69
MOTOR OVERLOAD RELAYS
Always refer to the manufacturer's literature applicable to the particular overload relay
before testing. The test operator should be familiar with the operating characteristics of
the relay, the tolerances applicable to the operating characteristics and the means of
adjusting the relay, if any.
The test usually performed on these devices is to verify the time delay characteristics of
the relay when subjected to an overload. One test point is usually suggested to
establish whether the relay is operating correctly and within the band of the time-current
curve for relay. The suggested test current is three times (3X) the normal current-rating
of thermal overload relays or three times (3X) the pick-up current (setting) of magnetic
overload relays.
It is, of course, easiest to make the connections and perform the tests on the relays if
they are removed from the starter. However, it is not necessary to remove the relay as
long as the test leads can be connected and the circuit de-energized. It should be
further noted that any leads already connected to the relay need not be removed when
conducting the tests. It is recommended that the optional high current leads, PN 2265,
be used to conduct this test. The high-current leads from the test set to the relay under
test should be kept as short as possible and should be twisted to minimize the losses
caused by inductive reactance.
Run the test (see
TESTING TIME DELAY OVERCURRENT DEVICES
) and note the
time required for the overload relay to trip. If the tripping time exceeds the desired
value, or if the relay does not trip at all, the relay may not be protecting the motor
properly. If the relay operates too quickly, it may result in unnecessary nuisance trips.
It should be remembered that these devices operate over a wide band and precise
results should not be sought. A tolerance of ±15% is usually acceptable.
If a thermal overload relay is not operating properly, tripping too soon or too late,
remove the heater element. Note its type, rating, etc., and compare with manufacturer's
data for operating characteristics of the motor. If correct for the application, substitute a
new heater of the same rating and retest. If improper heater elements are being used,
either under or oversized, replace with the proper sized heater and retest.
If a magnetic overload relay is not operating properly, refer to the relay manufacturer's
literature for instructions on making adjustments to the time delay. If the relay is
operating improperly, it may also be desirable to verify the pick-up point (minimum
operating point) of the relay. To perform this test, it is necessary to disengage the time
delay feature of the overload relay. Refer to the manufacturer's literature for detailed
instructions.
Summary of Contents for SR-98
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