Brochure

© 1998 Square D All Rights Reserved
4
5/98
Product Description
At current values greater than I
c
, the SCPD reacts quicker than the overload
relay. At current values less than I
c
, the overload relay reacts quicker. Articles
110 and 430 of the NEC provide guidance in the selection of the SCPD to
facilitate coordination of the components of a motor branch circuit (i.e.
location of point I
c
).
Withstand Ratings
Equipment withstand ratings are linked to branch circuit protection. The
same parameters that affect the trip point of a given protective device also
contribute to how much (or how little) let-through energy the device may be
exposed to and still function after the clearing of the fault. Withstand does not
explicitly show up in Figures 1 or 2. Traditional melting alloy and bi-metal
overload relays have been the “weak link” in motor branch circuit withstand
ratings. Since these devices employ sensing elements directly in the current
path, electrical faults leading to mechanical stresses are a concern. These
devices typically contain small mechanical parts than can quickly become
out-of-spec when exposed to let-through energy exceeding their withstand
capability. If the coordinated protection for the circuit operates properly (and
the SCPD protects the circuit), the motor and the controller will be protected.
The withstand rating of a branch circuit must account for the withstandability
of the lowest rated component in the circuit.
Thermal Overload Relay
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 overload are in the same
ambient).
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.
NOTE: The overload relay must be used in conjunction with a contactor. The
overload relay has no power contacts and cannot disconnect the motor by
itself. The control circuit contact must be wired in series with the coil of the
contactor so that the contactor will de-energize when an overload occurs.
Square D manufactures three types of overload relays, the melting alloy, the
bimetallic, and solid state. 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. Solid state overloads are discussed on Page 5.
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 and
Drawing shows operation of melting alloy overload relay. As heat melts alloy,
ratchet wheel is free to turn. The spring then pushes contacts open.
Melting Alloy Thermal Unit
Thermal Relay Unit
Motor
Magnet Coil
One Piece Thermal Unit
Solder pot (heat sensitive element)
is an integral part of the thermal unit.
It provides accurate response to overload
current, yet prevents nuisance tripping.
Heating winding (heat producing element)
is permanently joined to the solder pot, so
proper heat transfer is always insured.
No chance of misalignment in the field.