Overload Relays
Thermal overload relays sense motor current by
converting this current to heat in a resistance
element. The heat generated is used to open a
normally closed contact in series with a starter
coil causing the motor to be disconnected from
the line.
In spite of being relatively simple and inexpensive,
thermal overload relays are very effective in
providing motor running overcurrent protection.
This is possible because the most vulnerable part
of most motors is the winding insulation and this
insulation is very susceptible to damage by
excessively high temperature.
Being a thermal model of a motor, the thermal
overload relay will produce a shorter trip time at a
higher current similar to the way a motor will reach
its temperature limit in a shorter time at a higher
current. Similarly, in a high ambient temperature,
a thermal overload relay will trip at a lower current
or vice versa allowing the motor to be used to its
maximum capacity in its particular ambient
temperature (if the motor and the overload are in
the same ambient).
1998 Square D All Rights Reserved
12
5/98
Thermal Overload Relays – NEMA Rated
Application Data
Once tripped, the thermal overload relay will not
reset until it has cooled, automatically allowing the
motor to cool before it can be re-started.
Square D manufactures two basic types of
thermal overload relays, the melting alloy and the
bimetallic. In some types, the bimetallic is
available in both non-compensated and ambient
temperature-compensated versions. In both
melting alloy and bimetallic, single element and
three element overloads are available.
Motor Logic Solid State overload relays use an
electronic method of detection, which responds
directly to the motor current. When tripped the
overload relay may be reset either manually or
remotely using the Remote Reset Module. The
Solid State overload relay provides overload
protection only for three phase motors rated up to
600 volts AC.
The Solid State overload relay provides protection
for phase loss and phase unbalance. In addition,
this overload relay is ambient insensitive. The
Solid State overload relay is available as a
separate trip Class 10 or 20 and as a selectable
trip Class 10/20 device.
The Motor Logic solid state overload relay must
be selected based on the FLA of the motor it is to
protect. Applications involving a motor with an
FLA less than 1.5 amps can be addressed by
passing multiple looped turns of the motor leads
through the current transformer windows.
