When considering contactors, DC contactors are a completely different animal than AC contactors. This articles will discuss two types of DC contactors - DC Motor contactors and DC Drive contactors. We will discuss how each operates and provide some examples of when you need each type in your application.
DC Motor Contactors
DC Motor Contactors are for use as main line, reversing or accelerating contactors in DC motor starters and controllers. Other applications include switching resistive heating and battery loads. All DC motor starter Contactors are solenoid operated for fast, constant, long-life performance. Generous size double break silver cadmium oxide contacts in combination with built-in permanent magnet blowouts assure long contact life and positive circuit interruption.These contactors ideal for straight-through wiring and for wire channel layouts. The Joslyn Clark 7400 Series falls within the line DC Motor Contactors. Learn more.
DC Drive Contactors are designed for DC adjustable speed drive circuits using silicon controlled rectifier (SCR) switching and other controls, where a combination of normally open and normally closed power contacts are required and built-in dynamic braking. In SCR switching circuits, the SCR is normally phased back prior to the contactor opening. Therefore, the N.O. contacts are not required to interrupt any appreciable current. Under abnormal conditions, such as failure of the SCR or the control circuit, the DC contactor may be required to interrupt up to 200% current. The N.O. contacts are equipped with permanent magnet blowouts to meet this requirement. Magnetic blowouts use blowout coils to lengthen and move the electric arc. These are especially useful in DC power circuits. AC arcs have periods of low current during which the arc can be extinguished with relative ease, but DC arcs have continuous high current. So, blowing them out requires the arc to be stretched further than an AC arc of the same current.
The dynamic brake N.C. contact is required to insert the dynamic brake resistor. Therefore, it must make up to 200% of rated current each time the DC contactor is de-energized. Jogging is accomplished at low voltage and current, and thus, the dynamic brake contact is rated at 1/3 of rated voltage and dynamic brake current. The permanent magnet blowouts provide this interrupting ability.
Joslyn Clark specializes in DC Drive Contactors and DC Motor Contactors. To learn more about our 5DP Series and 7400 Series, please download our data sheets.
Read more about DC Contactors.