Relays and contactors

Applications of relays Relays are used in electropneumatic control systems to:

• multiply signals,

• delay and convert signals,

• link information,

• separate the control and main circuits.

They are also used in purely electrical control systems to separate the DC and AC circuits.

Structure of a relay

A relay is an electromagnetically-actuated switch where the control circuit and the controlled circuit are electrically separated from each other. It essentially consists of a coil with an iron core (see (3)(1) in Figure 3.12), an armature as a mechanical actuating element (4), a return spring (2) and switch contacts (6). When a voltage is applied to the solenoid coil, an electromagnetic field is generated. This causes the movable armature to move towards the coil core. The armature acts upon the relay contacts that are either closed or opened, depending on the arrangement. If the flow of current through the coil is interrupted, a spring returns the armature to its initial position

A relay coil can be used to switch one or more contacts. In addition to the relay type described above, there are also other designs of electromagnetically-actuated switches, for example the remanence relay, the time relay and the contactor.

Time relays

In the case of time relays, a distinction is made between relays with switch-on delay and relays with switchoff delay.

In a relay with switch-on delay, the armature switches on with a preset delay td; there is no switch-off delay. In a relay with switch-off delay, the reverse happens. The contacts switch accordingly (see Figures 3.13/3.14). The delay time td can be set as required