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DC Contactor Basics

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.

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.

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 ofmaintenance 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.