M.A. Vogelsberger

Life-Cycle Monitoring and Voltage-Managing Unit for DC-Link Electrolytic Capacitors in PWM Converters

A novel life-cycle monitoring and voltage-managing device for dc-link electrolytic capacitors in pulsewidth modulation converters is presented. The system performs online identification of the capacitors equivalent series resistance (ESR) in order to detect the life-cycle status and permit preventive maintenance. The ESR detection is based on the capacitors ac losses calculated from voltage/current measurements using a simple low-cost microcontroller. The unit is designed as small printed circuit board located directly at the capacitors screw terminals in order to simplify the required temperature sensing and to minimize wiring effort. The minimized energy consumption allows a power supply taken out of the capacitor to be tested. Besides life-cycle monitoring, the unit further facilitates energy-efficient voltage balancing for capacitors in series arrangements avoiding any power resistors typically used in balancing circuits. Instead, the unit controls the capacitor voltage by influencing its power consumption. Each individual monitoring unit (one per each power capacitor of the converter) transfers the ESR results to the converters main controller and receives control commands via a common optoisolated fieldbus. Alternatively, this data transfer is performed using wireless near-field communication leading to a completely autonomous monitoring unit without any wiring for power supply and data transmission.

Using PWM-Induced Transient Excitation and Advanced Signal Processing for Zero-Speed Sensorless Control of AC Machines

The sensorless control of induction machines, particularly for operation at low speed, has received significant attention in recent years. To realize a field-oriented control of AC machines that is able to work at zero speed, the most commonly used methods are either sensor-based models or transient-signal-excitation methods. The major disadvantage of present signal-injection methods is that they are intrusive to pulsewidth modulation (PWM). An additional switching sequence has to be embedded in the control that will cause a torque and current ripple. In order to overcome these problems, a new flux-estimation algorithm that uses the phase current derivative to extract the flux-position information is presented. In contrast to previously introduced methods, this new approach operates without additional transient excitation of the machine and requires only fundamental-wave excitation using standard PWM or slightly modified PWM. Furthermore, only the current response in the two active states of PWM is used. This makes it possible to use sensorless control for the whole speed range including overmodulation and removes the distortion and parasitic influence of the zero switching states during the estimation of the flux. Experimental results are presented to validate the applicability of the presented approach.

Adaptive Flux model for commissioning of signal injection based zero speed sensorless flux control of induction machines

Speed sensorless control of AC machines at zero speed so far is only possible using signal injection methods. Especially when applied to induction machines spatial saturation leads to a heavy dependence of the control signals on the flux/load level. This dependence has to be identified on a special test stand during a commissioning procedure. To avoid the usage of a speed sensor as well as load dynamometer coupled during the commissioning an adaptive flux model is proposed that delivers an accurate reference flux angle. After the commissioning this adaptive flux model is used in combination with the signal injection method to deliver the spatial flux position.

Autonomous Self Commissioning Method for Speed Sensorless Controlled Induction Machines

Speed sensorless control of ac machines at zero speed so far is only possible using signal injection methods. Especially when applied to induction machines the spatial saturation leads to a dependence of the resulting control signals on the flux/load level. Usually this dependence has to be identified on a special test stand during a commissioning procedure for each type of induction machine. In this paper an autonomous commissioning method based on a neural network approach is proposed that does neither depend on a speed sensor present as a reference nor on a load dynamometer coupled to the machine and guaranteeing constant speed. The training data for the neural network is obtained using only acceleration and deceleration measurements of the uncoupled machine. The reliability of the proposed autonomous commissioning method is proven by measurement results. When comparing the resulting sensorless control performance, the proposed commissioning method reaches the same level of performance as a manual identification using load dynamometer as well as speed sensor.

Comparison of Neural Network Types and Learning Methods for Self Commissioning of Speed Sensorless Controlled Induction Machines

Speed sensorless control of induction machines at zero speed is so far only possible using signal injection methods and exploiting non-fundamental wave effects. When applying such methods the resulting control signal shows a heavy dependence on the machines operating point, i.e., the flux and the load level. To achieve speed sensorless control around zero speed it is thus necessary to identify and eliminate the flux/load dependence. In this paper different neural network approaches are tested with respect to their ability to perform an autonomous self commissioning of the control. The multi layer perceptron (MLP), the functional link neural network (FNL), as well as the time delayed neural network (TDL) are all trained using the backpropagation algorithm. To avoid the necessity of a speed sensor during commissioning, a modified flux observer is applied to deliver the reference values of the training data. A machine with closed rotor slots was chosen for this investigation because this type of machine is considered the most difficult for zero speed sensorless control. The results show that for this specific problem, the MLP shows the best performance followed by the FNL whereas the TDL is only applicable using an extensive amount of training data.

Identification and compensation of inverter dead-time effect on zero speed sensorless control of AC machines based on voltage pulse injection

In this paper the influence of the non-ideal behavior of power electronics, sensors, and signal processing on pulse signal injection based speed sensorless control is addressed and investigated. The speed sensorless control signals are obtained from the transient current response resulting from an excitation of the machine with voltage pulses. The investigation is focused on sensorless control of induction machines however, the results are applicable also for other types of ac machines. By using pulse width modulation in combination with voltage source inverters the inverter interlock dead-time is essential necessary to prevent the short circuit of the dc link. A new approach to identify the interlook dead-time by utilizing the transient voltage pulses and the current response is presented. The disturbing influences and effects of the inverter interlock dead-time on the resulting signal include the deviation of the operating point during the pulse injection, as well as additional resulting harmonics in the control signal. For the specific problem of compensating these effects on the signal injection measurement a new approach is applied in this paper. After identifying the effective dead time the switching commands are adapted in order to clearly reduce the disturbing effects. Further simulation results and additionally measurement results are presented to show the influence on the resulting control signals.

Increasing Accuracy of Winding Insulation State Indicator of Three Phase Inverter-fed Machines using Two Current Sensors only

In modern traction drives the application of monitoring systems is growing to ensure continuous operability. Because of the voltage source inverters (VSI) and the high steep voltage change (dv/dt), increased stress of the winding insulation exist. Stator insulation faults are common reasons for a machine breakdown. Insulation health state can be examined by evaluating the transient reaction to a voltage step excitation. Using the inverter as a source of excitation, it is possible to perform an insulation test by evaluating the resulting transient current sensor signals. The trace of the machines transient current reaction depends on the state of the winding insulation system. If insulation degradation occurs the parameters like the parasitic winding capacitances are changing and influencing the trace of the ringing. Normally, for a three phase AC machine the state of every phase is analyzed with the corresponding current sensor. However, regarding the economic issue, the usage of system resources and additional components is restricted. With the proposed method the evaluation of the stator insulation condition is possible only with two current sensors. The state of the phase without a sensor can be analyzed by a special excitation sequence without significant deterioration of sensitivity compared to the results if a sensor is available. Because the transient reaction of non-excited phases is very small, enhanced signal preprocessing is required to prevent sensitivity losses.

Integration of transient and fundamental wave excitation for zero speed sensorless control of AC machines

To realize a field oriented control of ac machines that is able to work at zero speed the most commonly used methods are either sensor based models or transient signal excitation methods. In this paper a new evaluation algorithm based on phase current derivation to extract the flux position information is presented. This method is working without additional transient excitation of the machine and requires only the fundamental wave excitation using standard PWM or a slightly modified PWM. Only the current response in the active states of the PWM is exploited, to avoid parasitic influences during the zero switching state. Measurement results are presented to validate the applicability of the presented approach.