Figure 6-25.--A static logic panel for a cargo elevator.
unaffected by variations in the ambient (room)
operating coil of the main contactor and disconnects the
temperature. Different compensation methods are used
motor from the power supply. Since it takes time for the
for different types of thermal overload relays. Refer to
parts to heat up, the thermal overload relay has an
inherent time delay, which allows the motor to draw
the technical manual furnished with the equipment on
excessive current at start without tripping the motor.
which the controller is used for information on the
particular form of compensation provided. There are
You can make a coarse adjustment of the tripping
four types of thermal overload relays--solder pot,
current of thermal overload relays as follows:
bimetal, single metal, and induction.
Change the heater element.
SOLDER POT THERMAL OVERLOAD
RELAY.-- The heat-sensitive element of a solder-pot
Change the distance between the heater and the
relay consists of a cylinder inside a hollow tube. The
heat-sensitive element to make a fine adjustment.
cylinder and tube are normally held together by a film
An increase in this distance increases the tripping
of solder. In case of an overload, the heater melts the
current. (NOTE: Making fine adjustments
solder (thereby breaking the bond between the cylinder
depends on the type of overload relay.)
and tube) and releases the tripping device of the relay.
Change the distance the bimetal strip has to move
After the relay trips, the solder cools and solidifies. The
before the overload relay contact is opened.
relay can then be reset.
Check the related technical manual for additional
BIMETAL THERMAL OVERLOAD
information and adjustments.
RELAY.-- In the bimetal relay, the heat-sensitive
element is a strip or coil of two different metals fused
Thermal overload relays must be compensated; that
together along one side. When heated, the strip or coil
is, they are constructed so the tripping current is
6-17
