This course is not intended to make relay engineers out of you! The discussion is limited to naming some basic operating principles, and discussing some of the common relay applications.
All relays provide an output signal which can be used to initiate some action. This output signal is often used to close one or more set of contacts, operate a circuit breaker, or a switch. Some electronic relays provide an output current or a voltage signal, the more elaborate programmable controllers send digital signals which can be "read" by other devices. The basic electromechanical relays, however, simply operate to close a set of contacts when moved by either:
The simple electromagnetic attraction principle is illustrated in this diagram. The induction disc relay is pictured below in the Induction Disc Relay section. The relay will start to close whenever the current in its operating coil exceeds a threshold. It can be quite rapid since the force of attraction gets larger as the armature moves closer to the core. A mechanical dash-pot or mechanical escapement can be used to delay its operation. In some cases, the operating coil is provided with taps to permit a choice of the pick-up current setting. In other designs, the amount of the air-gap can be adjusted by turning a screw. These relays are usually considered to be "instantaneous," operating in about one cycle or less.
