Understanding Resistance Welding Stages and SCR Contactor Operation

Resistance welding is a process where two or more metal surfaces are joined by heat obtained from resistance to electric current flow. The process typically involves several distinct stages, each controlled precisely to ensure a strong and reliable weld. A key component in controlling the welding current is the SCR (Silicon Controlled Rectifier) contactor.

Key Stages in Resistance Welding

The standard resistance welding cycle includes the following stages:

• Squeeze Time: This is the period during which the welding electrodes come together and apply pressure to the parts being welded. This pressure is maintained before the welding current is initiated. The purpose is to ensure good contact between the workpieces and the electrodes, which is essential for proper current flow and heat generation. No current flows during this stage.
• Weld Time: This is the critical period when the welding current flows through the workpieces. The electrical resistance at the interface between the two metal surfaces generates heat, causing the metal to reach a plastic or molten state, forming the weld nugget. The SCR contactor is responsible for switching on the high welding current during this specific time.
• Hold Time: After the weld time, the welding current is switched off, but the electrodes remain closed, maintaining the applied pressure. This allows the weld nugget to cool and solidify under pressure, which helps to consolidate the weld and prevent cracking or distortion.
• Off Time: This is the time after the electrodes open and the workpiece is removed, before the start of the next welding cycle. This allows for reloading the machine or repositioning the workpiece.

SCR Contactor Function in Resistance Welding

The SCR contactor acts as a high-speed switch that controls the flow of the large welding current. For the welding current to flow and heat the material, the SCR contactor must be closed (turned on). Based on the stages described above, the current flow occurs only during the Weld time. Therefore, the SCR contactor will close and remain closed only during the weld time to allow the necessary current to pass through the joint.

Let's consider the options in relation to the SCR contactor closing:

• Squeeze time: Current is off. SCR contactor is open.
• Hold time: Current is off. SCR contactor is open.
• Weld time: Current is on. SCR contactor is closed.
• Weld as well as off time: Current is on during weld time but off during off time. SCR contactor is closed during weld time and open during off time. The contactor is not closed throughout both periods.

Therefore, the SCR contactor closes specifically during the Weld time.