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 From below left: basic diagram of a hybrid test bench with mechanical, electrical and bus data acquisition and real-time connection to an automation system; hybrid test bench with HBM test and measuring equipment at the Technical University Darmstadt’s Institute for Internal Combustion Engines (vkm); and integrated measurement system for electrical and mechanical variables

hybrid drive

power channels inmost cases.Whatmay just about suffice for three-phasemotors and an intermediate circuit becomes a problemwhen there are five- or six-phase motors or other complex systems. One of themost frequently

Measurement of the angle of rotation is

another special case. If the signals of the electricmotor are to be analysed later on, for example, in order to produce fluxmaps orMTPA (maximumtorque per ampere) curves, the position of the rotor is crucial for the necessarymathematical analysis. Electrical powermeasurement is a

completely new field: power analysers are normally used for this purpose, but they present the test bench of a dynamicmotor with an array of problems. Conventional power analysers are optimised for use in the network, or with white goods. The measurement cycles are suitably slow, so that greater accuracies can be achieved through averaging.However, it is precisely these slow data rates that stand in the way of dynamicmeasurement, or rapid progress through a characteristicmap with thousands ofmeasurement points. And in most cases, there is no connection to a fieldbus system. Yet another problemis the restriction of the number of channels to three or four

underestimated problems is that of complete traceability. Power analysers deliver ready calculated results, and are unable to store rawdata. Therefore, comprehensive tracing of themeasurement chain is not possible. Calibrated power analysersmay be used, but they are normally calibratedwith pure sinusoidal signals at 53Hz. But in the hybrid drive, PWMsignals in the range of several kHz have to bemeasured. In such cases, a calibration certificate is unable to demonstrate exactly howa power analyser can performmeasurements. In the aftermath of #dieselgate, this problemis obviously going to require evenmore attention in future. Another often underappreciated detail is

the synchronicity required for an efficiency measurement. If we intend to compare the electrical power input with themechanical power output – to calculate efficiency – these power values need to have been obtained and averaged in precisely the same measurement window. For dynamic measurements, even themost disparate sample rates and input filters influence the electrical andmechanical signals, andmay play a considerable part in possible measurement errors. The storage of raw data provides an

escape fromthe problemof traceability. Here, in addition to themeasured values for power, small extracts of raw data – current and voltage – are stored at a higher resolution. This permits the subsequent, renewed calculation of power values, such as active and reactive power and the mechanical power values, enabling the calculated efficiencymap to be verified. This verifiability extends back to the raw data and the sensors, not just to the power analysers.Of course, power analysers can also store raw data, but they have not been optimised for this task. A new approach is offered by the eDrive

GENDAQmeasurement systemfrom HBM. Controlled fromaWindows computer, thismodular device records all the necessary signals synchronously and simultaneously. To forward the computer results in real time, at the same time as raw data are stored, the EtherCAT bus – virtually standard in the automotive industry – is used. Plug-in cards for current and voltage

signals,mechanical variables such as vibration or pressure, and special satellites for recording bus signals and, as a special feature, temperatures with thermocouples with insulated inputs up to 1,000V. The rotational speed, torque and angle signal can be recorded directly and simultaneously for up to sixmeasuring points. Since temperatures and CANbus signals are also recorded, it is now possible to carry out temperature compensation. EE

December 2016 /// Environmental Engineering /// 49

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