<s>
A	O
contactor	B-Algorithm
is	O
an	O
electrically-controlled	O
switch	O
used	O
for	O
switching	O
an	O
electrical	O
power	O
circuit	O
.	O
</s>
<s>
A	O
contactor	B-Algorithm
is	O
typically	O
controlled	O
by	O
a	O
circuit	O
which	O
has	O
a	O
much	O
lower	O
power	O
level	O
than	O
the	O
switched	O
circuit	O
,	O
such	O
as	O
a	O
24-volt	O
coil	O
electromagnet	O
controlling	O
a	O
230-volt	O
motor	O
switch	O
.	O
</s>
<s>
Unlike	O
general-purpose	O
relays	O
,	O
contactors	B-Algorithm
are	O
designed	O
to	O
be	O
directly	O
connected	O
to	O
high-current	O
load	O
devices	O
.	O
</s>
<s>
Devices	O
switching	O
more	O
than	O
15	O
amperes	O
or	O
in	O
circuits	O
rated	O
more	O
than	O
a	O
few	O
kilowatts	O
are	O
usually	O
called	O
contactors	B-Algorithm
.	O
</s>
<s>
Apart	O
from	O
optional	O
auxiliary	O
low-current	O
contacts	O
,	O
contactors	B-Algorithm
are	O
almost	O
exclusively	O
fitted	O
with	O
normally	O
open	O
(	O
"	O
form	O
A	O
"	O
)	O
contacts	O
.	O
</s>
<s>
Unlike	O
relays	O
,	O
contactors	B-Algorithm
are	O
designed	O
with	O
features	O
to	O
control	O
and	O
suppress	O
the	O
arc	O
produced	O
when	O
interrupting	O
heavy	O
motor	O
currents	O
.	O
</s>
<s>
Contactors	B-Algorithm
come	O
in	O
many	O
forms	O
with	O
varying	O
capacities	O
and	O
features	O
.	O
</s>
<s>
Unlike	O
a	O
circuit	O
breaker	O
,	O
a	O
contactor	B-Algorithm
is	O
not	O
intended	O
to	O
interrupt	O
a	O
short	B-Application
circuit	I-Application
current	O
.	O
</s>
<s>
Contactors	B-Algorithm
range	O
from	O
those	O
having	O
a	O
breaking	O
current	O
of	O
several	O
amperes	O
to	O
thousands	O
of	O
amperes	O
and	O
24	O
V	O
DC	O
to	O
many	O
kilovolts	O
.	O
</s>
<s>
The	O
physical	O
size	O
of	O
contactors	B-Algorithm
ranges	O
from	O
a	O
device	O
small	O
enough	O
to	O
pick	O
up	O
with	O
one	O
hand	O
,	O
to	O
large	O
devices	O
approximately	O
a	O
meter	O
(	O
yard	O
)	O
on	O
a	O
side	O
.	O
</s>
<s>
Contactors	B-Algorithm
are	O
used	O
to	O
control	O
electric	O
motors	O
,	O
lighting	O
,	O
heating	O
,	O
capacitor	O
banks	O
,	O
thermal	O
evaporators	O
,	O
and	O
other	O
electrical	O
loads	O
.	O
</s>
<s>
A	O
contactor	B-Algorithm
has	O
three	O
components	O
:	O
</s>
<s>
The	O
contacts	O
are	O
the	O
current-carrying	O
part	O
of	O
the	O
contactor	B-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
motor	O
circuit	O
contactors	B-Algorithm
may	O
use	O
contacts	O
made	O
of	O
silver	O
with	O
cadmium	O
oxide	O
added	O
to	O
improved	O
durability	O
and	O
arc	O
resistance	O
.	O
</s>
<s>
Open-frame	O
contactors	B-Algorithm
may	O
have	O
a	O
further	O
enclosure	O
to	O
protect	O
against	O
dust	O
,	O
