lengthens when heated and opens the overload
Overload Relays
relay contacts.
The heater in the INDUCTION type has a coil
Nearly all shipboard motor controllers pro-
in the motor load circuit and a copper tube
vide overload protection when motor current
inside the coil. The tube acts as the short-circuited
is excessive. This protection is provided by
secondary of a transformer. It is heated by the
THERMAL or MAGNETIC overload relays.
current induced in it. The heat-sensitive element
Those relays disconnect the motors from their
is usually a bimetal strip or coil.
power supply. This prevents them from over-
MAGNETIC OVERLOAD RELAYS.--The
Overload relays in magnetic controllers have
magnetic overload relay has a coil connected in
normally closed contacts. The contacts are opened
series with the motor load circuit and a tripping
by a mechanical device that is tripped by an
armature or plunger. When motor current exceeds
overload current. Opening the overload relay
the tripping current, the armature opens the
contacts breaks the circuit through the operating
overload relay contacts. This type of magnetic
coil of the main contactor. This causes the main
overload relay operates instantly when the motor
contactor to open, cutting off the power to the
current exceeds the tripping current. Another type
motor. Overload relays in manual controllers
of magnetic overload relay is delayed a short time
operate mechanically to trip the main contacts and
when motor current exceeds tripping current. This
allow them to open.
type is essentially the same as the instantaneous
relay except for the time-delay device.
THERMAL OVERLOAD RELAYS.-- T h e
thermal overload relay has a heat-sensitive
OVERLOAD RELAY RESETS.--After an
element. It has an overload heater that is
overload relay has operated to stop a motor, it
connected in series with the motor load circuit.
must be reset before the motor can be run again
When the motor current is excessive, heat from
with overload protection. Magnetic overload
the heater causes the heat-sensitive element to
relays can be reset immediately after tripping.
open the overload relay contacts. This breaks the
Thermal overload relays must be allowed to cool
circuit through the operating coil of the main
a minute or longer before they can be reset. The
contactor. The motor is also disconnected from
type of overload reset is manual, automatic, or
the power supply. Since it takes time for parts to
electric.
heat, the thermal overload relay has an inherent
The manual, or hand, reset is located in the
time delay. This permits the motor to do
controller enclosure that contains the overload
maximum work at any reasonable current. It does
relay. This reset usually has a hand-operated rod,
this only as long as the motor is not being
lever, or button that returns the relay tripping
overheated. When it is, the overload relay
mechanism to its original position.
disconnects the motor.
The automatic type of reset, usually a spring-
There are four types of thermal overload
or gravity-operated device, resets the overload
relays: solder pot, bimetal, single metal, and
relay without the help of an operator. The electric
induction. The heat-sensitive element of a
reset is operated by an electromagnet controlled
SOLDER-POT type is a cylinder inside a hollow
by a push button. This form is used when it is
tube. The cylinder and tube are normally held
desired to reset an overload relay from a remote
together by a film of solder. If an overload
operating point.
occurs, the heater melts the solder (this breaks the
bond between the cylinder and tube). Then the
SHORT CIRCUIT PROTECTION.--Over-
tripping device of the relay is released. After the
load relays and contactors are usually not designed
relay trips, the solder cools and solidifies. You
to protect motors from currents greater than
can then reset the relay.
about six times the normal rated current of ac
In the BIMETAL type, the heat-sensitive
motors. Since short-circuited currents are much
element is a strip or coil of two different metals.
higher, protection against short circuits in motor
They are fused together along one side. When
controllers is obtained through other devices.
heated, the strip or coil deflects because one metal
Navy practice is to protect against these short
expands more than the other. The deflection
circuits with CBs placed in the power supply
causes the overload relay contacts to open. The
system. In this way, both the controller and motor
heat-sensitive element of the SINGLE-METAL
are protected. The cables connected to the
type is a tube around the heater. The tube
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