Selector, Electromechanical

a switching device that connects subscribers’ lines in automatic telephone and telegraph exchanges. It has a single input and several outputs; together with a control unit, it can select and connect one of the outputs to the input. An electromechanical selector consists of a rotor (a series of wipers made of a flexible material that are rigidly attached to a gear or ratchet wheel and constitute the input to which the hunting line is connected), a stator (a series of fixed brass contact segments or wires, insulated from one another, that constitutes the outputs or bank of the selector to which the desired lines are connected), and a drive (a device for moving the wipers).

Electromechanical selectors differ with respect to the type of motion and the number of wipers and the design of the bank and drive system. Some selectors have a single circular (rotary) or rectilinear motion of the wipers; others have two motions (one circular and the other rectilinear). The number and type of the wiper motions are determined by the design and the bank capacity of the selector. The most frequently used selector has three-wire inputs and outputs: a low-capacity unit may have 10–15 outputs, and a high-capacity unit may have 100, 200, 300, or 500 outputs. Low-capacity units have one wiper motion, an individual bank in the form of a set of contact segments located along an arc at 120° or 180°, and three- or two-pronged wipers to reduce the time of rotation to their initial position. They are driven by an electromagnet through which a series of DC control pulses are passed (for example, when dialing a number). Each time the armature of the electromagnet is attracted (direct drive) or released (revertive drive), the wipers are moved by steps along the contact segments until a connection to the desired line is made. Upon clearing, the wipers are returned to their initial positions by current pulses generated in the selector’s control units. This type is called an individual control, and a selector with such control is called a step-by-step selector.

The bank of a high-capacity selector is divided into groups of outputs, so that the wipers must perform two motions: one to select a group and the other to select an output in the group. The bank may be individual or multiple, for groups of up to 40–60 selectors, and made in the form of a set of contact segments (for selectors having an individual bank) or a set of contact wires (for selectors having a multiple bank). The drive used for each type of motion may be individual or common to a group of selectors. In an individually controlled selector with a bank consisting of 10 groups (decades) of 10 outputs each, the wipers select a group during the first (vertical) motion and an output in the group during the second (rotary) motion. These are called two-motion selectors.

A common control is a system of horizontal and vertical shafts with spur pinions that are continuously rotated by an electric motor. The selector’s wipers are moved by a meshing of the selector pinion and the spur pinion of the vertical shaft when an individual clutch electromagnet is operated. This is known as a machine control, and selectors thus equipped are called machine selectors. The bank on a machine selector is divided into 25 groups (multiple frames) with 20 outputs each. During the first (circular) motion of the wipers a group with 20 outputs is selected, and during the second (radial) motion an output in the group is selected. Electromechanical selectors are also used in other automatic equipment.