Tuesday, December 16, 2014

Testing electric bikes!

Test instruments supplied by Yokogawa are playing a key role in the final testing of motors for electric bikes manufactured by Heinzmann in the village of Schönau in Germany’s Black Forest region.

Heinzmann has been developing and manufacturing electric drives for over 30 years. The company’s drives are used in a range of industrial applications as well as in various types of electric vehicles.

The first products for electric bikes were introduced by the company in 1994 and one of its great successes was the development of motors for the cargo bikes used by the German mail carrier Deutsche Post AG.

This particular application uses mainly DC hub motors with gearing to ensure a high torque during acceleration. However, in electric bikes for leisure purposes, Heinzmann installs brushless synchronous disk motors. These are not only maintenance-free, but distinguish themselves compared to a conventional electric motor by being based on a flat design which offers the benefits of small size and low weight. In addition, these motors offer high efficiency and quiet running. They can be used in both front- and rear-wheel drive systems, and can also facilitate energy recovery during braking or downhill running.

A full Heinzmann electric bike drive system includes the electric motor, a controller, a battery pack, a torque sensor in the bottom bracket and a display/control unit on the handlebars. Heinzmann delivers the drive systems to various bicycle manufacturers, but they are also available as a retrofit kit and can also be incorporated into customers’ own electric bikes.

Engine assembly and final test
In Schönau the disk motors are assembled and a functional test is carried out. At the test stand the assembled motor is fixed into a mount, the power and sensor cables are connected and the motor shaft is connected via an adapter coupling and torque shaft to a brake motor. The operator then scans the identification label which is fixed to the motor. On the basis of the type of motor, the test system retrieves the appropriate motor data from a database and determines the respective test parameters and limits for the individual tests. For each motor a high-voltage test and power measurements are performed in the idle state and under load conditions, allowing the efficiency of the motor to be calculated from the measured values. Short-circuit tests are also carried out at random. In the event of failure, the test is immediately aborted and an error log is produced. Following each faultless run, a test label is printed. All measured data, along with the serial number of the motor, are stored in a database to ensure traceability.

The test rig was completely designed and produced to Heinzmann’s specifications by ETU. It includes, among other things, a Yokogawa WT1800 power analyser, a programmable DC power supply for voltages up to 80 V and currents up to 60 A, a high-voltage tester and a measuring range extension for the power analyser with three current transformers. The interface of the test rig operates intuitively via a PC with a touch screen, whereas for the total control of the system a PLC is used. In order to provide realistic control of the motors, different electric bike motor controllers can be integrated into the system.

“ETU chose the WT1800 power analyser largely based on previous experience, but also because it could be upgraded to six channels and combined with a torque sensor to measure the mechanical performance of the drive units. This way, we are ready to meet future challenges”, says Jürgen Bläsi, Managing Director of ETU GmbH.

The laboratory power supply device simulates the battery and feeds a current of up to 50 A into the motor controller. The output current of the power supply is measured by the fourth channel of the WT1800. The motor controller generates a corresponding 3-phase signal to drive the motor. The actual power delivered to the motor is measured using the current transformer and power analyser, and the mechanical power on the torque transducer is detected by the WT1800. In this way, the efficiencies of both the individual powertrain components as well as the overall efficiency of the system can be determined.

“We offer a wide range of motor types and variants with a wide range of performance information. It is therefore necessary to carry out the tests quickly”, says Bernd Becker, Managing Director of Electrical Drives for Heinzmann GmbH & Co.KG.: “For us, it is important that the tests are modular, scalable and flexible, so that, in case of capacity expansion, we can quickly install another system. ETU satisfied all these requirements very well,”

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