Priniciples of Electromechanical Relay Operation
3.21 Introduction to Power Relays or Contactors
In industrial and military applications, power relays or contactors perform a
wide variety of functions. They are used for switching heavy contact loads
that may be highly inductive, such as motor, generator, and transformer loads.
These devices are also used to switch the heavy resistive and lighting
loads.
In both military and industrial motor control systems, power relays are used
for "across-the-line" applications; that is, the relay contact closure places
the motor directly across the source power line. Across-the-line industrial
motor starters are made in sizes up to those capable of carrying 600 amperes.
Industrial motors are also started at reduced voltages, using series
resistance, autotransformer action, or field control (in the case of dc
motors) to restrict current inrush. An additional set of relay contacts is
required for reduced voltage starting.