oil	O
,	O
explosion	O
hazards	O
and	O
weather	O
.	O
</s>
<s>
The	O
magnetic	O
blowouts	O
in	O
the	O
pictured	O
Albright	O
contactor	B-Algorithm
(	O
which	O
is	O
designed	O
for	O
DC	O
currents	O
)	O
more	O
than	O
double	O
the	O
current	O
the	O
contactor	B-Algorithm
can	O
break	O
,	O
increasing	O
it	O
from	O
600A	O
to	O
1,500	O
A	O
.	O
</s>
<s>
Sometimes	O
an	O
economizer	O
circuit	O
is	O
also	O
installed	O
to	O
reduce	O
the	O
power	O
required	O
to	O
keep	O
a	O
contactor	B-Algorithm
closed	O
;	O
an	O
auxiliary	O
contact	O
reduces	O
coil	O
current	O
after	O
the	O
contactor	B-Algorithm
closes	O
.	O
</s>
<s>
A	O
somewhat	O
greater	O
amount	O
of	O
power	O
is	O
required	O
to	O
initially	O
close	O
a	O
contactor	B-Algorithm
than	O
is	O
required	O
to	O
keep	O
it	O
closed	O
.	O
</s>
<s>
Economizer	O
circuits	O
are	O
nearly	O
always	O
applied	O
on	O
direct-current	O
contactor	B-Algorithm
coils	O
and	O
on	O
large	O
alternating	O
current	O
contactor	B-Algorithm
coils	O
.	O
</s>
<s>
A	O
basic	O
contactor	B-Algorithm
will	O
have	O
a	O
coil	O
input	O
(	O
which	O
may	O
be	O
driven	O
by	O
either	O
an	O
AC	O
or	O
DC	O
supply	O
depending	O
on	O
the	O
contactor	B-Algorithm
design	O
)	O
.	O
</s>
<s>
The	O
coil	O
may	O
be	O
energized	O
at	O
the	O
same	O
voltage	O
as	O
a	O
motor	O
the	O
contactor	B-Algorithm
is	O
controlling	O
,	O
or	O
may	O
be	O
separately	O
controlled	O
with	O
a	O
lower	O
coil	O
voltage	O
better	O
suited	O
to	O
control	O
by	O
programmable	B-Architecture
controllers	I-Architecture
and	O
lower-voltage	O
pilot	O
devices	O
.	O
</s>
<s>
Certain	O
contactors	B-Algorithm
have	O
series	O
coils	O
connected	O
in	O
the	O
motor	O
circuit	O
;	O
these	O
are	O
used	O
,	O
for	O
example	O
,	O
for	O
automatic	O
acceleration	O
control	O
,	O
where	O
the	O
next	O
stage	O
of	O
resistance	O
is	O
not	O
cut	O
out	O
until	O
the	O
motor	O
current	O
has	O
dropped	O
.	O
</s>
<s>
When	O
current	O
passes	O
through	O
the	O
electromagnet	O
,	O
a	O
magnetic	O
field	O
is	O
produced	O
,	O
which	O
attracts	O
the	O
moving	O
core	O
of	O
the	O
contactor	B-Algorithm
.	O
</s>
<s>
When	O
the	O
contactor	B-Algorithm
coil	O
is	O
de-energized	O
,	O
gravity	O
or	O
a	O
spring	O
returns	O
the	O
electromagnet	O
core	O
to	O
its	O
initial	O
position	O
and	O
opens	O
the	O
contacts	O
.	O
</s>
<s>
For	O
contactors	B-Algorithm
energized	O
with	O
alternating	O
current	O
,	O
a	O
small	O
part	O
of	O
the	O
core	O
is	O
surrounded	O
with	O
a	O
shading	O
coil	O
,	O
which	O
slightly	O
delays	O
the	O
magnetic	O
flux	O
in	O
the	O
core	O
.	O
</s>
<s>
Because	O
arcing	O
and	O
consequent	O
damage	O
