RA-70M Theory of Operation

The RA-70M operates in a similar manner to the RA-70A and RA-70P. The basic differences are that the

RA-70M is designed to provide a three-wire potentiometer compatible output and is used in conjunction

with the Basler Manual Voltage Controls to provide an autotracking capability for “bumpless” transfer

between the Auto and Manual operating modes of many exciter systems. A diagram showing the RA-70M

function blocks is shown in Figure 5.

Nulling/Autotracking Input Circuits

This sensing circuit connects to the F+ and F− outputs of the AVR and to the F+ and F− outputs of the

MVC. Both inputs are compared and the difference is sent to the microprocessor for processing and

output as the nulling signal. If Autotracking is enabled, the null output will read zero as long as tracking is

maintained. The field voltage range (32 Vdc, 63 Vdc, or 125 Vdc) is automatically selected. A front panel

LED lights whenever the AVR and MVC are nulled.

Pushbutton/Contact Inputs Circuit

The pushbuttons (front panel switches) provide for front panel control over the Raise, Lower, Pre-position

1. Pre-position 2. Mode Select, and Programming functions. External dry contacts provide the same

functions as the first four pushbuttons on the front panel except for the Programming function, which is

available only by the use of the front panel pushbuttons. The contact inputs allow remote control of the

reference adjuster.