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KiTorq sensor proves its worth on electric motor test standRegenics is the Formula Student Electric team at the Regensburg University of Applied Sciences. Since 2010 the student team has used its electric vehicle to take part in Formula Student Electric Germany, a competition for young engineers. Most recently, the 50 students went wih their vehicle to the Hockenheimring racing track for a test run.
Josef Engl, a student at Regensburg and the team leader for electric drive trains at Regenics, is in charge of the race car's electric motors. In his development work, he uses a test stand which is equipped with the KiTorq torque measuring flange from Kistler with a measuring range of 500Nm. "Thanks to the design of the torque flange along with its very short construction, we were able to make do without a bearing system between the two motors on our test stand," he says. "Since we could bold the two motors close together on the flange using the coupling, we were able to use smaller dimensions for the shafts with a correspondingly lower weight."
Since the CoMo Torque evaluation unit is mobile, the unit that had already been available at the university for some time could also be used for the measurement with the KiTorq. The stator's sensor outputs were connected to a power analyser with additional analogue and digital inputs in order to read out the speed and torque and display it with a separate LabVIEW interface. This online tool helps to configure the measuring flange and also to store the torque alarm limits and various measuring ranges.
In this manner, it was possible to verify the actual speed using the measuring flange. "Otherwise, we were only able to measure the speed via CAN with the power electronics," reports Engl. "Howver, it is a very important control measuring instrument when setting up the test stand. Measurement of the torque on the shaft using the measuring flange replaces the purely computational determination of the torque. Based on the torque and speed, we can automatically compute the power too."
The motor torque constant (k = torque t/phase current I) actually acts as a constant over a wide range. "However, a small non-linearity does creep in, especially for larger values of the torque," observes Engl. Using the measuring flange, he was able to determine when the current no longer behaves directly proportionally to the torque. Then he could create a characteristic and store it in the power electronics. By measuring the delay between the specified torque value (via CAN to the power electronics) until reaching the torque value on the shaft, it was possible to test various parameters in the power electronics and obtain ideal settings.
Based on the comparison between the electric power and the measured torque (conversion to power using the speed) on the mechanical shaft, it was possible to determine the efficiency of the motor, the inverter and ultimately the overall system. Furthermore, the torque/speed characteristic was recorded at load steps.
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