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Measuring up a low-cost electric vehicle

15 February 2015

Test instruments from Yokogawa are playing a key role in the development of a new family of cost-effective electric vehicles being developed jointly by two German companies.

Leading fork-lift truck manufacturer Linde Material Handling GmbH of Aschaffenburg, together with Karabag, based in Hamburg, have converted a conventional Fiat 500 into an electric car. The Karabag 500E is a conventional Fiat 500 retrofitted with an electric drive. The vehicle has a 3-phase asynchronous motor, providing 20kW of nominal power, and a 125V 11kW lithium polymer battery. It can reach a top speed of 105km/h with a range of 100km. 

The conversion components come from a Linde forklift with appropriate modifications. Linde develops and manufactures the electronics and motors for all its vehicles, producing around 50,000 inverters and 400,000 electric motors per year, and is the market leader in Europe in its performance class.

Karabag was searching for a partner to convert a conventionally powered car into an electric vehicle and approached Linde at the end of 2010. Being a Fiat dealer, Karabag had already been offered a solution by an Italian supplier; however, this was found to be too expensive.

The first conversion was done very pragmatically. Linde simply tried most things and changed as little as possible on the car. After four days the prototype was running. Linde was able to use existing forklift components such as the motor, inverter, controller, fuse and charging circuits, and basically left them unchanged. 

Next, came the definition of driving behaviour, the risk analysis, EMC testing and manufacturer’s declaration. After nine months the project was complete.

To reduce costs, the battery capacity was cut by a factor of two, yet despite the small battery, the range was only reduced from 140km to 100km. With the optimal balancing of the motor, inverter and controller, losses could be minimised and, moreover, the driving behaviour is now many times better than before.

Compared to the previous conversion kit, Linde was able to dispense with the complete cooling system. In addition, the current system is smaller and lighter; the electronics fit into a single box and can be easily replaced if necessary.

For testing purposes, Linde uses a Yokogawa DL850V ScopeCorder and a WT1800 power analyser connected to the electric vehicle. The ScopeCorder is a multi-channel recording instrument for analogue and digital signals, with sampling rates as high as 100MS/s, 16-bit resolution and input channel isolation providing a withstand voltage up to 1 kV. In addition, it is able to monitor the CAN bus and decode the payload. By direct storage of the data on its internal hard drive, longer duration recording is possible. 

The DL850V ScopeCorder is used to monitor the starting (inrush) currents and voltages of the three phases of the motor. For voltage measurements, a 100 MS/s plug-in module is used. The three voltages can be directly connected using standard measuring cables, and current transformers are used for current measurement. In parallel, the transmitted CANbus payload is recorded. 

Using the manufacturer-specific CAN DBC files, the status of various vehicle functions is extracted from the CANbus messages and viewed. This data can be shown as waveforms or as a multimeter display. The queried vehicle functions include: direction indicators, lights, parking brake, doors, charging status, battery temperature, battery voltage and battery current.

The WT1800 power analyser has six current and voltage inputs, so that the current and voltage characteristics of all three phases of the inverter can be captured and displayed. The synchronous measurement of electrical and mechanical power allows an accurate calculation of efficiency.

Because of the 5 MHz bandwidth, a dual harmonic analysis on two phases up to the 500th order is possible. In addition, a wide range of options for representing the measured values on the display are available. Currents up to 50A rms and voltages up to 1,000V rms can be measured directly. Current transformers are required for higher currents.

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