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Synchro Symbols

A synchro consists of a rotor (R) and a stator (S). The letters R and S are used to identify rotor and stator connections both on the synchro and in wiring diagrams and schematics. Synchros are represented by the symbols shown in figure 5-20. The symbols shown in parts (A) and (B) are used when it is necessary to show only the external connections to a synchro, while those shown in parts (C), (D), and (E) are used when it is important to see the positional relationship between rotor and stator. The small arrows on the rotors in parts (C), (D), and (E) indicate angular displacement of the rotor; in this illustration the displacement is zero.

Synchro Terms

Some standard synchro terms that you will use are defined as follows:

Rotor position: Amount of rotor offset from zero position, measured in degrees, minutes, or seconds

Electrical zero: Standard position used as the electrical reference point from which all angular displacements are measured (not necessarily the zero position of the dial)

Angular position: Counterclockwise (viewed from the shaft extension end) angular rotor displacement from electrical zero position

Direction of rotation: Clockwise or counterclockwise rotor rotation, determined when facing the shaft extension end of the synchro

Increasing reading: Reading transmitted to a synchro when numerical value of the information transmitted increases


Synchros are electromagnetic devices; therefore, a review of magnetism will be necessary to understand synchro principles.

A bar magnet illustrates the magnetic field and pole relationship of the synchro. The lines of force flow from the south pole to the north pole inside the magnet, as shown in figure 5-21.

Two bar magnets shown in figure 5-21 illustrate the actions of like and unlike poles in bar magnets.

Three bar magnets, spaced 120 degrees apart, and a removable bar magnet free to pivot within the ring of mounted magnets show basic synchro principles (fig. 5-22). If the ring of three magnets is fixed, the single pivoted magnet moves so that its south pole is in line with the north pole of magnet No. 1. Since its north pole is attracted equally by the south poles of magnets 2 and 3, it will remain between the two. The pivoted magnet, therefore, aligns itself with magnet No. 1. The three magnetic fields combine to form one resultant

Figure 5-20.-Schematic symbols for synchros. 5-20

Figure 5-21.-Repulsion and attraction.

magnetic field. If the three magnets are now rotated 120 degrees and held in that position, the resultant magnetic field is also rotated through 120 degrees. The pivoted bar magnet will turn in the same direction so that it remains aligned with the resultant magnetic field of the three stationary magnets. This alignment illustrates the action of a torque synchro receiver.

Three dc electromagnets could be used in place of the three permanent magnets mounted on the ring, and the effect on the magnet pivoted in the center would be the same. By feeding the proper amount of current in, the proper bar magnet can be made to rotate in either direction. The permanent magnet pivoted in the center could also be replaced by an electromagnet (fig. 5-23).

Because synchros operate on ac voltages, the magnet (R1, R2) pivoted in the center is energized by an ac source; the fixed magnets (S1, S2, S3) are also

Figure 5-23.--Synchro primary magnetic field.

Figure 5-22.-Pivoted bar magnet.

Figure 5-24.-Effective stator voltages.

energized by the ac source through another set of coils. The arrangement is shown in figure 5-23.

The pivoted electromagnet will react in the same manner as the bar magnet did when dc voltage was applied. The pivoted electromagnet will assume a position that depends upon the magnetic field established by the stator coils.

If the 115 VAC is applied to the rotor, at a given instant the flux takes the directions shown by the arrows in figure 5-24. Both flux loops cut the S2 winding, but only one cuts the S1 and S3 windings. If a voltmeter could be placed directly across the S2 winding, it would indicate 52 volts; across the S1 and S3 windings, it would indicate 26 volts.

The coils of the stator are Y-connected (shown upside down in these illustrations). The stem of the Y is one coil and the branches of the Y are two other coils. They have a common connection, but no lead is brought out from this point.

Figure 5-25.-Torque synchros used to position a dial.

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