Power Control 413
vice. The electrical current developed as a result of the mechanical move-
ment is used to limit movement of the machine, or to make some change
in its operational sequence. Limit switches are often used in sequencing,
routing, sorting, or counting operations in industry. Ordinarily, they are
used in conjunction with hydraulic or pneumatic controls, electrical re-
lays, or other motor-operated machinery, such as drill presses, lathes, or
conveyor systems.
In its most basic form, a limit switch converts mechanical motion
into an electrical control current. Part of the limit switch is the cam, an
external part which is usually mounted on a machine. The cam applies
force to the actuator of the limit switch. The actuator is the part of the limit
switch that causes the internal NO or NC contacts to change state. The ac-
tuator operates by either linear or rotary motion of the cam, which applies
force to the limit switch. Two other terms associated with limit switches
are pretravel and overtravel. Pretravel is the distance that the actuator must
move to change the normally open or normally closed state of the limit-
switch contacts. Overtravel is the distance the actuator moves beyond the
point at which the contacts change state. Both pretravel and overtravel
settings are important in machine setups where limit switches are used.Temperature Switches
Temperature switches are among the most common types of con-
trol devices used in industry. The control element of a temperature switch
contains a specific amount of liquid. The liquid increases in volume when
the temperature increases. Thus, changes in temperature can be used to
change the position of a set of contacts within the temperature-switch en-
closure. Temperature switches may be adjusted throughout a range of
temperature settings.Figure 15-4. Rotary switch