For medium voltage systems it is common to use two or three autoreclose shots, and, for specific applications, four shots. Using four shots, final dead time can be set for a time long enough to allow thunderstorms to stop before definitive final reclose. This scheme prevents unnecessary lockout caused by consecutive transient faults.
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.
5
5/98
1998 Square D All Rights Reserved
Heater Coil
85%-115%
Trip Adjustment
Bimetal
Strip Contact
Product Description
solder 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.
