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DC
Contactor Functionality
Trombetta DC contactors provide a robust and economical
means to switch high currents in low voltage DC circuits.
These DC contactors are suited for applications involving
voltages up to 48VDC. Current capacity varies by family with
inrush capacity reaching 800 amps and continuous carry
capacity reaching 225 amps for the larger units. The DC
contactors allow these high currents to be turned on and off
by switching current to the DC contactor's coil. Coil
current requirements range from less than .5 amps for
continuous duty 48 volt DC contactors to several amps for
low voltage intermittent duty DC contactors. It is generally
the case that the coil voltage is derived from the same
source as the load current and therefore coil voltage rating
equals the voltage level being switched by the contacts.
This is not a requirement however.
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DC
Contactor Construction and Basic Operation
DC Contactors comprise a high current switch and a solenoid
in a single enclosure. The switch provides the desired
function, to turn current flow on and off. The solenoid
serves as the actuator for the switch, thereby allowing the
switch to be controlled remotely via a light duty (low
current) control device and light gage wiring. Most
commonly, the switch portion is a Normally Open (NO) switch
of the Single Pole Single Throw (SPST) variety. In operation
the switch contacts are open with no power applied to the
solenoid and are actuated to the closed condition when power
is applied to the solenoid. The other common but less used
configuration involves a switch configuration that has two
contact sets, one NO and one Normally Closed (NC). This
configuration is commonly referred to as Single Pole Double
Throw (SPDT). In an SPDT unit, one contact set, the NC set,
is closed with no power applied to the solenoid while the
other, the NO set, is open. When power is applied to the
solenoid, the NC contacts open and the NO contacts close.
SPDT is not an entirely accurate description for this
contact configuration. However in most cases where these are
used, one terminal each from the NO and NC contact sets are
wired together resulting in the same functionality as an
SPDT switch, hence the common acceptance of that
designation. SPDT contactors are most often used in sets of
two to compose a motor reversing control.
Trombetta Reversing Polarity (RP) series DC contactors
comprise two SPDT contactors in one common housing. This
provides motor reversing control with one device and no
extra wiring. Polarity of output and hence motor direction
is selected by applying power to the appropriate solenoid
coil. With no power applied to either of the two coils,
there is no power output to the motor.
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DC
Contactor Application Considerations
Accurately matching a DC contactor to an application
requires proper attention to a number of details that define
conditions of operation. These generally fall under the
three major divisions of voltages, currents and
environmental factors. All three involve very important
operating parameters. Defining the parameters can range from
a simple to a complex task. Unfortunately, the task often
involves complexities that are not immediately apparent.
Addressing voltages must give consideration to the voltage
being switched by the contacts and the voltage applied to
the coil. These voltages need to be defined not only in
their nominal condition but also at their extremes. Voltage
is often a very dynamic parameter affected not only by
system design but also by state of maintenance and dynamic
parameters such as the current load imposed on the system at
any given moment. Furthermore, when dealing with the
extremes, consideration must be given to additional
influential factors such as duration of extreme conditions
and also what is the worst-case status of the other
operational parameters that might occur coincident with
operation at voltage extremes. For example, in a 12 volt
(nominal) engine starting application the voltage applied to
the coil of the solenoid can momentarily dip into the
proximity of 6 volts during starter motor inrush when
contact current load is maximal. Inductive loads, which are
very common, impose another voltage dynamic, the voltage
imposed on the contacts during opening rises to a magnitude
significantly greater than the nominal system voltage.
Coil current requirements for the solenoid portion of the DC
contactor, after brief review, typically are not of
significant concern because the device controlling coil
current is found to be more than amply rated. Usually
consideration for current focuses on the current to be
switched by the DC contactor. It is necessary to
characterize the dynamics of the current to be controlled.
Usually the inrush current, the current occurring
immediately upon closure of the contacts is most critical.
It is necessary to know the peak magnitude it rises to and
the time it takes for the current to rise to its peak value.
Sustained current load imposed on the contacts may be
another parameter of major importance as well as the current
being carried at the moment the contacts are required to
open. These three current parameter subgroups, inrush,
sustained carry and break (or interrupt) relate most
directly to issues of contact welding, contact temperature
rise and contact erosion respectively. Their relative
importance varies with the type of application.
Environmental considerations need to address operating
temperatures, especially the extremes, humidity, exposure to
splash and spray of contaminants and what those contaminants
might include, as well as shock and vibration
parameters.
Adequate definition of an application specification can be a
challenging task. Significant effort in creation of the
application specification is justified because its' accuracy
is of critical importance in achieving satisfactory
durability and longevity with the most economical design.
Trombetta can provide expert assistance in this most
important activity.
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DC
Contactor Application Guidelines
As with most electrical and electronic apparatus,
cool and dry is best, albeit not always practical. As much
as is practical, the DC contactor should be mounted away
from sources of heat and where the potential for splash and
spray is minimal. In applications where pressure washing
occurs, if at all possible, mount the DC contactor in an
area where it will not be hit with direct spray. Pressure
washers are very capable of driving water and detergents
through minuscule passages into areas from which it is
difficult or impossible to subsequently drain. They are also
capable of deforming or displacing elastomer seals to the
point of driving solution past them. On outdoor equipment,
unless using a fully sealed DC contactor, it is recommended
not to mount in locations that are directly open to the
ground below and restricted from airflow above. These
locations tend to trap humidity rising from the ground. As
the DC contactor temperature changes due to self heating
from operation or from outside influence, air is exchanged
between the interior of the DC contactor and its'
surroundings. As the DC contactor heats, some air is
evacuated. Later when it cools, internal pressure falls and
air is drawn back inside bringing moisture due to humidity
with it. With sufficient temperature drop, the entrained
moisture will condense out leaving liquid in the solenoid.
What gets inside often stays inside because the temperature
doesn't rise adequately to re-vaporize the liquid and drive
it out again. On equipment running on turf, the moisture in
the air may contain traces of corrosive elements form
fertilizers or other chemical treatments. Mount the DC
contactor in a location where it is not exposed to severe
vibration and shock. Horizontal mounting results in the
least risk of shock inducing unintentional movement of the
contacts. Where imposition of significant shock is possible
it is usually best to mount the contactor in a manner such
that the most significant shock loads are transverse to the
axis of contact motion. Where that is not practical it is
usually best to mount in a manner that the most significant
shock load will act in a direction that influences the
contacts in the opening direction. This is to minimize risk
of unintentional closure of the contacts. When wiring
connections to the control (coil) terminals use robust
terminals with appropriate matching of the terminal crimp
feature to the wire being used. Follow the terminal
manufacturers recommendations for crimping tools and
techniques. Use a wire gage sufficient to limit voltage loss
in the coil circuit wiring to 1Ž2 volt or less. Usually 18
gage wire is sufficient to accomplish this. When using DC
contactors with grounded coils, it is critically important
that the ground path connection be high integrity and robust
at time of installation and that conditions are such that
the integrity of the connection will not degrade over time.
Factors that can hamper ground connections include paint or
corrosion between surfaces and loose fasteners. When wiring
connections to the high current terminals, use wire with a
current rating appropriate for the sustained current to be
carried. Use appropriate size ring terminals and follow the
terminal manufacturers recommendations for crimping tools
and techniques. Insufficiently rated or poorly installed
wiring and terminations can cause excessive voltage drop
that results in heat being driven into the DC contactor.
Connections that run hot tend to self degrade such that the
losses and related temperature rises worsen with each
electrical cycle. Over time excessive temperature rise may
distort or destroy the insulation materials around the high
current terminals, thereby causing
failure.
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