Priniciples of Electromechanical Relay Operation
3.3 Mechanical Requirements for Operation
The switch usually consists of combinations of movable and fixed, low
resistance contacts which do the actual circuit opening and closing. The
fixed contacts may be mounted on compliant springs or fixed brackets. The
movable contacts are mounted on some form of spring that can be deflected or
on a hinge arm. The force and travel needed for these motions must serve a
number of purposes.
Before the armature is actuated for a relay with double throw contacts, the
movable contacts must be held against the normally closed fixed contacts by a
spring force sufficient to establish good electrical connection. When the
armature is actuated, a number of things happen. By some means, each movable
contact is pushed or pulled away from the corresponding, normally closed,
fixed contact. This requires a force sufficient to overcome one or more
springs. Also, there is friction between the contacts if they slide before
they separate, and in actuator pivots, if they are present. As contact motion
takes place, various springs deflect according to Hook's low, and inertial
forces must be overcome.
After the accelerated motion of contact transfer, there is an impact and
deceleration as the movable contact reaches its normally open fixed contact.
Both must deflect or deform to some degree as the desired contact force builds
ups. In many designs, some further overtravel will occur to provide contact
cleaning action, through sliding, and to compensate for contact wear or
erosion. Also, when there are multiple sets of contacts, allowance must be
made for manufacturing tolerances for the various stages of travel required
for and the spring forces associated with them.
Understanding these forces and travels requires a consideration of the contact
mechanism and the motions involved. The discussion below will deal, in an
elementary way, with both the static and the dynamic characteristics required
of a mechanism for providing these forces. Detailed analysis of design
consideration are available in the reference mentioned at the end of this
section.