Page 79 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 4 Electrical Controls 41

switches should not be confused with on/off switches using
Actuator Handle
push actuators. Momentary switches are available in multi-
pole, normally open, normally closed, or a combination of
Housing
both.
Figure 4-15 shows a simple leaf spring momentary
switch. The leaf is simply a bent strip of copper with an insu-
lating button affixed to one end and the opposite end screwed
down to the base. The contact is a second strip, that is also
FORWARD
screwed to the base. The terminals are at the ends of the
copper strips.
Front View Side View
Figure 4-13 Drum Switch Terminal
Button
Base
Leaf
Pole 3
Contact
Pole 2
Terminal
Motor
Pole 1 Figure 4-15 Leaf-Spring Push Button Switch
Forward
Axle
Figure 4-16 shows an exploded view of the leaf spring
switch and just how simple the construction is.
Figure 4-17 shows a sectional view of a commercial, nor-
mally open, momentary switch. This particular unit incorpo-
rates two contact sets. The fixed contacts are set into the body
of the switch and the floating contacts are affixed to the
bridge. When the button is pressed, the bridge assembly is
Off
forced down to the fixed contacts and the circuit is closed.

Button

Reverse

Figure 4-14 Drum Switch Schematic
Contact

Leaf
Three-phase motors can be reversed by switching two of the
three power feeds. The drum switch is a three position, three
pole unit with the center position off. Pole one is on/off/on, Base
poles two and three are configured for a reversing function.
Figure 4-14 shows a schematic representation of a drum switch
function. In the forward position the three power feeds are fed
directly through to the motor. In the off position the three feeds
are disconnected. In the reverse position, pole one is fed
directly to the motor, while poles two and three are reversed.

Push Buttons

Push buttons, or momentary switches, are very common
items. A perfect example is your door bell button. These Figure 4-16 Leaf-Spring Switch Wxploded View