Priniciples of Electromechanical Relay Operation
3.27 Introduction to Thermal Relays
Thermal relays provide operate time delays of 0.1 second to 5 minutes, the
operate time for a particular design being a function of adjustment and power
dissipation or applied voltage. The basic relay consists of a heater element,
a moving bi-metallic heated member, and an actuating linkage that operates
normally open or normally closed contacts. Because this device depends on
thermal expansion and contraction effects to obtain the desired operate time
delay, there is also an appreciable inherent delay on release. Rapid
reoperation, therefore, is possible only with special circuit arrangements.
Thermal relays employ only inorganic materials in their construction, and
hermetically sealed units are, therefore, relatively free from contact
activation effects. The heater elements are wound with low temperature
coefficient wire to minimize resistance changes with temperature. Being
dependent upon heating for operation, thermal relays may be used for direct
current, or the equivalent alternating current, rms operation.
The terms associated with the design and application of thermal relays are
peculiar to these devices. These terms are defined in Chapter 1.