Lincoln Electric SVM170-A Thermal and Overload Protection, Wire Feed Overload Protection

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E-6	THEORY OF OPERATION	E-6

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THERMAL AND OVERLOAD PROTECTION

The POWER MIG 255C has built-in protective ther- mostats that respond to excessive temperatures. One is located on the output choke. The other thermostat is located on the SCR heat sink assembly. They open the wire feed and welder output circuits if the machine exceeds the maximum safe operating temperature. This can be caused by a frequent overload, or high ambient temperature.

The thermostats are self-resetting once the machine cools sufficiently. If the thermostat shutdown is caused by excessive output or duty cycle and the fan is oper- ating normally, the power switch may be left on and the reset should occur within a 15 minute period. If the fan is not turning or the air intake louvers are obstructed, then the input power must be removed and the fan problem or air obstruction be corrected.

OVERCURRENT PROTECTION

The machine will automatically reduce the output if the load on the machine exceeds 300 to 320 amperes. This protects the welding power SCR’s from excessive short circuit currents and from exceeding their temper- ature rating before the thermostats can react.

WIRE FEED OVERLOAD PROTECTION

The POWER MIG has solid state overload protection of the wire drive motor. If the motor becomes over- loaded, the protection circuitry turns off the wire feed speed and gas solenoid. Check for proper size tip liner, and drive rolls, for any obstructions or bends in the gun cable, and any other factors that would impede the wire feeding. To resume welding, simply pull the trigger. There is no circuit breaker to reset, as the protection is done with reliable solid state electronics.

POWER MIG 255C

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Lincoln Electric SVM170-A service manual Thermal and Overload Protection, Wire Feed Overload Protection

Contents Power MIG 255C Safety California Proposition 65 WarningsElectric Shock can kill Welding Sparks can cause fire or explosion IiiPrécautions DE Sûreté Sûreté Pour Soudage a L’ArcMaster Table of Contents For ALL Sections Installation Table of Contents Installation SectionTechnical Specifications Power MIG 255C Input POWER, Grounding and Connection Diagrams Safety PrecautionsUncrating the Power MIG 255C LocationInstallation Figure A.2 Triple Voltage Machine Input Connections Shielding GAS GUN and Cable InstallationAuxiliary Power Receptacles Table of Contents Operation Section OperationOPERATIONB-2 Wear eye, ear and body protec- tionWelding Capability Product DescriptionRecommended Processes and Equipment LimitationsDescription of Controls Wire Size Conversion PartsWire Drive Roll Drive and Idle Roll Sets Procedure for ChangingWire Reel Loading Readi REELS, Spools or Coils Wire Drive Configuration Feeding Wire ElectrodeIdle Roll Pressure Setting To Start the WelderFigure B.4 Instructions to Enter Fast RUN-IN When Timer Option is not installedInstructions to Enter Slow RUN-IN Making a WeldFAN Control Avoiding Wire Feeding ProblemsWire Feed Overload Protection Input Line Voltage ProtectionWelding Thermal Overload Protection Overcurrent ProtectionAccessories Table of Contents Accessories SectionK363P READI-REEL Adapter Drive Roll KitsMagnum GUN Connection KIT Dual Cylinder Mounting KIT K1702-1K1738-1 Spool Gun Hanger Accessory Operating Instructions for Timer KITConnecting the Spool GUN to the Power MIG 255C Spool GUNMaking a Weld with the Prince XL Spool GUN Making a Weld with the Magnum SG Spool GUNSG Spool Gun Wire Feed Speed Power MIG Maintenance Table of Contents Maintenance SectionContact TIP and GAS Nozzle Installation GUN Tubes and Nozzles General MaintenanceGUN Cable Cleaning Liner Removal and ReplacementGUN Handle Disassembly Table D.1MAINTENANCED-4 English Metric DescriptionTable of Contents Theory of Operation Section Theory of OperationInput Line Voltage Main TransformerOutput Rectification Feedback ControlConstant Voltage Output Figure E.4 Constant Voltage OutputWire Drive Motor FeedbackWire Feed Overload Protection Thermal and Overload ProtectionSCR Operation Figure E.6 SCR OperationPower MIG 255C Troubleshooting and Repair Table of Contents Troubleshooting and Repair Section2TROUBLESHOOTING and REPAIRF-2 HOW to USE Troubleshooting GuidePC Board Troubleshooting Procedures Troubleshooting Guide Output ProblemsTrolyte in these capacitors is Problems Possible AreasSymptoms Misadjustments Course of Action Toxic. Avoid contact with anyPerform the SCR Rectifier Function Problems Wire Feeding Problems Welding Problems Welding Problems Test Description Test ProceduresMain Transformer Test Materials NeededTest Procedure ON/OFF Power Switch will be hot during these tests115VAC 16J5 8J5206 3J5 Rectifier Diode Bridge Test 16TROUBLESHOOTING and REPAIRF-16 Figure F.2 G3521 Rectifier Diode Bridge Location208 206 28 VAC 209 354 206 36 VDCPower MIG 255C Static SCR Rectifier Assembly Test Static SCR Rectifier Assembly Test Active SCR Rectifier Assembly Test Power MIG 255C Active SCR Rectifier Assembly Test Active SCR Rectifier Assembly Test Figure F.7 Location Leads X2 Figure F.8 Wire Drive Motor and Tachometer Feedback Test Wire Drive Motor and Tachometer Feedback Test 4J1Test for Supply Voltage to Tachometer 515B 206B 15 VDC 5J1 1J1Test for Feedback Voltage to Control Board 555 206B To 3.5 VDC 6J1 1J1Normal Open Circuit Voltage Waveform VoltsMachine Loaded to 250 Amps AT 26 VDC Scope Settings Typical Output Voltage Waveform Machine LoadedMachine Loaded to 220 Amps AT 22 VDC Scope Settings Troubleshooting and Repair Typical SCR Gate Voltage Waveform Power MIG 255C Description Component Replacement ProceduresControl PC Board Removal and Replacement Removal and Replacement ProcedureFigure F.11 Control Board Mounting Power MIG 255C Wire Drive Assembly Removal and Replacement Wire Drive Assembly Removal and Replacement Procedure Power MIG 255C SCR Output Rectifier Removal and Replacement SCR Output Rectifier Removal and Replacement Figure F.15 Right Heat Sink Lead Disconnection Power MIG 255C Capacitor Bank Removal and Replacement Capacitor Bank Removal and Replacement Main Transformer and Output Choke Removal and Replacement 52TROUBLESHOOTING and REPAIRF-52 Main Transformer and Output ChokeTOC Power MIG 255C FAN Motor Assembly Removal and Replacement FAN Motor Assembly Removal and Replacement Open Circuit Voltage Wire Speed RangeRetest After Repair Input Volts/HertzPower MIG 255C Electrical Diagrams Section Section G-1Electrical Diagrams Wiring Diagram Entire Machine Code 11192 L12464Wiring Diagram Entire Machine Code 11218 L12506 Schematic Entire Machine G4752 Schematic Control PC Board G4823-2D1 Sheet Electrical Diagrams Electrical Diagrams PC Board Assembly Control G4824-2D1 Item Used with QTY Part Number DescriptionSchematic Display PC Board L10951 PowermigPC Board Assembly Display L10952 Lincoln Electric COSVM Error Reporting Form