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tripping armature. More current is needed to move the
deflects because one metal expands more than the other.
armature when the distance is increased. Compensation
The deflection causes the overload relay contact to open.
for changes in ambient temperature is not needed for
SINGLE METAL.­ The heat-sensitive element of
magnetic relays because they are practically unaffected
the single-metal relay is a tube around the heater. The
by changes in temperature.
tube lengthens when heated and opens the overload
relay contact.
Overload Relay Resets
INDUCTION.­ The heater in the induction relay
After an overload relay has operated to stop a motor,
consists of a coil in the motor circuit and a copper tube
it must be reset before the motor can be started again.
inside the coil. The tube acts as the short-circuited
Magnetic overload relays can be reset immediately after
secondary of a transformer and is heated by the current
tripping. Thermal overload relays must cool a minute or
induced in it. The heat-sensitive element is usually a
longer before they can be reset. The type of overload
bimetal strip or coil. Unlike the other three types of
reset may be manual, automatic, or electric.
thermal overload relays that may be used with either ac
or dc, the induction type is manufactured for ac use only.
The manual, or hand, reset is usually located in the
controller enclosure, which contains the overload relay.
Magnetic Overload Relays
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 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 operates instantly when the motor current
The automatic type of reset usually has a spring- or
exceeds the tripping current. This type must be set at a
gravity-operated device, resetting the overload relay
higher tripping current than the motor-starting current
without the help of an operator. The electric reset is
because the relay would trip each time you start the
actuated by an electromagnet controlled by a push
motor. One use of the instantaneous magnetic overload
button. This form of overload reset is used when it is
relay is in motor controllers used for reduced voltage
desired to reset an overload relay from a remote
starting where the starting current peaks are less than the
operating point.
stalled rotor current.
The operation of a second type of magnetic overload
Overload Relay­Emergency Run
relay is delayed a short time when the motor current
exceeds the tripping current. This type of relay is
Motor controllers having emergency-run features
essentially the same as the instantaneous relay except
are used with auxiliaries that cannot be stopped safely
for the time-delay device. This is usually an oil dashpot
in the midst of an operating cycle. This type of feature
with a piston attached to the tripping armature of the
allows the operator of the equipment to keep it running
relay. Oil passes through a hole in the piston when the
with the motor overloaded until a standby unit can take
tripping armature is moved by an overload current. The
over, the operating cycle is completed, or the emergency
size of the hole can be adjusted to change the speed at
passes.
which the piston moves for a given pull on the tripping
NOTE: Use this feature in an emergency only. Do
armature. For a given size hole, the larger the current,
not use it otherwise.
the faster the operation. The motor is thus allowed to
Three methods of providing emergency run in
carry a small overload current. The relay can be set to
magnetic controllers are an emergency run pushbutton,
trip at a current well below the stalled rotor current
a reset-emergency run lever, or a start-emergency run
because the time delay gives the motor time to accelerate
push button. In each case, the lever or push button must
to full speed before the relay operates. By this time, the
be held closed manually during the entire emergency.
current will have dropped to full-load current, which is
well below the relay trip setting.
Figure 1-1 is a schematic diagram of a controller
showing a separate EMERGENCY RUN push button
In either the instantaneous or time-delay magnetic
with normally open contacts in parallel with the
overload relays, you can adjust the tripping currents by
normally closed contact of the overload relay. For
changing the distance between the series coil and the
1-4

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