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As a technology partner, Computer Controls supports the Akademischer Motorsportverein Zürich (AMZ) with professional knowledge and high-precision instrumentation. The AMZ was founded in 2006 by students at ETH Zurich, who develop a prototype vehicle for the various “Formula Student” races in Europe each year. The association gives students an opportunity to put their theoretical engineering knowledge into practice in a very complex product.
The power dissipation of an inverter can be determined with the aid of a power analyzer. On their motor test stand the AMZ Racing Team uses a Hioki PW6001 Power Analyzer to carry out validation tests for power circuitry and motors. The power dissipation of the components can be determined quickly and easily with the measurement setup described here. The power analyzer computes the Fourier components of the measured signals and directly outputs the derived effective, apparent and reactive power values. Based on the measured torque and motor speed, it can also indicate the motor efficiency by calculating the difference between the input power and the effective mechanical output.
Figure 1: Test setup for making measurements with the power analyzer on the AMZ test stand
Figure 1 shows the AMZ motor test stand on the right, and in the middle (from top to bottom) two low-voltage power supplies (12 V and 24 V), the Hioki Power Analyzer, the high voltage power supply and the motor connectors, with an inverter from Lenze Schmidhauser behind the connectors. On the left there is a battery which is connected in parallel with the power supply and absorbs the residual energy in the system when it is switched off.
The motor test stand consists of two motors coupled through a torque sensor. The motor under test (A) is operated in torque mode and can be adjusted to any desired torque by setting the corresponding inverter current level. The other motor (B) acts as a brake motor and is speed regulated, which means it runs at a specific controlled constant speed.
Figure 2: Detail view of the AMZ motor test stand with brake motor B (left), test motor A (right) and torque sensor
To measure the efficiency of the two inverters (implemented as dual inverters in a single unit) and the efficiency of the motor under test, the power analyzer (which has six inputs for current and voltage measurement as well as two additional inputs for motor measurement) is supplied with the various signals. The DC input current is measured directly at the input of the dual inverter using a clamp-on current probe. The input voltage is probed at the output of the power supply and connected to input 6 of the power analyzer. Because the device under test is a dual inverter, both motors mounted on the test stand are connected to the same inverter.
The phase currents and voltages of the two motors are measured directly at the converter outputs using clamp-on current probes and voltage probes. Because the power analyzer has only six inputs, signals are taken from all three phases of the test motor A (inputs 1 to 3) and from two phases of the brake motor B (inputs 4 and 5), with the signal for the third phase calculated by the instrument under the assumption of ideal symmetry. The sixth input for voltage and current is used to determine the total dissipation from the power fed into the system from the AC line. The signals from the torque sensor, which measure the torque and the motor speed, are fed to the corresponding inputs of the power analyzer. The measured data is saved on a USB stick plugged into the power analyzer, which allows individual measurement sessions to be saved directly as .csv files.
Computer Controls also outfitted the AMZ with a practical carrying case containing a PicoScope, a clamp-on current probe, passive probes and a software CD. It is now used everywhere in the AMZ, especially for conducting tests at various test stations. This allows measurement data to be displayed, saved and processed as necessary on any computer after installation of the corresponding software. Test time is very valuable in the AMZ, and the PicoScope is especially helpful for quickly detecting and correcting errors. Thanks to its compact size and computer-aided measurement data evaluation, the PicoScope is also a valuable tool for making measurements in the workshop, for example to analyze electromagnetic interference.
Figure 3: The portable PicoScope in use in the workshop