When used in a welding machines they turn on (or off) the welding powerof the machine. A welder will tell you the lead is 'hot' (has weld poweravailable or cold (won't arc).
Millermatic 35 Welder Specs
The contactor is an electro-mechanical, remotely controlled switch. Whenused to control a welding power supply, it only degrades, when it 'opens' underload. When the arc is broken, with the power supply, the contacts (inside thecontactor) arc as they open. Eventually they burn black and will not conduct or they willweld together.
Most machines break the input AC line. Either a 2-Pole (Single Phase) or3-Pole (Three Phase) are the most common contactors normally found in the field. Note: Somemachines use a 4-Pole contactor, the Linde® UCC-305 Tig powersupply is the most common. The UCC-305 was one of the most dependable Tig power suppliesever made. The input power is single phase, but the contactor is a 4-Pole, 50 amps (OEMONLY) unit!
Some machines break the power between the the main power transformer andthe diode bridge. This is normally done for design reasons (Miller®CP-200 & CP-300 are the most common examples).
Millermatic 35 Welder Manual Generator
Many older Hobart® & Lincoln®power supplies used a contactor in the 'welding circuit'. They actually breakthe welding power output. These contactors were normally large with very large contacts.Many newer Mig power supplies (Esab® Migmaster®250, 251, & Millermatic ®200) use a three pole contactor,with the three legs connected in parallel, to break the welding power. This is a veryeconomical design, and very easy to replace.