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Test bench technology for the future

New EMC test system for hybrid, electric and fuel cell drives: 0 – 1000 V, ± 300 A, 100 kW

In order to ensure electromagnetic compatibility, hybrid, electric and fuel cell automobile drives require new test systems. To this end, Mooser EMC Technik GmbH in Ludwigsburg have commissioned what is currently probably the most modern and powerful EMC testing centre. It fulfils all requirements current or to be expected in the next years.

Due to high time and cost pressure in the automotive industry, EMC testing is usually not done in the car but as early as possible on component level. This also applies to the new high voltage technology in hybrid, electric and fuel cell drives, where supply voltages of several hundred volts, currents of several hundred amperes and electric motors with outputs of 50 kW and more are no longer a rarity.

With a test range of up to 1000 V, 300 A and 100 kW output of the high voltage supply and the brake motor, Mooser's test centre offers optimal conditions for EMC tests of inverters, motors, HV batteries and complete systems. Conventional EMC measurement cabins designed for 12 V and 24 V vehicle power systems only are not suited to this purpose. Since handling such high voltages and currents needs some getting used to, at least initially, the facilities were appropriately secured, and the staff was specially trained.

Coupled yet decoupled
For testing the inverters, motors, high-voltage batteries and complete systems under realistic conditions, in addition to the external power supply (a battery simulation with a facility for feeding back electric energy), an engine test bench in the cabin and an external motor are required. During drive operations, the external motor serves as brake, and during braking operations (recuperation phase) as drive for the DUT. The mounting attachment for the DUT is deliberately designed to be universal. In addition, there is an external liquid cooling system present for the DUTs. For tests where there is no electric motor available yet, there is the possibility of testing an inverter with a load simulation.

An important requirement is the mechanical coupling of both motors without affecting the shielding effect of the EMC cabin. To this end, however, the motors must be vibration-cushioned and decoupled, which is achieved by two separate motor bases with a total weight of several tons.

The absorber cabin with the special high-voltage filters and the high-voltage bushings was constructed by Albatross Projects, and so was the mechanical grommet from motor 2 (DUT) to motor 1 (braking and drive device). The requirements for these grommets were unexplored territory. With a speed of several thousand rotations per minute and a torque of up to 300 Nm, a shielding effectiveness of the cabin of ≥ 100 dB is advisable. Thanks to the filter design and the special grommet for the motor drive, the ripple currents of the external motor are likewise prevented from entering the cabin.

Completely shielded
In order to ensure unhindered radio reception in the vehicle, the components must comply with certain limit values. For the electric drives, high-voltage DC is converted into a three-phase current with variable frequency, resulting in very high and wide high-frequency noise spectra. The considerably higher voltages alone produce interferences which are about 50 times (34 dB) higher than with conventional automobile electronics. Therefore the complete high voltage area (HV) including the connectors is shielded. The closed shielding concept is a basic requirement for EMC measurements and entails further special developments, such as impedance adjustments of the HV DC supply lines.

There are numerous functional connections between the electric drive (HV area) and the 12 V vehicle power system (low voltage area, LV), necessitating consistent separation of the two areas by shields and filters as well. This separation also forms a part of the EMC tests. Therefore for tests the entire system is set up, consisting of HV and LV components.

Please do not interfere!
The basic conditions for interference emission measurements as defined in CISPR 25 apply analogously to the procedures for measuring interference resistance. The ISO norms 11452 define the basic measurement procedures for interference resistance of automobile components. The required interference resistance is usually 100 to 300 V/m, in some cases up to 600 V/m.

The international norms CISPR 25 (Comité International Spécial des Perturbations Radioélectriques) and the ISO norms 11452 also specify the basic setup for component testing of conventional car electronics. Of course, the measuring systems of the EMC testing centre meet these requirements as well. The mounting for the electric motors is a massive steel construction on a machine bed with several tons of weight. The construction is designed to affect the measurement results only insignificantly.

Supplied with energy
The energy supply for the DUTs is a tried and slightly modified system by the testing technology vendor KRATZER AUTOMATION, Munich. Key parameters are variable voltage from zero to 1000 V, currents from zero to 300 A and an electric motor with 100 kW of output. The system supplies energy for the DUTs and can absorb the energy fed back during recuperation phase. Quick load steps are also possible. Together with the external 100 kW motor and its control system by ABB, the energy supply, the external control computer and the testing software by KRATZER AUTOMATION form a complete engine test bench in an EMC cabin. Via the interfaces of the control computer, the engineers can run the test programs on the EMC test bench.
Concomitantly with the absorber chamber, Mooser have set up a test bench for electrical tests of HV components. On this test bench, HV devices are tested for resistance against residual alternating voltages, dips and drops and other contingencies. These tests may also be performed in a 1000 l temperature chamber with a temperature range from -45 °C to +185 °C.

Conclusions
With the new EMC test centre for hybrid, electric and fuel cell motors, Mooser EMC Technik GmbH are currently running what is probably the most modern and powerful facility of its kind. Thanks to its forward-looking design, it fulfils all requirements current or to be expected in the next years for an EMC test centre for HV applications. The insights from EMC tests of inverters, motors, HV batteries and complete systems are used not only for the customers' development projects but also for the ongoing development of national and international norming.