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DEVELOPING AN AC CURRENT GENERATOR

by Ernesto Gradin & Aubrey Kagan

Start ý Features ý Magnetics ý Primary and Secondary Turns ý Primary and Secondary Winding ý Primary Inductance ý Hardware ý Firmware ý Analog to Digital Conversion ý User Interface ý Sources and PDF

Weidmuller manufactures a wide range of industrial signal conditioners. Among them is a series of modules that translate AC current to a standard control signal of about 4 to 20 mA or 0 to 10 V. Integral to the manufacturing process is calibration and test, which requires an AC current to be generated to stimulate the input of the module under test.

The component structure of the project allows for any reasonable current to be generated, and because the control is hosted on a PC, it is possible to configure any kind of user interface. Because we have no intention of making this into a commercial product, all the relevant information has been supplied.

The currents that need to be generated for our products can, in some cases, exceed 100-A AC. As a result, the use of resistive techniques was discounted because they generate excessive heat. In addition, they are potentially dangerous to the test technician because there could be 120 V present at the module inputs. A further disadvantage is that the test is restricted to the line frequency and subject to unpredictable changes of the line voltage.

A common solution in the industry is to use a toroidal transformer with only a few turns on the secondary. For a transformer, the power presented at the primary is equal to the power delivered at the secondary, and the output voltage is proportional to the turns ratio of the secondary winding to the primary winding. It is possible, by using these two properties, to construct a transformer to generate a high current at a low output voltage.

Initially, we tried this with a variac driving the primary, but found that the line voltage variations were too great to be practical. In addition, the output frequency was still limited to the line value.

The next attempt was to use a sinewave oscillator driving a power amplifier that was connected to the primary of the transformer. In this configuration, it was possible to change the AC frequency, but it suffered from another problem. The transformer's secondary current is affected by the loop resistance that includes the terminal resistance of the module and the wire length. For each module calibrated, the gain setting of the oscillator would have to be changed, resulting in a lengthened calibration time.

Clearly the solution was to create a closed-loop system. Once implemented, additional features became available, such as surge testing and step responses. We decided not only to design a controller to accomplish this goal, but to use the project as a vehicle to learn new technologies such as Accel (schematic capture and layout), Visual Basic, and a software-based PID control loop.

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