By: David Howard
About Electrical ContactsLet's begin with the basics about contacts. It's also called a tip, point, finger, wedge, button and segment. A single or multi-pole product that contains movable and stationary contacts, often including screws, springs and possibly other hardware is called a kit or set.
A contact opens and closes (makes and breaks) to engage or interrupt an electrical circuit. It's made to be replaced. It wears electrically (burning) and mechanically. It is found in switches, relays, contactors and motor starters.
When electricity is supplied to the coil in the contactor, it acts like a magnet. This magnetic action pushes or pulls a magnetic plate that slides or claps the movable contact together with the stationary. Please see the illustration below.
Copper only is often the material of choice when making/breaking more than one time per hour. You will often find copper-only contacts in DC applications, particularly cranes. They don't appear to be copper, because in the great majority of cases, they have been plated. The plating can be with silver, nickel or cadmium. The plating helps prevent the build up of copper oxide (green rust), which is non-conductive.
When closed for long periods of time (8 hours) copper only contacts have a tendency to 'stick' or weld and need silver pads of varying thickness. Generally speaking, the more amps, the thicker the pad will be. Pure silver pads are found sometimes in DC applications. The material of choice, however, is most often silver-cadmium oxide. The mix is usually 85/15 or 90/10. These contacts are found in most AC contactors for NEMA size 00-6. A 50/50 silver tungsten mix is used for high-amp applications. The tungsten is very hard and often used for circuit breaker applications.
Below you'll find photos of the basic parts of a contactor. This particular example is a GE size 2, CR305D0** in the 100, 200, 300 Line. The replacement contact set is found on page 53 in the Repco Electrical Contact catalog. The photos are from the top down, as you would take the contactor apart. The sizes of the contactor and various parts are not in relational proportion to each other.
GE Contactor CR305D0**
Top Magnet Assembly
Lower Magnet Assembly
Top View - Movable Contact Support
Bottom View - Movable Contacts in Support
Stationary Contacts in Support
Repco Replacement 3 Pole, Size 2 Contact Set - 9723CG
Very informative and helpful. Thanks
5/15/2012 1:36:30 PM