines—With or Without Signal Injection?

Controlled induction motor drives without mechanical speed sensors at the motor shaft have the attractions of low cost and high reliability. To replace the sensor, information on the rotor speed is extracted from measured stator currents and from voltages at motor terminals. Vector-controlled drives require estimating the magnitude and spatial orientation of the fundamental magnetic flux waves in the stator or in the rotor. Open-loop estimators or closed-loop observers are used for this purpose. They differ with respect to accuracy, robustness, and sensitivity against model parameter variations. Dynamic performance and steady-state speed accuracy around zero speed range are achieved by signal injection, exploiting the anisotropic properties of the machine. The overview in this paper uses signal flow graphs of complex space vector quantities to provide an insightful description of the systems used in sensorless control of induction motors.

Sensorless vector control of induction motors at very low speed using a nonlinear inverter model and parameter identification

The performance of vector controlled induction motor drives without speed sensor is generally poor at very low speed. The reasons are offset and drift components in the acquired feedback signals, voltage distortions caused by the nonlinear behavior of the switching converter, and the increased sensitivity against model parameter mismatch. New modeling and identification techniques are proposed to overcome these problems. A pure integrator is employed for stator flux estimation which permits high estimation bandwidth. Compensation of the drift components is done by offset identification. The nonlinear voltage distortions are corrected by a self-adjusting inverter model. A further improvement is a novel method for on-line adaptation of the stator resistance. Experiments demonstrate smooth steady-state operation and high dynamic performance at extreme low speed.

On continuous control of PWM inverters in the overmodulation range including the six-step mode

The power output and the dynamic performance of PWM (pulse width modulated) controlled AC motor drives can be improved by increasing the inverter output voltage through overmodulation. Two different solutions are proposed to increase the output voltage in a continuously controllable fashion up to maximum possible value, which is reached in the six-step mode. The solutions differ in their approaches. A space vector strategy is used for high dynamic performance, high switching frequency drives, while a field-oriented PWM method is used for low switching frequency, high power inverters. Experimental results are presented. >

Drift- and parameter-compensated flux estimator for persistent zero-stator-frequency operation of sensorless-controlled induction motors

The performance of sensorless controlled induction motors is poor at very low speed. The reasons are the limited accuracy of stator voltage acquisition and the presence of offset and drift components in the acquired signals. To overcome these problems, a pure integrator is employed for stator flux estimation. The time-variable DC offset voltage is estimated from the flux drift in a parallel stator model and used to eliminate the offset by feedforward control. Residual high-frequency disturbances are compensated by feedback flux amplitude control. A linearization of the PWM inverter transfer function and an improved stator resistance estimation scheme further enhance the system performance. Experiments demonstrate high dynamic performance of sensorless control at extreme low speed and zero stator frequency.

Pulsewidth modulation-a survey

Pulse-width modulation (PWM) is surveyed with reference to performance criteria, feedforward schemes, and feedback PWM control. It is stressed that the implementation of PWM techniques in the design of AC motor drive systems depends on the machine type, the power level, and the semiconductor devices used in the power converter. It is ultimately performance and cost criteria which determine the choice of a PWM method in a specific application.<<ETX>>

Identification and compensation of torque ripple in high-precision permanent magnet motor drives

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments.

Sensorless speed and position control of synchronous machines using alternating carrier injection

High frequency carrier injection is a promising approach solving high performance sensorless drive demands. Position control at low and zero speed is only possible using anisotropic effects considered in high-frequency models. The usually open loop carrier signal injection is impacted by nonlinear inverter properties like the dead-time effect. This paper discusses the influence of the dead time effect on the carrier signal excitation comparing alternating and revolving injection principles. To overcome disturbing effects an alternating injection procedure is proposed using a predefined injection angle. The approach reduces the effects of the inverter distortion voltages. As a result it is possible to track even small saliencies typical for surface mounted permanent magnet synchronous machines. For processing the high frequency current for position estimation, there is no additional hardware necessary within a standard drives with field oriented control. The paper presents theoretical analysis and experimental results.