Machine-internal stator winding overvoltages are based on resonance effects caused by interaction of the stepwise voltage changes due to the inverter switching actions and the winding properties, and the parasitic capacitances between stator windings and machine housing. These overvoltages are typically of highest amplitude at the star point of the stator windings inside the machine, and can result in severe insulation damage and failure of the machine. In this paper we propose a simple equivalent circuit model to describe the effect, and provide equations to calculate the maximum insulation stress analytically. We verify the model with measurements on a 1.5MW traction motor, and discuss a circuit simulation of a full drive system which implements the proposed model. The main advantage of the proposed model is its simplicity combined with good accuracy.

Sensorless control offers many advantages for the operation of electrical machines. Being able to operate the machine via field-oriented control without relying on the encoder greatly increases the reliability of the drive. In the low to zero speed region, saliency based control strategies are well suited. For several standard industrial motor designs, induction machine can exhibit multiple inherent spatial saliencies linked to position. For such multi-salient induction machines, the intermodulation saliency has shown good characteristics for sensorless control. Transient excitation can be used to extract said intermodulation saliency. However, it requires intervention in the current controller pulse-width-modulation sequence and greatly oversampled current. Alternatively, rotating high frequency injection can be used to extract the intermodulation saliency, which offers simpler implementation possibilities. The rotating voltage space vector can simply be superimposed onto the current controller output. Additionally the current does not need to be oversampled. However, since the resulting high frequency current is not controlled, it is distorted by the inverter nonlinearities. Compensating the unwanted saliencies and inverter-related harmonics, the intermodulation saliency will be used in this paper to extract the rotor positon of multi-salient induction motors from the high frequency current. The experimental position estimation quality is tested at different stationary loads and transient load steps. Applicability is verified on a traction machine designed for railway application.

In electric rail transportation, traction motors are responsible for propulsion and transmit torque to the bogie wheelsets through a gear drive system. Typically, each traction motor is fixed to the bogie frame using a suspension system and thus motors might withstand a large amount of shock and vibration arising from wheel to track contact. With developing fatigue, the motor-bogie suspension might fail. To prevent destructive contact between motor and track, bogies are often equipped with an additional safety apparatus: a safety bar fixed to the bogie frame that is axially aligned with a motor-fixed safety catch when suspension is healthy. This is the last protective measure that prevents the motor from falling to the track and, thus, it is of tremendous importance to reliably and quickly determine suspension faults. While the motor-bogie suspension is broken, the motor can move up or down within the safety catch. This results in small but abrupt angular movements of the rotor after torque transients take place. Thus, detection of motor-bogie suspension fault can be detected by inspection of the shaft encoder signal or speed sensor signal under load changes. In this work, a novel detection method for motor-bogie suspension fault is presented, using spatial saliency information extraction thus only requiring current transducers and no traditional shaft encoder nor additional sensors. The transient spectra of the motor saliencies are used to define a trigger that serves to detect motor suspension failure during a test. The method is experimentally validated in a test bench equipped with a 1.6MW locomotive traction motor and a suspension-fault emulation system.

Due to the fast slew rates of modern power switching devices, inverter-fed motors experience intensified insulation stress. In addition, inverter switching with fast slew rates excites several high frequencies including multiple antiresonance frequencies of the winding and supply cable, further suffocating winding insulation at localized winding spots. As a result of the wide range of excited frequencies, high frequency currents and voltages can be used to track insulation state. In particular, the voltage at the star point of inverter-fed motors can show the degree of motor ground-wall insulation aging. However, accessing star point might not be feasible in some applications. In this paper, a special switching scheme is proposed, turning the star neutral voltage visible at the terminal voltage without the need for additional equipment or disassembling. Via solely sensing the terminal voltage, ground-wall insulation will be monitored. In addition, the proposed switching scheme elicits additional motor resonance modes, which extends the variety of current and voltage analysis for insulation condition monitoring. Experimental validation using an inverter-fed 2MW medium-voltage traction induction motor is provided.

The extraction of motor spatial saliencies permits stable control at standstill and nearby speeds. One technique to extract saliencies is to excite the motor with voltage steps and process the resulting phase current slopes to form a saliency vector. Yet, the additional voltage steps produce unwanted current ripple and increase audible noise. Therefore, voltage steps have been recently integrated into the space vector PWM symmetrical switching scheme, defined at each period by two adjacent active states and two inactive states. In this paper, only the longest active state is used for saliency extraction. The novelty of this paper lies in the application of the one-active SVPWM excitation to a parallel dual motor configuration supplied by a single inverter and using a reduced current sensor configuration. This configuration consists of only three current sensors, two sensors attached to two phases of one motor, and a single sensor attached to the second motor. A novel signal processing scheme is presented in this paper to extract the saliencies of each motor individually under one-active SVPWM excitation. Experimental results demonstrate the ability of the proposed technique for dual motor saliency extraction under one-active space vector PWM excitation and using the special current sensor configuration.

The development of new technologies in the field of power electronics implies higher switching frequencies. Such frequencies are associated with higher transient voltages. Higher voltages mean stress on the insulation material. As result of this increased load, partial discharge (PD) probability can be increased, which may push the insulation material to its limits. Therefore, it is of high importance to recognize PDs. It is already known that PDs occur not only on the rising edge, but also in the steady state of a voltage pulse. However, double PDs may occur, too. Using a special test arrangement, it is possible to detect also weak PD activity, to localize the time instant and to determine changes in voltage and current. These changes are characterized e.g. by a sudden drop in pulse voltage.

