Electric Vehicle: The need for multiphase measurements and motor evaluations

by Andreas Maushammer, Yokogawa T&M Germany

The mobility of the future is powered by electricity. This is quiet, environmentally friendly and means less dependence on fossil fuels. The continuous development of this technology with the goal of further energy savings as well as to inspire the customers with innovations and the new technology again and again are the challenges of today’s automotive manufacturers. 

Industry Response & New Challenges 

At the International Motor Show (IAA) 2017, Matthias Müller, the CEO of Volkswagen AG, announced the “Roadmap E”, the to its own statements most comprehensive electrification offensive in the automotive industry. More than 80 new electrified models are expected to be delivered to customers by 2025, including nearly 50 pure electric vehicles and 30 plug-in hybrids. By 2030, the company also wants to offer at least 1 e-vehicle from each of the approximately 300  Group models, providing up to € 50 billion in direct investment for this purpose.

The other manufacturers are also preparing comprehensively for the change in the vehicle market. Daimler, for example, has been realigning its internal structures, processes and procedures with the “Leadership 2020” program since 2016, and BMW is also setting the course to electromobility. However, until e-vehicles have replaced the classic drive system, the automotive industry still faces major challenges. New systems, drives and components must be developed to production maturity with ever stricter energy regulations and guidelines. 

The significance of such regulations and guidelines for the environment is undisputed. Nevertheless, in addition to the development of new technology concepts and the need for innovations, manufacturers are faced with a difficult task, today and in the future. To solve this problem, the car manufacturers are testing their new developments on test benches. In order to gain more detailed results, the test stands are usually structured according to task areas such as basic development, pre-series and series development, and are tailored to the respective application, e.g. to engine, inverter, or powertrain testing. 

The results of the measurements are then used on the one hand for the further development of the test items such as motor or converter, on the other hand also for the specification of the vehicle and its components. Overall efficiencies, which reflects the highest efficiency of the vehicle and thus guarantee low consumption and a high range, is therefore one of the most important goals of the manufacturers and for many customers one of the main criteria when making a purchase decision. 

The heart of test benches for e-mobility and its needed properties 

The elementary component of a test stand for e-vehicles is therefore the precision power analyzer. In comparison to an oscilloscope or a ScopeCorder  the power analyzer measures the voltages and currents with high precision and is able to calculate the power and enables the determination of efficiencies in real time. At the end of the development process, the traceable measurement data can also be used to specify the product. With regard to the efficiency designation of the product but also to the ever stricter energy and quality regulations, every company should invest in metrology with the utmost accuracy and thus in the continued existence of the company. This enables engineers to design products that, in addition to being able to measure high levels of efficiency in a traceable way, already comply with the stringent standards of the future. After all, a product can only ever be as good as the measuring technology used. 

Looking more closely at the powertrain of an e-vehicle, one quickly realizes that a power analyzer must offer the test engineer more than just highest precision. He needs a high number of inputs for voltages, currents and mechanical performances to record, e.g.  via a torque sensor on the test shaft the mechanical performance. Only then, can drives and their components be completely measured, and partial and overall efficiency determined. 

In the diagram on the left, a powertrain with two 3-phase electric motors, an inverter and a DC power source (e.g. a battery in the car) is shown as an example. To determine the overall efficiency here, the test engineer needs at least one input module for voltage and current as well as two motor functions (mechanical inputs) to record the torque via external sensors of the two motors. Thus, the efficiency over the entire system (total efficiency = mechanical power / input power) can be determined. 

The overall efficiency gives now information about the efficiency of the powertrain, how can the powertrain with this information be improved? This is precisely why it is crucial for developers to know what is happening within the system and exactly where the losses occur. For this purpose, the two motors must be electrically measured, creating a total need of 7 electrical input modules – 1 module for the DC source (e.g. the battery) and a total of 6 modules for the two 3-phase motors – each with an input for voltage and current. Of course, the aforementioned motor functions (mechanical inputs) are additionally required for the detection of torque, speed and the rotation direction.  

Together with the mechanical performance, the currents and voltages can simultaneously be analyzed and evaluated in real time and the results displayed in a numerically or graphically way. A high sampling rate as well as functions like a FFT- or dual harmonic analysis help the engineers to coordinate the individual components in the best possible way. In addition to the power loss of the inverter and the two motors, the individual efficiency as well as the overall efficiency as well as further technical parameters of the drive train should be able to be calculated with a mathematical function in real time. 

The most important properties of a Power Analyzer. 

In summary, in the future, a large number of input modules for recording voltages and currents will play an important role for multiphase measurements and motor evaluations beside the need for highest accuracy. Attention should also be paid to the variety of motor functions, more precise the number of mechanical inputs available. Because of the intensive research on the drive concepts, it might be interesting to measure four instead of two electric motors (e.g. when driving a four-wheel drive with hub motors) simultaneously in order to determine the overall efficiency. Functions for the exact analysis as well as a suitable software package should then round off the offer.

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