Magnetic Overload Relays

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