Priniciples of Electromechanical Relay Operation
3.1 Introduction
This section is concerned with the fundamental operating principles of "light
duty" electromagnetic relay. Such relays are for use in communication,
control, monitoring, or alarm switching circuits in which load currents are
normally fractions of an ampere to 25 amperes. The questions of importance in
the use of such relays tend to be not only how much power they can switch but
more importantly how often, how fast, how reliably, or how consistently. For
the best understanding of the electrical and mechanical characteristics of
such relays, one should consider separately the electromagnetic switching
"actuator" and the contact performance. Principles of operation for solid
state relays are discussed in Chapter 14.
The principles of operation discussed here are those that might reasonably be
of concern to a user of relays or a circuit designer faced with the selection
and specification of a relay that will best satisfy his needs in terms of
functions to be performed and modes of operation and environments to be
encountered. Since a relay usually performs a switching function, an
understanding of contact phenomena should also aid in the selection of a relay
well suited to a particular application in terms of reliability and optimum
life. Relay performance follows precise rules of physics, chemistry, and
metallurgy. When these rules are observed or violated, the relay behaves
accordingly.
The texts listed in the bibliography provide a comprehensive treatment of
relay design and contact theory.