Maintenance And Inspection Of Thermal Overload Relay - Mitsubishi Electric MS-T Series Instruction Manual

4. Maintenance and Inspection of Thermal Overload Relay

(1) Heater rating and adjustment
For the settling current of the thermal overload relay, select one that matches the motor's full load
current. Make fine adjustments by rotating the knob. For example, if the motor's full load current is 8A,
use a 9A thermal overload relay, and turn the knob as shown below to set it to the 8A scale position.
Note that an unfavorable trip could occur due to the ambient temperature, wire size, set value error,
and time transition changes, etc. Turn the knob to adjust the set position.
Knob adjustment (for 8A)
If the heater fuses, the bimetal in the thermal overload relay may be curved in the opposite of the
correct direction. This is a result of a large amount of heat by the heater fusing, which abnormally
heats the bimetal and creates large internal stress, causing permanent deformation.
(3) Contact configuration
For the MS-T series, the thermal overload relay contact is configured as 1NO/1NC contact as shown
below. The NO and NC contacts can be used as independent contacts, and differing voltages can also
be used.
20
[Reset]
If an overcurrent flows to the motor, the thermal overload relay will trip.
Check the cause of the overcurrent, take measures, and then press
the reset bar to reset the relay. There may be cases when the relay
cannot be reset immediately after a trip, but resetting will be possible
after the bimetal cools off. For the automatically reset type, the relay
will be reset automatically after a short time. (This will differ according
to the temperature that the bimetal is heated to, but will be just over 10
seconds to 10 minutes.)
[Do not disassemble]
Never touch the inside of the thermal overload relay. The thermal overload
relay has been configured precisely at Mitsubishi before shipping.
Reset
Reset bar
(2) Fusing of thermal overload relay
The thermal overload relay is used to
protect the motor from burning. It cannot be
used as protection against short circuits, so
if a short circuit occurs, replace the thermal
overload relay.
If a current larger than the heater fuse's I
value flows due to a short circuit, etc., the
thermal overload relay heater will fuse
before it actually operates. To prevent
fusing of the heater, use the MCCB of the
correct capacity on the power supply side of
the magnetic motor starter, or use a
saturable reactor on the thermal overload
relay to coordinate protection.
95
97
96 b(NC) contact
98 a(NO) contact
2
t