422 Electrical Power Systems Technology
cal Code for metal dust, carbon black, coal, coke, flour, starch, or grain dusts.
The enclosure is cast aluminum with machined surfaces between cover and
base to provide seal.
NEMA TYPES 1B1, 1B2, IB3 Flush Types provide behind-the-panel mounting into
machine bases, columns, or plaster walls to conserve space and to provide
a more pleasant appearance. NEMA 1B1 mounts into an enclosed machine
cavity, NEMA 1B2 includes its own enclosure behind the panel to exclude
shavings and chips that might fall from above. NEMA IB3 for plaster walls
includes an adjustment to compensate for wall irregularities.
OTHER ELECTROMECHANICAL POWER CONTROL EQUIPMENT
There are so many types of electromechanical power control equip-
ment used today that it is almost impossible to discuss each type. How-
ever, some of the very important types will be discussed in the following
paragraphs.
Relays
Relays represent one of the most widely used control devices avail-
able today. The electromagnet of a relay contains a stationary core. Mount-
ed close to one end of the core is a movable piece of magnetic material
called the armature. When the coil is activated electrically, it produces a
magnetic field in the metal core. The armature is then attracted to the core,
which in turn produces a mechanical motion. When the coil is de-ener-
gized, the armature is returned to its original position by spring action.
The armature of a relay is generally designed so that electrical con-
tact points respond to its movement. Activation of the relay coil will cause
the contact points to “make” or “break,” according to the design of the
relay. A relay could be described as an electromagnetic switching mecha-
nism. There are an almost endless number of special-purpose relays and
switch combinations used for electrical power control. Figure 15-5 shows
a simplified diagram of the construction of a relay that is used to control a
motor.
Relays use a small amount of current to create an electromagnetic
field that is strong enough to attract the armature. When the armature is
attracted, it either opens or closes the contacts. The contacts then either
turn on or turn off circuits that are using large amounts of current. The
minimal current that must flow through the relay coil, in order to create
a magnetic field strong enough to “attract” the armature, is known as the