Thermal Overload Relays

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

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