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 eventually the motor
will burn out due to the accumulated heat from the repeated inrush and
overload current. More important is the possibility of danger to personnel.
The unexpected restarting of a machine may find the operator or
maintenance man in a hazardous situation as he attempts to find out why his
machine has stopped.
Ambient Temperature Compensated Bimetallic
Ambient-compensated bimetallic overload relays are designed for one
particular situation; that is, when the motor is at a constant ambient
temperature and the controller is located separately in a varying ambient
temperature. In this case, if a standard thermal overload relay were used, it
would not trip at the same level of motor current if the controller temperature
changed. The standard thermal overload relay is always affected by the
surrounding temperature. To compensate for temperature variations, an
ambient-compensated overload relay is used. Its trip point is not affected by
temperature, and it performs consistently at the same value of current.
Thermal Overload Relay Trip Characteristics
Melting alloy and bimetallic overload relays are designed to approximate the
heat actually generated in the motor. As the motor temperature increases, so
does the temperature of the thermal unit. The motor and relay heating curves
(left) show this relationship. From this graph we can see that no matter how
high the current drawn, the overload relay will provide protection yet will not
trip unnecessarily