occurs	O
just	O
as	O
the	O
contacts	O
are	O
opening	O
or	O
closing	O
,	O
contactors	B-Algorithm
are	O
designed	O
to	O
open	O
and	O
close	O
very	O
rapidly	O
;	O
there	O
is	O
often	O
an	O
internal	O
tipping	O
point	O
mechanism	O
to	O
ensure	O
rapid	O
action	O
.	O
</s>
<s>
The	O
first	O
to	O
make	O
contact	O
and	O
last	O
to	O
break	O
will	O
experience	O
the	O
greatest	O
contact	O
wear	O
and	O
will	O
form	O
a	O
high-resistance	O
connection	O
that	O
would	O
cause	O
excessive	O
heating	O
inside	O
the	O
contactor	B-Algorithm
.	O
</s>
<s>
This	O
technique	O
is	O
only	O
effective	O
if	O
the	O
contactors	B-Algorithm
disengage	O
in	O
reverse	O
order	O
that	O
they	O
engaged	O
.	O
</s>
<s>
Otherwise	O
the	O
damaging	O
effect	O
of	O
arcing	O
will	O
be	O
split	O
evenly	O
across	O
both	O
contactors	B-Algorithm
.	O
</s>
<s>
Another	O
technique	O
for	O
improving	O
the	O
life	O
of	O
contactors	B-Algorithm
is	O
contact	O
wipe	O
;	O
the	O
contacts	O
move	O
past	O
each	O
other	O
after	O
initial	O
contact	O
in	O
order	O
to	O
wipe	O
off	O
any	O
contamination	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
properly	O
applied	O
contactor	B-Algorithm
will	O
have	O
a	O
life	O
span	O
of	O
10,000	O
to	O
100,000	O
operations	O
when	O
run	O
under	O
power	O
;	O
which	O
is	O
significantly	O
less	O
than	O
the	O
mechanical	O
(	O
non-powered	O
)	O
life	O
of	O
the	O
same	O
device	O
which	O
can	O
be	O
in	O
excess	O
of	O
20	O
million	O
operations	O
.	O
</s>
<s>
Most	O
motor	O
control	O
contactors	B-Algorithm
at	O
low	O
voltages	O
(	O
600	O
volts	O
and	O
less	O
)	O
are	O
air	O
break	O
contactors	B-Algorithm
;	O
air	O
at	O
atmospheric	O
pressure	O
surrounds	O
the	O
contacts	O
and	O
extinguishes	O
the	O
arc	O
when	O
interrupting	O
the	O
circuit	O
.	O
</s>
<s>
Modern	O
medium-voltage	O
AC	O
motor	O
controllers	O
use	O
vacuum	O
contactors	B-Algorithm
.	O
</s>
<s>
High	O
voltage	O
AC	O
contactors	B-Algorithm
(	O
greater	O
than	O
1,000	O
volts	O
)	O
may	O
use	O
vacuum	O
or	O
an	O
inert	O
gas	O
around	O
the	O
contacts	O
.	O
</s>
<s>
High	O
voltage	O
DC	O
contactors	B-Algorithm
(	O
greater	O
than	O
600	O
V	O
)	O
still	O
rely	O
on	O
air	O
within	O
specially	O
designed	O
arc-chutes	O
to	O
break	O
the	O
arc	O
energy	O
.	O
</s>
<s>
Contactors	B-Algorithm
are	O
rated	O
by	O
designed	O
load	O
current	O
per	O
contact	O
(	O
pole	O
)	O
,	O
maximum	O
fault	O
withstand	O
current	O
,	O
duty	O
cycle	O
,	O
design	O
life	O
expectancy	O
,	O
voltage	O
,	O
and	O
coil	O
voltage	O
.	O
</s>
<s>
A	O
general	O
purpose	O
motor	O
control	O
contactor	B-Algorithm
may	O
be	O
suitable	O
for	O
