Position indicating instrumentation is used in DOE nuclear facilities to provide remote indication of equipment positions including control rods and major valves.

EO 1.1DESCRIBE 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 for safety.

Synchro Equipment

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 indication.

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 rotor's 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.

Transmitters, Receivers Control Transformers

Differentials

Figure 1 Synchro Schematics

The receiver, or synchro motor, is electrically similar to the synchro generator. The synchro receiver uses the voltage generated by each of the synchro generator windings to position the receiver rotor. Since the transmitter and receiver are electrically similar, the angular position of the receiver rotor corresponds to that of the synchro transmitter rotor. The receiver differs mechanically from the transmitter in that it incorporates a damping device to prevent hunting. Hunting refers to the overshoot and undershoot that occur as the receiving device tries to match the sending device. Without the damping device, the receiver would go past the desired point slightly, then return past the desired point slightly in the other direction. This would continue, by smaller amounts each time, until the receiver came to rest at the desired position. The damper prevents hunting by feeding some of the signal back, thus slowing down the approach to the desired point.

Differential synchros are used with transmitter and receiver synchros to insert a second signal. The angular positions of the transmitter and the differential synchros are compared, and the difference or sum is transmitted to the receiver. This setup can be used to provide a feedback signal to slow the response time of the receiver, thus providing a smooth receiver motion.

Control transformer synchros are used when only a voltage indication of angular position is desired. It is similar in construction to an ordinary synchro except that the rotor windings are used only to generate a voltage which is known as an error voltage. The rotor windings of a control transformer synchro are wound with many turns of fine wire to produce a high impedance. Since the rotor is not fed excitation voltage, the current drawn by the stator windings would be high if they were the same as an ordinary synchro; therefore, they are also wound with many turns of fine wire to prevent excessive current.

During normal operation, the output of a control transformer synchro is nearly zero (nulled) when its angular position is the same as that of the transmitter.

A simple synchro system, consisting of one synchro transmitter (or generator) connected to one synchro receiver (or motor), is shown in Figure 2.

Figure 2 Simple Synchro System

When the transmitter's shaft is turned, the synchro receiver's shaft turns such that its "electrical position" is the same as the transmitter's. What this means is that when the transmitter is turned to electrical zero, the synchro receiver also turns to zero. If the transmitter is disconnected from the synchro receiver and then reconnected, its shaft will turn to correspond to the position of the transmitter shaft.

Summary

Synchro equipment is summarized below.

Synchro Equipment Summary

A basic synchro system consists of a transmitter (synchro generator) and receiver (synchro motor).

When the transmitter's shaft is turned, the synchro motor's shaft turns such that its "electrical position" is the same as the transmitter's.