Position indicating instrumentation is used in DOE nuclear facilities to provide
remote indication of equipment positions including control rods and major valves.
DESCRIBE the synchro position indicators to include
the basic construction and theory of operation.
Position indicating instrumentation is used in nuclear facilities to provide remote indication of
control rod position with respect to the fully inserted position, and remote indication of the open
or shut condition of important valves. This remote indication is necessary for the monitoring of
vital components located within inaccessible or remote areas. Remote position indication can be
used at any DOE facility, not only nuclear facilities, where valve position indication is required
Remote indication or control may be obtained by the use of self-synchronizing motors, called
synchro equipment. Synchro equipment consists of synchro units which electrically govern or
follow the position of a mechanical indicator or device. An electrical synchro has two distinct
advantages over mechanical indicators: (1) greater accuracy, and (2) simpler routing of remote
There are five basic types of synchros which are designated according to their function. The
basic types are: transmitters, differential transmitters, receivers, differential receivers, and control
transformers. Figure 1 illustrates schematic diagrams used to show external connections and the
relative positions of synchro windings. If the power required to operate a device is higher than
the power available from a synchro, power amplification is required. Servomechanism is a term
which refers to a variety of power-amplifiers. These devices are incorporated into synchro
systems for automatic control rod positioning in some reactor facilities.
The transmitter, or synchro generator, consists of a rotor with a single winding and a stator with
three windings placed 120 degrees apart. When the mechanical device moves, the mechanically
attached rotor moves. The rotor induces a voltage in each of the stator windings based on the
rotors angular position. Since the rotor is attached to the mechanical device, the induced voltage
represents the position of the attached mechanical device. The voltage produced by each of the
windings is utilized to control the receiving synchro position.