Hans Tischmacher

Bearing currents in converter operation

The basic inter-relationships, which are required to investigate the cross-system subject of “bearing currents in converter operation”, are discussed in this paper. In order to obtain an integrated overview of the situation, it is decisive that the main points of investigation that are discussed are combined and viewed holistically. Results from the parameter test area and their link with the overall situation are shown. A higher-level systematic to simulate electric currents flowing through a bearing is outlined, taking into account the various processes that take place in the bearing. The paper is completed by presenting a new type of bearing current sensor system. A review of the steps still required is provided in the conclusion.

Capacitances and lubricant film thicknesses of motor bearings under different operating conditions

Motor bearings in industrial converter-fed threephase motors are mostly equipped with grease-lubricated roller bearings. It is known that under certain operating conditions bearing currents flow that can damage the bearings. When estimating the danger of damaging bearing currents it is important to look at the thickness and the capacitance of the lubricant film as well as the electrical parameters of the drive system. Calculating the thickness of the lubricant film for grease-lubricated bearings is problematical. This paper discusses how this quantity can be determined based on the bearing capacitance. After a brief introduction into the theory of lubricant film formation, the results of extensive tests are presented regarding the dependency of the bearing capacitance on temperature and speed.

Prediction of motor bearing currents for converter operation

The mechanisms that lead to bearing currents have been known for several years. Empirical results have been collected for a few combinations comprising converter, motor and motor feeder cable. Although there are a series of remedial measures, the basic parameters which favor subsequent bearing damage, especially corrugation, have still not been adequately clarified. After a brief introduction into the theory of motor bearing currents, this document will present two models that allow these currents to be calculated in advance for any drive system. An algorithm is subsequently described which can be used to estimate the potential danger for an induction motor taking into account the temperature and the speed.

Influence of the Inverter's Modulation Technique on the Audible Noise of Electric Motors

Noise emissions of inverter-driven electric motors are highly influenced by additional electromagnetic noise generated by the current harmonics from inverter operation. The amplitude and the frequency of these harmonics depend on the modulation technique applied in the power electronic converter. The purpose of this paper is to highlight the important role that modulation techniques play in noise generation in variable-speed drives and to analyze the different characteristics of current harmonics and generated noise. A good understanding of the available choices and their characteristics, advantages, and limitations is necessary in order to optimize the acoustic design of the “motor-converter” system.

Case Studies of Acoustic Noise Emission From Inverter-Fed Asynchronous Machines

The issue of noise emission from electric drives is becoming increasingly important. Motor manufacturers have to comply with certain standards in order to assure the high competitiveness of their products. At the same time, with todays variable speed drives, which are supplied with nonsinusoidal voltages, the issue of noise reduction has become more complex. This is because the influence of additional factors, compared to machines supplied with sinusoidal voltage, must be considered over a wide speed range. The key to optimizing the machines acoustic behavior is the thorough knowledge of the influence of the different noise sources and the excitation mechanisms over the complete speed range. Apart from the theoretical analysis and the simulation, an experimental investigation is necessary to obtain a better understanding of the previously mentioned factors and to minimize the machines acoustic noise. This paper presents some characteristic case studies of acoustic noise emission in asynchronous machines supplied from voltage source inverters in order to examine the influence of diverse factors on the total noise level.

Sound optimization of a converter‐fed drive system using an acoustic camera in combination with modal analysis

Purpose – Modern variable‐speed drive systems using frequency converters generate additional audible noise as a result of the voltages that are no longer sinusoidal. Using suitable measures, the purpose of this paper is to find an optimum for minimum noise radiation in the complete drive system.Design/methodology/approach – The main areas to concentrate on to achieve an optimum are the source of excitation itself – the frequency converter, as well as the actual source of the noise, the motor. Optimization measures are drawn‐up using soundscaping with subsequent modal analysis of the actual state. The effectiveness of any changes made can be estimated using simulation techniques. This approach is confirmed by subsequently implementing the selected measures and verifying them by performing the appropriate measurements.Findings – The paper takes care of the increasingly important field of converter‐fed drive systems and their special acoustical challenge. It shows a practical way to reduce audible noise comb...

Bearing wear caused by converter-induced bearing currents

The basic interrelationships that are required to investigate the cross-system subject of “bearing currents in converter operation”, are discussed in this paper. In order to obtain an integrated overview of the situation, it is decisive that the main points of the investigation are not discussed individually, but must be considered from an overall point of view. Results from the parameter test area and their link to the overall situation are shown. Higher-level systemology to simulate electric currents flowing through a bearing is outlined, taking into account the various processes that take place in the bearing. The paper is completed by presenting a new type of bearing current sensor system. A review of the steps still required is provided in the conclusion.

A new 7.2kV medium voltage 3-Level-NPC inverter using 6.5kV-IGBTs

The SINAMICS GM150 IGBT, a new 3-level-neutral-point-clamped (3L-NPC) Voltage-Source- Converter (VSC) with series-connected 6.5 kV-IGBTs for industrial Medium Voltage Drives is presented. It covers the output voltage range from 6 kV up to 7.2 kV without the need for a step-up output transformer. The SINAMICS GM150 IGBT is based on the proven basic design of its predecessor SIMOVERT MV while the output voltage and current ratings have been extended. The maximum available output power of the water cooled version with a parallel connection of two inverters is 8.7 MVA at an output voltage of 7.2 kV. For the power section state of the art IGBT modules with AlSiC base plates are used. The dynamic voltage sharing for the IGBTs is done by the gate drive units without any additional passive snubber circuits. For the NPC-diodes small passive snubber circuits are used to ensure the dynamic voltage balancing. The mechanical design, the semiconductor ratings and the available converter performance of the SINAMICS GM150 (IGBT) is described. An additional focus is put on the semiconductor switching characteristics in series-connection as well as on the design and the performance of the sinusoidal output filter which is required in retrofit applications with conventional line motors.

The European Standard EN 50598-2: Efficiency classes of converters and drive systems

A new Standard that defines, for the first time, efficiency classes for converters and electric drive systems will take effect in 2014. This paper focuses on the drive system, which is referred to as Power Drive System (PDS) in the Standard. The paper also addresses the procedure for determining the drive system efficiency classes, referred to as IES classes, as well as the possible loss determination methods - such as measurement, calculation, etc. Finally, the authors provide an example of the losses distribution in a typical PDS and highlight the key factors that should be considered when undertaking a system optimization.

Probability model for discharge activities in bearings of converter-fed electric motors

Voltage source inverters in electrical drive systems are the main sources of possible mechanical degeneration inside the anti-friction bearings of downstream drive train components. In order to find the right countermeasures to avoid these parasitic effects, knowledge of the switching conditions of the converter and the subsequent effects are an important issue. This paper focuses on the converters inherent common-mode voltage and its impact on the mechanical drive train elements. Using an electrical bearing model, a method will be presented and discussed to approximate the probability for discharges in the bearings of converter-fed electric motors.