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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

FEATURES

The ACCG can produce an AC current in several modes:

It is possible to generate other outputs through manipulation of the ACCG by the software on the host PC. Functions such as a ramp may be achieved. Associated with processes are a number of parameters (see Communications Protocol Sidebar Table 3). These parameters are saved in the EEPROM of the ACCG and can be read or changed.

The host software is written in Visual Basic, so it may be easily customized for any application without changing the firmware (which requires detailed knowledge of the hardware operation).

SYSTEM OVERVIEW

Power for the AC current generator controller is derived from two 24-VDC power supplies (see Figure 1). They are connected in series to generate a positive and negative supply. The positive supply also provides power for the true RMS AC current monitor. Based on the work we did for 10- and 100-A outputs, 60-W power supplies are all that are necessary.

(Click to enlarge)

Figure 1ýSystem overview showing the interconnection between the significant elements of the project.

The AC current generator controller is a module developed especially for this project. We will describe the component parts and functions later in the article. In essence, it is a microcontroller, A/D converter, AC waveform generator, and power amplifier. The AC output from the power amplifier drives the primary of the current generating transformer. The current generating transformer must be wound for a particular current range.

The secondary of the current generating transformer is used to drive the module that is being tested. The secondary loop passes through a shunt (for calibration purposes) and the primary of a current measuring transformer before being closed by the load itself (R_L). The output of the current measurement transformer is converted to a 4- to 20-mA signal through a true RMS current monitor.

We selected Weidmullerýs Bicron B5303 current transformer to drive the Weidmuller 991064 true RMS AC current monitor module. The 4- to 20-mA signal can then be processed by the AC current generator controller. Calibration is not an issue with this module because the system software is arranged to allow a current to be generated in an open loop, and then the corresponding output, as measured by the AC current generator controller A/D converter, is "learned." The closed-loop control is executed around that value.

To calibrate the unit during the "learn" process, the actual AC current must be known. This is achieved by measuring the RMS voltage across the shunt (R_S). Using Ohmýs Law, it is possible to calculate the RMS current in the loop.

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