heavy	O
starting	O
duty	O
on	O
large	O
motors	O
;	O
so-called	O
"	O
definite	O
purpose	O
"	O
contactors	B-Algorithm
are	O
carefully	O
adapted	O
to	O
such	O
applications	O
as	O
air-conditioning	O
compressor	O
motor	O
starting	O
.	O
</s>
<s>
North	O
American	O
and	O
European	O
ratings	O
for	O
contactors	B-Algorithm
follow	O
different	O
philosophies	O
,	O
with	O
North	O
American	O
general	O
purpose	O
machine	O
tool	O
contactors	B-Algorithm
generally	O
emphasizing	O
simplicity	O
of	O
application	O
while	O
definite	O
purpose	O
and	O
European	O
rating	O
philosophy	O
emphasizes	O
design	O
for	O
the	O
intended	O
life	O
cycle	O
of	O
the	O
application	O
.	O
</s>
<s>
The	O
current	O
rating	O
of	O
the	O
contactor	B-Algorithm
depends	O
on	O
utilization	O
category	O
.	O
</s>
<s>
NEMA	O
contactors	B-Algorithm
for	O
low-voltage	O
motors	O
(	O
less	O
than	O
1,000	O
volts	O
)	O
are	O
rated	O
according	O
to	O
NEMA	O
size	O
,	O
which	O
gives	O
a	O
maximum	O
continuous	O
current	O
rating	O
and	O
a	O
rating	O
by	O
horsepower	O
for	O
attached	O
induction	O
motors	O
.	O
</s>
<s>
NEMA	O
standard	O
contactor	B-Algorithm
sizes	O
are	O
designated	O
00	O
,	O
0	O
,	O
1	O
,	O
2	O
,	O
3	O
to	O
9	O
.	O
</s>
<s>
Contactors	B-Algorithm
for	O
medium-voltage	O
motors	O
(	O
greater	O
than	O
1,000	O
volts	O
)	O
are	O
rated	O
by	O
voltage	O
and	O
current	O
capacity	O
.	O
</s>
<s>
Auxiliary	O
contacts	O
of	O
contactors	B-Algorithm
are	O
used	O
in	O
control	O
circuits	O
and	O
are	O
rated	O
with	O
NEMA	B-Algorithm
contact	I-Algorithm
ratings	I-Algorithm
for	O
the	O
pilot	O
circuit	O
duty	O
required	O
.	O
</s>
<s>
Contactors	B-Algorithm
are	O
often	O
used	O
to	O
provide	O
central	O
control	O
of	O
large	O
lighting	O
installations	O
,	O
such	O
as	O
an	O
office	O
building	O
or	O
retail	O
building	O
.	O
</s>
<s>
To	O
reduce	O
power	O
consumption	O
in	O
the	O
contactor	B-Algorithm
coils	O
,	O
latching	O
contactors	B-Algorithm
are	O
used	O
,	O
which	O
have	O
two	O
operating	O
coils	O
.	O
</s>
<s>
It	O
includes	O
a	O
contactor	B-Algorithm
as	O
an	O
essential	O
component	O
,	O
while	O
also	O
providing	O
power-cutoff	O
,	O
under-voltage	O
,	O
and	O
overload	O
protection	O
.	O
</s>
<s>
Vacuum	O
contactors	B-Algorithm
utilize	O
vacuum	O
bottle	O
encapsulated	O
contacts	O
to	O
suppress	O
the	O
arc	O
.	O
</s>
<s>
As	O
the	O
contacts	O
are	O
encapsulated	O
,	O
vacuum	O
contactors	B-Algorithm
are	O
used	O
fairly	O
extensively	O
in	O
dirty	O
applications	O
,	O
such	O
as	O
mining	O
.	O
</s>
<s>
Vacuum	O
contactors	B-Algorithm
are	O
also	O
widely	O
used	O
at	O
medium	O
voltages	O
