deflects because one metal expands more than the other.
In either the instantaneous or time-delay magnetic
The deflection causes the overload relay contact to open.
overload relays, you can adjust the tripping currents by
changing the distance between the series coil and the
SINGLE-METAL THERMAL OVERLOAD
tripping armature. More current is needed to move the
R E L A Y . -- The heat-sensitive element of the
armature when the distance is increased. Compensation
single-metal relay is a tube around the heater. The tube
for changes in ambient temperature is not needed for
lengthens when heated and opens the overload relay
magnetic relays because they are practically unaffected
contact.
by changes in temperature.
INDUCTION THERMAL OVERLOAD
Overload Relay Resets
RELAY.-- The heater in the induction relay consists of
a coil in the motor circuit and a copper tube inside the
After an overload relay has operated to stop a motor,
coil. The tube acts as the short-circuited secondary of a
it must be reset before the motor can be started again.
transformer and is heated by the current induced in it.
Magnetic overload relays can be reset immediately after
The heat-sensitive element is usually a bimetal strip or
tripping. Thermal overload relays must Cool a minute
coil. Unlike the other three types of thermal overload
or longer before they can be reset. The type of overload
relays that may be used with either ac or dc, the
reset may be manual, automatic, or electric.
induction type is manufactured for ac use only.
The manual, or hand, reset is usually located in the
Magnetic Overload Relays
controller enclosure, which contains the overload relay.
This type of reset usually has a hand-operated rod, lever,
or button that returns the relay tripping mechanism to its
The magnetic overload relay has a coil connected in
original position, resetting interlocks as well, so that the
series with the motor circuit and a tripping armature or
motor can be run again with overload protection. (An
plunger. When the normal motor current exceeds the
interlock is a mechanical or electrical device in which
tripping current, the contacts open the overload relay.
the operation of one part or mechanism automatically
Though limited in application, one type of magnetic
brings about or prevents the operation of another.)
overload relay is the instantaneous overload relay. This
type operates instantly when the motor current exceeds
The automatic type of reset usually has a spring- or
the tripping current. It must be set at a higher tripping
gravity-operated device, resetting the overload relay
current than the motor starting current because the relay
without the help of an operator. The electric reset is
would trip each time you start the motor. Instantaneous
actuated by an electromagnet controlled by a push
magnetic overload relays are used in motor controllers
button. This form of overload reset is used when it is
for reduced voltage starting where the starting current
desired to reset an overload relay from a remote
peaks are less than the stalled rotor current.
operating point.
The second type of magnetic overload relay is
EMERGENCY RUN FEATURE
time-delay magnetic overload relay. It delays a short
time when the motor current exceeds the tripping
Motor controllers having emergency-run features
current. This type of relay is essential y the same as the
are used with auxiliaries that cannot be stopped safely
instantaneous relay except for the time-delay device.
in the midst of an operating cycle. This type of feature
This is usually an oil dashpot with a piston attached to
allows the operator of the equipment to keep it running
the tripping armature of the relay. Oil passes through a
with the motor overloaded until a standby unit can take
hole in the piston when the tripping armature is moved
over, the operating cycle is completed, or the emergency
by an overload current. The size of the hole can be
passes
adjusted to change the speed at which the piston moves
for a given pull on the tripping armature. For a given
size hole, the larger the current, the faster the operation.
CAUTION
Therefore, the motor is allowed to carry a small overload
USE THIS FEATURE IN AN
current. The relay can be set to trip at a current well
EMERGENCY ONLY. DO NOT USE IT
below the stalled rotor current because the time delay
OTHERWISE.
gives the motor time to accelerate to full speed before
the relay operates. By this time the current will have
There are three methods of providing an emergency
dropped to full-load current, which is well below the
relay trip setting.
run in magnetic controllers-an emergency run push
6-18
