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fit the motor it is intended to protect, it can be reset after
circuits the reactor to apply line voltage to the motor.
The reactor controller is not widely used for starting
tripping so the motor can be operated again with
large ac motors. It is smaller than the closed-transition
overload protection. Some controllers feature an
emergency-run button that enables the motor to be run
compensator and does not have the high transition
without overload protection during an emergency.
currents that develop in the open-transition
compensator.
Thermal Overload Relays
TYPES OF MASTER SWITCHES
A master switch is a device, such as a pressure or a
The thermal overload relay has a heat-sensitive
thermostatic switch, that governs the electrical
element and an overload heater that is connected in
operation of a motor controller. The switch can be
series with the motor load circuit. When the motor
manually or automatically actuated. Drum, selector, and
current is excessive, heat from the heater causes the
push-button switches are examples of a manual master
heat-sensitive clement to open the overload relay
switch. The automatic switch is actuated by a physical
contact. This action breaks the circuit through the
force, not an operator. Examples of automatic master
operating coil of the main contractor and disconnects the
switches include float, limit, or pressure switches.
motor from the power supply. Since it takes time for the
parts to heat up, the thermal overload relay has an
Depending on where it is mounted, a master switch
inherent time delay, which allows the motor to draw
is either local or remote. A local switch is mounted in
excessive current at start without tripping the motor.
the controller enclosure; a remote switch is not.
To make a coarse adjustment of the tripping current
Master switches may start a series of operations
of thermal overload relays, change the heater element.
when their contacts are either closed or opened. In a
Fine adjustment depends on the type of overload relay.
momentary contact master switch, the contact is closed
To make a fine adjustment, change the distance between
(or opened) momentarily; it then returns to its original
the heater and the heat-sensitive element. An increase in
condition. In the maintaining contact master switch, the
this distance increases the tripping current. You can
cent act does not return to its original condition after
make another form of adjustment by changing the
closing (or opening) until it is again actuated. The
distance the bimetal strip has to move before the
position of a normally open or normally closed contact
overload relay contact is opened. Check the related
in a master switch is open or closed, respectively, when
technical manual for additional information and
the switch is de-energized. The de-energized condition
adjustments.
of a manual controller is considered to be in the off
position.
Thermal overload relays must be compensated; that
is, constructed so the tripping current is unaffected by
OVERLOAD RELAYS
variations in the ambient (room) temperature. Different
means are used for different types. Refer to the technical
Nearly all shipboard motor controllers provide
manual furnished with the equipment on which the
overload protection when motor current is excessive.
controller is used for information on the particular form
This protection is provided by either thermal or
of compensation provided. There are four types of
magnetic overload relays, which disconnect the motor
thermal overload relays: solder pot, bimetal, single
from its power supply, thereby preventing the motor
metal, and induction.
from overheating.
SOLDER POT. The heat-sensitive element of a
Overload relays in magnetic controllers have a
solder-pot relay consists of a cylinder inside a hollow
normally closed contact that is opened by a mechanical
tube. The cylinder and tube are normally held together
device that is tripped by an overload current. The
by a film of solder. In case of an overload, the heater
opening of the overload relay contact de-energizes the
melts the solder (thereby breaking the bond between the
circuit through the operating coil of the main contactor,
cylinder and tube) and releases the tripping device of the
causing the main contactor to open, securing power to
relay. After the relay trips, the solder cools and solidifies.
the motor.
The relay can then be reset.
Overload relays for naval shipboard use can usually
BIMETAL. In the bimetal relay, the heat-sensitive
be adjusted to trip at the correct current to protect the
element is a strip or coil of two different metals fused
motor. If the rated tripping current of the relay does not
together along one side. When heated, the strip or coil
1-3

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