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TRANSPORT: MEASUREMENT Test bench for


Drive hybridisation means that requirements for test bench equipment have become considerably more stringent. Klaus Lang, Business Development Manager at HBM in Darmstadt, explains how the eDrive test system for inverter-fed electrical machines benefits system integrators by simplifying measurement strategies


F


ormany years, drive test benches in automotive engineering have always had a very similar design. After all, the object under test was always the same: a combustion


engine and possibly a transmission. The focus has beenmostly on optimising test routines, shortening test times and therefore cutting costs. Technicalmodifications were brought about by the increasing number of bus systems in test specimens and the introduction of real-time fieldbuses for automation. But if themore stringent requirements


for hybrid drives are to be satisfied by measurement systems for different tasks, the complexity of the test bench increases dramatically. The eDrive fromHBMoffers turnkey test andmeasuring equipment for even themost complex test benches, all froma single source, including the real-time transfer of data to the automation system. The first group ofmeasurement signals


are rather slow variables – pressure, vibration and temperature, for instance. As


48 /// Environmental Engineering /// December 2016


a rule, these are used to determine the general status of the test bench and test specimen, to ensure the correct conditions for actual testing.However, things are now renderedmore difficult by the need to insulate these inputs – sometimes up to 1,000V – for technical or safety reasons. If the temperature ismeasured directly at an inverter of the auxiliary drive, for example, it would be advisable froma technical safety perspective to insulate these temperature channels. This protects themeasuring equipment and the operators in the event of amalfunction in the inverter. Even if the winding temperature of themotor also needs to bemeasured on a development test bench, the sensormust be insulated, just as whenmeasuring the temperature on the HV battery. After the fundamentalmechanical


variables,mechanical power variables now come into play – first rotational speed and then torque.Here, too, hybrid test benches facemore exacting requirements. For example, the rotational speed of an electric


motor can bemuch higher than that for a combustion engine, and torque fluctuations can also lie in a higher frequency range. This is due to the number of pole pairs of themotor which, together with the magnets, is responsible not just for the rotationalmotion but also for the torque ripple. This is an interfering signal thatmust be


recorded in order to understand its effects on the test specimen, the test bench and, of course, the drive train itself.Here, measurement needs to bemore dynamic than on a pure combustion engine test bench, on which so-called torque peaks are only generated by the combustion process, which is of amuch lower frequency than the torque ripple. Things also getmore complicated when themechanical power needs to bemeasured several times – if the power produced by the combustion engine and the electricmotor have to be analysed separately, for example. Then, two measurement flanges and two rotational speedmeasuring systems have to be used.


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