Part
12470
Rev. 2 3/25/99
10
common type of magnetic overload relay utilizes a plunger or iron core piston which extends
from an oil filled dashpot into the operating coil of the relay.
When the electromagnetic field produced by the operating coil is strong enough, the piston
moves through the oil and opens the contacts of the relay. A time-delay is achieved by the
oil in the dashpot, retarding the movement of the piston. Usually magnetic overload relays
with oil dashpots have facilities which permit adjusting their minimum operating current
(pick-up point) and their time delay characteristics.
PLANNED MAINTENANCE PROGRAM
A scheduled program for maintenance of motor overload relays consists primarily of "good
housekeeping" in conjunction with visual inspections and electrical tests. A brief outline is given
below:
1.
Clean
All types of motor overload relays should be cleaned periodically to ensure continued,
reliable operation. It is possible for dirt or dust, created by conditions in the plant, to prevent
parts of the relay from moving. These same conditions can also prevent the proper
dissipation of normal heating, resulting in unnecessary operation of thermal type overload
relays.
2.
Tighten Connections
This is particularly important in thermal overload relays. Loose electrical connections can
cause extra heat which may result in a nuisance operation of the relay.
3.
Inspect Heater Size
Determine that the specified heater is used in thermal overload relays. Too often, oversized
heaters are arbitrarily installed to eliminate unexplained trips. Actually, the original heaters
may have oxidized after a period of time and become smaller in cross section. In that event,
the heat required to operate the relay is provided by a smaller amount of current than that
intended by the original design. This may make the relay trip prematurely and the heater
appear undersized.
4.
Inspect Settings (Where applicable)
Most magnetic overload relays have adjustable settings for minimum operating current and
time delay characteristics. These should be adjusted to the specified settings.