from	O
1000-5000	O
volts	O
,	O
effectively	O
displacing	O
oil-filled	O
circuit	O
breakers	O
in	O
many	O
applications	O
.	O
</s>
<s>
Vacuum	O
contactors	B-Algorithm
are	O
only	O
applicable	O
for	O
use	O
in	O
AC	O
systems	O
.	O
</s>
<s>
Vacuum	O
contactors	B-Algorithm
are	O
therefore	O
very	O
efficient	O
at	O
disrupting	O
the	O
energy	O
of	O
an	O
electric	O
arc	O
and	O
are	O
used	O
when	O
relatively	O
fast	O
switching	O
is	O
required	O
,	O
as	O
the	O
maximum	O
break	O
time	O
is	O
determined	O
by	O
the	O
periodicity	O
of	O
the	O
AC	O
waveform	O
.	O
</s>
<s>
A	O
mercury	B-Algorithm
relay	I-Algorithm
,	O
sometimes	O
called	O
a	O
mercury	O
displacement	O
relay	O
,	O
or	O
,	O
mercury	O
contactor	B-Algorithm
,	O
is	O
a	O
relay	O
that	O
uses	O
the	O
liquid	O
metal	O
mercury	O
in	O
an	O
insulated	O
sealed	O
container	O
as	O
the	O
switching	O
element	O
.	O
</s>
<s>
A	O
mercury-wetted	O
relay	O
is	O
a	O
form	O
of	O
relay	O
,	O
usually	O
a	O
reed	B-Algorithm
relay	I-Algorithm
,	O
in	O
which	O
the	O
contacts	O
are	O
wetted	O
with	O
mercury	O
.	O
</s>
<s>
These	O
are	O
not	O
considered	O
contactors	B-Algorithm
because	O
they	O
are	O
not	O
intended	O
for	O
currents	O
above	O
15	O
amps	O
.	O
</s>
<s>
When	O
a	O
series	O
of	O
contactors	B-Algorithm
is	O
to	O
be	O
operated	O
in	O
sequence	O
,	O
this	O
may	O
be	O
done	O
by	O
a	O
camshaft	B-Application
instead	O
of	O
by	O
individual	O
electromagnets	O
.	O
</s>
<s>
The	O
camshaft	B-Application
may	O
be	O
driven	O
by	O
an	O
electric	O
motor	O
or	O
a	O
pneumatic	O
cylinder	O
.	O
</s>
<s>
Before	O
the	O
advent	O
of	O
solid-state	O
electronics	O
,	O
the	O
camshaft	B-Application
system	O
was	O
commonly	O
used	O
for	O
speed	O
control	O
in	O
electric	O
locomotives	O
.	O
</s>
<s>
In	O
addition	O
to	O
their	O
current	O
ratings	O
and	O
rating	O
for	O
motor	O
circuit	O
control	O
,	O
contactors	B-Algorithm
often	O
have	O
other	O
construction	O
details	O
not	O
found	O
in	O
relays	O
.	O
</s>
<s>
Unlike	O
lower-powered	O
relays	O
,	O
contactors	B-Algorithm
generally	O
have	O
special	O
structures	O
for	O
arc-suppression	O
to	O
allow	O
them	O
to	O
interrupt	O
heavy	O
currents	O
,	O
such	O
as	O
motor	O
starting	O
inrush	O
current	O
.	O
</s>
<s>
Contactors	B-Algorithm
usually	O
have	O
provision	O
for	O
installation	O
of	O
additional	O
contact	O
blocks	O
,	O
rated	O
for	O
pilot	O
duty	O
,	O
used	O
in	O
motor	O
control	O
circuits	O
.	O
</s>
<s>
Relays	O
often	O
have	O
normally	O
closed	O
contacts	O
;	O
contactors	B-Algorithm
usually	O
do	O
not	O
(	O
when	O
de-energized	O
,	O
there	O
is	O
no	O
connection	O
)	O
.	O
</s>