The investigation presented in this paper aimed to analyze the temperature dependences of induction machine parameters. More specifically, the focus was placed on the temperature behavior of magnetic properties. The machine under test is a 100kW traction motor used in standard railway applications. Knowledge of parameters temperature dependences is necessary to ensure precise operation of the induction machine over a wide temperature range. Therefore, measurements for different stator and rotor temperatures of the machine were conducted, which revealed that the permeability of the stator and rotor side electrical steel sheet show a clear temperature dependence. Moreover, a reduction of flux density due to increasing temperature was determined. The purpose of these results is to improve the accuracy of stator or rotor equation-based flux models.

The basic spatial modulation effects of induction machines caused by slotting and saturation, have been analyzed for exploitation in speed sensorless control. With increasing load level, additional spatial harmonics arise, acting as disturbance and deteriorating sensorless control performance. In view of the fact, that induction machines for application in traction systems (e.g. public or cargo transportation) are operated at a high level of excitation, higher harmonic numbers especially of the spatial saturation become dominant and need to be compensated in order to establish speed sensorless operation. It can be observed, that especially the first order harmonic of spatial saturation shows a significant rise in magnitude when load current is increased. The presented paper thus focus on the investigation of high order spatial modulations and their deterministic properties with the goal to establish an efficient compensation of their disturbing effects. Based on the identified deterministic properties a compensation method is proposed and tested through measurements on a 15kW induction machine.

In this paper a method to consider saturation behavior of induction machines during the finite element (FE) analysis of high frequency excitations is developed. This saturation behavior manifests itself in an additional saturation and intermodulation harmonic, when evaluating the current response related to a high frequency voltage excitation. In order to correctly reproduce these harmonics within FE-analysis, the fundamental wave as well as high frequency magnetic properties of the lamination material must be represented in correct proportion. To achieve this, relative permeability is adjusted to create asymmetries in the high frequency magnetic properties. An adaptation method for the transient permeability based on the fundamental wave flux level was developed. Comparison of measurement and FE simulation shows good agreement of all main high frequency modulations present in induction machines.

Condition monitoring of inverter-fed drives is gaining a more important role. Inverter operation enables maximum utilization of a drive. However, some parasitic phenomena may occur, e.g. bearing currents, temperature increase due to additional losses and stress caused by fast voltage rise times. With focus in this work on stator winding insulation an online monitoring method is presented to detect changes in the insulation strength. The insulation health state is assessed at a thermal aged machine by voltage step excitation of the inverter. A change of the electrical strength is evaluated with the transient current response measured for each phase with the built-in sensors of the inverter. The proposed method is able to detect insulation deterioration before an actual short circuit occurs. The results are compared with the insulation condition parameters resulting from additional off-line low-voltage and high-voltage measurements.

Stator insulation defects in the machine winding of traction drives can be analyzed by evaluating the current response resulting to inverter induced voltage injection. Due to effects occurring during the aging of the insulation, e.g. change of parasitic electrical components, deviations of the high frequency components of the current response can be monitored. The insulation degradation of a stator winding system is a long developing process and usually takes place over a long period. With the proposed method stator insulation degradation can be detected already at an incipient stage by comparing the high frequency transient current response of the drive system from the first initial startup with continuous measurements during operation. No additional current transducers are necessary than the transducers already applied for the control of the machine. Investigations are conducted with a 1.4 MW induction machine designed for a traction application having form-wound coils equipped with fiber-insulated winding taps and a silicon carbide voltage source inverter enabling high dv/dt rates of up to 25kV/μs. Additionally, in order to investigate insulation degradation of the stator winding system, results of measurements of a form wound coil specimen are presented.

In the electrical machines, operated in railway traction drives, the growth of power density requires overproportional more heat to be discharged which leads to the need for more and more advanced cooling techniques. A novel optimization and evaluation approach for internal air cooled traction machines will be presented in this paper. It analyses and investigates the thermo-aero- and thermodynamic cooling performance combined with the electromagnetic performance. As a first step, analytical models are used to speed up calculation time for obtaining pressure drop, volume flow distribution and heat transfer as well as iron stack temperature. The results are validated using CFD (computational fluid dynamics simulations) of several iron stack cooling duct geometries. Finally, an approach is presented about on the optimization can be implemented, by coupling the analytic thermal model, CFD tool and electromagnetic losses obtained by finite element analysis.

In this work a method to detect insulation degradation of an adjustable speed drive (ASD) based on analyses of the transient current reaction caused by inverter switching is proposed. The drive system consists of an inverter fed three phase induction machine with 1.4MW. The standard current sensors are used to analyze the transient signal ringing to extract an insulation state indicator of the winding system and for the control of the machine as well. Using the inverter as a source of excitation, it is possible to perform an insulation test by evaluation of the resulting transient current sensor signals. The influences of the fundamental wave portion and transient component of the current signal on the insulation state indicator are analyzed. An effective separation of these components is investigated with an algorithm applicable for real time evaluation of the insulation state.

The fields of application of adjustable speed drives fed by voltage source inverters is constantly increasing. For better exploitation of the system's capabilities all drive components are more and more operated near and even above their rated values. This leads to more strains for the drive. However, at the same time the demands for reliability are also increasing. To reach this requirement different strategies like fault tolerant design, fault detection and condition monitoring can be implemented. However, to implement such strategies a deep understanding of the effects that lead to problems or faults in the drive system is necessary. The fast switching of modern voltage source inverters leads to transient overvoltage stressing the machine's insulation system. A short literature study presenting the characteristic of and the parameters influencing the non-linear voltage distribution in the stator winding of inverter-fed machines will be presented. Furthermore the findings will be compared with experimental results on an induction machine with tapped windings.