MDA AUG-SEP 2018 FINAL AUG-SEP 2018 DIGITAL | Page 29

Article
Some basics about
Contactor:
Contactor is basically a switching device
and is used for switching the loads
where frequency of switching loads i.e.
number of switching operations are very
high. Any basic contactor will consist of
fixed contacts, moving contacts, moving
armature, moving core, fixed core and
a coil. Moving contacts, armature and
moving core are connected to each
other, held off by using spring over the
fixed core.
As soon as supply is given to the
coil of the contactor, fixed core gets
magnetised due to electromagnetic
effect and it pulls the moving core,
armature and hence, moving contacts
towards the fixed
contacts to close the
contactor and power
supply is fed to the
load. As soon as
supply is removed to
coil, spring between
the moving and
fixed core forces
the moving core to
move away from
fixed core and as
a result, moving
contacts moves away
from fixed contacts and circuit will get
disconnected. Graphical representation
of contactor
Arc formation during
opening of contacts in AC
& DC switching
As the contacts starts opening, the
air between the contacts acts as an
insulator and dielectric strength of
air causes the drop in the voltage.
This drop in voltage increases the
current momentarily, results in high
temperature and causes the contact
material to get melts and vaporize to
form the arc. As the contacts continues
to separate and dielectric of air medium
increases, current flow drop rapidly and
voltage increases as the current reaches
close to zero. This increase in voltage
will cause the ionization of surrounding
air and causes the arc to sustain for long
time. It is when contacts separated by a
wide gap that dielectric strength of the
air gap increases up to an extent that it
overcome the ability of arc to sustain
and hence, results on stop of current
flow finally. So basically, how efficiently
you are able to increase the dielectric
strength of air gap, while opening of
contacts in a contactor, is the one of
the key issues while designing any
contactor.
Arc quenching
DC current switching is more severe
as compared to AC as it does not get
benefit of current going through a zero,
as in the case of sinusoidalcycle of AC
current, which helps deterioration of
arc in AC contactor. So extinguishing of
arc in DC depends more on contactor
mechanism and technology used in it to
break the current flow.
A sustained arc increases the erosion of
contact material and more likely to get
the contact welded and it will put many
things at risk.
Depending upon the rating of contactor,
there are different methods, by which
arc can be quenched in a DC contactor.
Contactors upto 50A, 600VDC rating,
an increased air gap (i.e. increased
dielectric strength) can be achieved
by having a ‘strong spring’ to open
the contact. Here in this case, coil
of contactor also has to be equally
strong to keep the contactor in closed
condition against spring pressure.
Above method of arc quenching is not
sufficient for higher rating contactors,
there is a need to use other methods of
arc quenching. The most efficient one
is ‘Magnetic blow out’ method. Here a
stray magnetic field is created around
the area of contacts where arc is formed
& this magnetic field blows the arc into
arc chutes. The arc chutes are designed
in such a way that it breaks the arc paths
by increasing the dielectric resistance.
The arc chutes are held in its position by
leaf spring which resists violent shock
or vibration as mounted on equipment
such as overhead cranes etc.
DC mill duty contactors, having two
coils, normal closing coil & holdout
coil are known as DC inductive time
limit (Ltl) contactors. When closing
coil is energised,
the contactor will
not pick up until the
magnetism of holdout
coil has decayed
sufficientlyto permit
the contactor to close.
Ltl contactors are
used for controlling
the acceleration or
plugging
functions
in hoist & travel
applications.
DC contactors work in the most
challenging environment and they
are finding new application in wind
and solar, acting as a main switch for
solar panels to central inverter where
they need to break DC current at high
voltages.
DC switching is one of the most critical
applications and there is a need to use
tested products only that meet all the
requirements of IEC standards. Any low
cost contactor used here would carry
the risk of contacts getting welded and
putting the system & personal life at
risk.
BCH DC contactors already proven
for years in Steel, cement & crane
applications.
https://bchindia.com
august-september 2018 | Global MDA Journal | 29