Melting Alloy
In melting alloy thermal overload relays, the motor current passes through a
small heater winding. Under overload conditions, the heat causes a special
solder to melt allowing a ratchet wheel to spin free thus opening the control
circuit contacts. When this occurs, the relay is said to “trip”. To obtain
appropriate tripping current for motors of different sizes, or different full load
currents, a range of thermal units (heaters) is available. The heater coil andsolder pot are combined in a one piece,
nontamperable unit. Melting alloy thermal overload relays must be reset by a deliberate hand operation after
they trip. A reset button is usually mounted on the cover of enclosed starters.
Thermal units are rated in amperes and are selected on the basis of motor
full load current, not horsepower.
Non-Compensated Bimetallic
Bimetallic thermal overload relays employ a U-shape bimetal strip
associated with a current carrying heater coil.When an overload occurs, the
heat will cause the bi-metal to deflect and operate a control circuit contact.
Different heaters give different trip points. In addition, most relays are
adjustable over a range of 85% to 115% of the nominal heater rating.
Bimetallic overload relays are used where the controller is remote or difficult
to reach. Three wire control is recommended when automatic restarting of a
motor could be hazardous to personnel.
Automatic Reset
These relays are field convertible from hand reset to automatic reset and
vice-versa. On automatic reset after tripping the relay, contacts will
automatically reclose when the relay has cooled down. This is an advantage
when the relays are inaccessible. However, automatic reset overload relays
should not normally be used with 2-wire control. With this arrangement, the
motor will restart when the overload relay contacts reclose after an overload
relay trip, and unless the cause of the overload has been removed, the
overload relay will trip again. This cycle will repeat and event
Shipping Port: Xiamen, China
