Current rating for an electromechanical contactor

  • #1
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I have a elctromechanical contactor (contactor is an electrically operated switch - It is larger version of relay) which has following specification,
Inductive full load current - 30A
Resistive full load current - 40A

Why is rated inductive current less than rated resistive current?
 

Answers and Replies

  • #2
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526
Could you supply the manufacturer and part number of the contactor? Is the controlled load AC or DC?

Generally speaking, inductive current rating is lower because inductive loads such as motors experience significant inrush current.
Another specification to pay attention to is the kW or HP power rating.

Contactors for DC loads require more scrutiny during selection, and are often designed specifically for DC operation.
 
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  • #5
779
526
I have a elctromechanical contactor (contactor is an electrically operated switch - It is larger version of relay) which has following specification,
Inductive full load current - 30A
Resistive full load current - 40A

Why is rated inductive current less than rated resistive current?
This has been answered in the posts above, but another, and perhaps more important question is, "What do you intend to use the contactor for?" A particular make and model contactor will be rated differently depending upon the application.

In the IEC world the nature of the application is called it's utilization category. Categories prefaced with "DC" describe DC load applications. AC-1 (switching AC to resistive loads), AC-3 (switching AC to squirrel cage induction motors in typical start-stop service), and AC-4 (same as AC-3, except the contactor must withstand locked rotor amp breaking currents encountered when the application includes inching and/or jogging service) are the most common. Transformer, capacitor, and various forms of lighting present different challenges to the contacts, and have their own utilization categories.

Unusual operating conditions must also be accounted for. For example, a manufacturer's specifications may be valid for 50 operations or less per hour, and a higher-rated contactor will likely be required to provide more frequent operation. Same goes for operation in high ambient temperature environments, and so on.

Siemens, ABB, Eaton-Cutler Hammer, Allen-Bradley, and other contactor manufacturers provide detailed information in their specifications, and typically offer additional sizing and application guidelines, for instance, "Low-Voltage Switchgear and Controlgear - An Application Guide", document # LVSAM-RM003A-EN-P from Allen-Bradley.
 
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