may be worn away before the contact of two associated
surfaces becomes inadequate to carry rated current.
In an electric-motor contactor, the wear allowance
of the power pole contacts is usually related to the
closed position of the magnetic operator. The wear
allowance of the power pole contacts of a magnetic
contactor is the amount of silver that can be worn away
without resulting in failure of the contacts to touch when
the magnetic operator is at its closed position.
Blowout coils seldom wear out or give trouble
when used within their rating. However, if they are
required to carry excessive currents, the insulation
becomes charred and fails, causing flashovers and
failure of the device.
Arc shields are constantly subjected to the intense
heat of arcing and may eventually burn away, allowing
the arc to short-circuit to the metal blowout pole pieces.
Therefore, arc shields should be inspected regularly and
renewed before they burn through.
Arc barriers provide insulation between electrical
circuits and must be replaced if broken or burned to a
degree where short circuits are likely to occur.
The importance of having clean, tight electrical
connections must be emphasized. Where practical, it is
a good idea and common practice to solder electrical
Excessive slamming on closing, particularly on ac
magnetic-operated devices, will eventually damage the
laminated face of the magnetic armature and may
damage the shading coil.
Magnetic coils should be kept dry. Wet coils should
always be dried out before using. They may be dried by
baking them in a well-vented oven at not more than
194°F to prevent water from boiling in the insulation.
The length of time in the oven depends on the size of the
coil. If an oven isnt available, place the unit under a
canvas cover roomy enough for hot air to be circulated
within. Another alternative is to direct infrared lamps
on the windings.
The closed pressure of contacts is an important
factor in their ability to carry current. A small contact
with proper contact pressure carries more current than a
large one with little pressure. Contact springs must be
kept in condition. Replace them when they have been
damaged or have lost temper by exposure to high
Connections should always be clean and tight.
Loose connections result in overheated parts that
eventually need replacing. Periodic inspections are
necessary because temperature changes, vibration, and
carelessness may loosen the connections.
Inspect the movable core of a controller for
cleanliness. Accumulated dirt causes sluggish
mechanical action which impairs the opening and
closing of the contact.
Noise results if the movable and stationary pole
pieces dont fit together well when the contactor is
closed or when dirt or rust prevents proper closure. The
most prominent noise produced in a controller comes
from a broken shaded pole, which is a single turn of wire
or strap, imbedded in part of the laminated magnetic
Check the cabinet which houses the controller for
cleanliness. Make sure the cover fits properly to keep
moisture, dirt, and dust from entering. Check for
corrosion of all metal parts. Table 7-5 is a guide for
troubleshooting ac controllers.