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EMC TEST HOUSES


Drive system upgrade saves energy for low voltage test lab


During modernisation of its low voltage test system, a German test house company has renewed the drive on its excitation generator, using an asynchronous motor with a frequency converter. This has increased the energy efficiency of the system, as well as its flexibility. Andy Pye discovers how this is achieved.


T


he Institute for International Product Safety GmbH (I2


PS


GmbH) carries out tests and trials in the electrical engineering, electronics and


environmental specification fields in accordance with national and international standards. Based in Bonn, Germany, the test laboratory uses modern testing equipment and measurement techniques and holds DIN EN ISO/IEC 17025 accreditation. Testing services include the areas of industrial low voltage devices and systems (which boasts one of the world’s most efficient surge generator systems), the safety of electrical equipment and electromagnetic compatibility (EMC), as well as environmental tests. The testing of equipment for low voltage technology with short-circuit and internal arc tests with currents up to 300kA is also conducted . The surge generator features a special control device, which allows the adjustment of the impulse excitation generated by the exciter, even during short-circuits. In this way, a constant short-circuit current is available with short current flow times in the range of 100ms and with longer current flow times up to a few seconds. This makes it possible to reproduce the short circuit conditions required by the standards to test the generator or transformer. The exciter of the original surge


generator was driven by a slip ring motor


❱❱ The surge generator in the test laboratory of I2


PS featuring the new asynchronous motor


and switching cabinet with the Eaton drive ❱❱ Flexible system configuration for lower


oscillations: A range of SPI inverters can be


connected to the respective motor via the SPA regenerative unit


40 years old, so maintenance and spare parts were becoming issues. An alternative system needed to


function reliably, be energy efficient and allow restarts without long waits. In collaboration with power management company Eaton, I2


PS developed an


asynchronous replacement for the slip ring motor. It is driven by an electronic drive unit consisting of a frequency converter and regenerative braking unit. In this instance, the SPI300A0- 4A3N1 inverter unit and the air-cooled regenerative SPA300A0-4A3N1 unit are used. The AFE SPA is a bi-directional voltage converter for the front-end of a common DC bus. It converts alternating current or voltage to direct current or voltage.


When used on the exciter of the surge


with oil-cooled starting resistors. These oil-cooled resistors heat up markedly during start-up; similarly, when the motor decelerates, the braking energy heats up the oil. Before restarting the motor, the machine operator had to wait a while to allow the oil to cool down. To save this time, often the surge generator was kept running during longer conversion phases of the experimental set-up – however, this wastes energy. The slip ring rotor motor was already over


generator, the Eaton-Regenerative unit supplies the kinetic energy that is released during braking back into the grid. Previously, when the exciter was shut down it came to a complete standstill after about 30 minutes; now, standstill is reached within two minutes, while from a standstill, normal speed is restored within two minutes. The upstream LCL (inductance- capacitance-inductance) filter corrects any waveform distortions on the output voltage, thus producing clean power with low harmonics. The Total Harmonic Distortion (THD) is below 5%. For comparison, normal six-pulse frequency converters have a THD of approximately


EMC Testing 2017 /// 7


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