414 Electrical Power Systems Technology
Float Switches
Float switches are used when it is necessary to control the level of a
liquid. The float switch has its operating lever connected to a rod and float
assembly. The float assembly is placed into a tank of liquid, where the mo-
tion of the liquid controls the movement of the operating lever of the float
switch. The float switch usually has a set of NO and NC contacts, which
are controlled by the position of the operating lever. The contacts are con-
nected to a pump-motor circuit. In operation, the NO contacts are con-
nected in series with a pump-motor control circuit. When the liquid level
is reduced, the float switch will be lowered to a point where the operating
lever will be moved far enough that the contacts will be caused to change
to a closed state. The closing of the contacts will cause the pump motor to
turn on. More liquid will then be pumped into the tank until the liquid
level has risen high enough to cause the float switch to turn the pump mo-
tor off.
Pressure Switches
Another type of electrical control device is called a pressure switch.
A pressure switch has a set of electrical contacts that change states as the
result of a variation in the pressure of air, hydraulic fluid, water, or some
other medium. Some pressure switches are diaphragm operated. They rely
upon the intake or expelling of a medium, such as air, which takes place
in a diaphragm assembly within the pressure-switch enclosure. Another
type of pressure switch uses a piston mechanism to initiate the action of
opening or closing the switch contacts. In this type of switch, the move-
ment of a piston is controlled by the pressure of the medium (air, water, et
cetera).
Foot Switches
A foot switch is a switch that is controlled by a foot pedal. This type
of switch is used for applications in which a machine operator has to use
both hands during the operation of the machine. The foot switch provides
an additional control position for the operation of a machine, for times
when the hands cannot be used.
Drum-controller Switches
Drum controllers are special-purpose switches, which are ordinarily
used to control large motors. They may be used with either single-phase
or three-phase motors. The usual functions of a drum-controller switch