Introduction
Welding Power Source = Equipment that provides electrical energy for welding
Key Functions:
- Convert input power to suitable welding voltage/current
- Provide appropriate output characteristics
- Control arc stability
- Enable process control
Power Source Types
By Output Type
1. Constant Current (CC) / Drooping Characteristic
- Current remains relatively constant despite voltage changes
- Steep voltage-current curve
- Used for: SMAW, GTAW
2. Constant Voltage (CV) / Flat Characteristic
- Voltage remains relatively constant despite current changes
- Flat voltage-current curve
- Used for: GMAW, FCAW, SAW
3. Constant Power
- Power (V×I) remains constant
- Specialized applications
Volt-Ampere Characteristics
Constant Current (CC) Characteristic
Curve: Steep downward slope
Behavior:
- Arc length changes → voltage changes significantly
- Current changes minimally
- Self-adjusting for manual processes
Example:
- Arc length increases → voltage increases
- Current decreases slightly
- Arc maintained
Applications:
- SMAW: Operator controls arc length manually
- GTAW: Precise current control needed
Advantages:
- Stable arc with varying arc length
- Suitable for manual welding
- Operator skill compensates for variations
Constant Voltage (CV) Characteristic
Curve: Nearly flat (slight downward slope)
Behavior:
- Arc length changes → current changes significantly
- Voltage changes minimally
- Self-regulating with wire feed
Self-Regulation:
- Arc length increases → current decreases
- Wire melts slower → wire feeds into arc
- Arc length returns to normal
Applications:
- GMAW: Continuous wire feed
- FCAW: Flux-cored wire
- SAW: Submerged arc
Advantages:
- Automatic arc length control
- Stable with continuous wire feed
- Consistent weld quality
Power Source Components
1. Transformer
- Steps down voltage (230V/400V → 20-80V)
- Steps up current (low → high amperage)
- AC output
2. Rectifier
- Converts AC to DC
- Diodes or thyristors
- Provides DC output
3. Inverter
- High-frequency switching (20-100 kHz)
- Compact, lightweight
- Precise control
- High efficiency
4. Control Circuit
- Regulates output
- Adjusts parameters
- Provides feedback control
Power Source Technologies
1. Transformer-Based (Conventional)
AC Transformer:
- Simple, robust
- Heavy, bulky
- Low cost
- AC output only
Transformer-Rectifier:
- AC transformer + rectifier
- DC output
- Heavier than inverter
- Reliable
Advantages:
- Robust, durable
- Simple maintenance
- Lower initial cost
Disadvantages:
- Heavy, bulky
- Less efficient
- Limited control features
2. Inverter-Based (Modern)
Process:
- AC input rectified to DC
- DC inverted to high-frequency AC (20-100 kHz)
- Stepped down by small transformer
- Rectified to DC output (if needed)
Advantages:
- Lightweight: 50-70% lighter
- Compact: Smaller transformer
- Efficient: 85-95% efficiency
- Precise control: Digital control
- Better arc characteristics: High-frequency output
- Multi-process: One machine, multiple processes
Disadvantages:
- Higher initial cost
- More complex electronics
- Sensitive to harsh environments
Duty Cycle
Duty Cycle = Percentage of 10-minute period machine can operate at rated output
Formula: Duty Cycle (%) = (Operating Time / Total Time) × 100
Example Ratings:
- 300A at 60% duty cycle
- Means: 6 minutes on, 4 minutes off in 10-minute period
Typical Duty Cycles:
- Industrial: 60-100%
- Professional: 40-60%
- Hobby: 20-40%
Relationship:
- Higher current → lower duty cycle
- Lower current → higher duty cycle
Calculation for Different Current: I₂ = I₁ × √(DC₁/DC₂)
Where:
- I₁, I₂ = Currents
- DC₁, DC₂ = Duty cycles
Open Circuit Voltage (OCV)
OCV = Voltage when no welding (no load)
Typical Values:
- SMAW: 50-80V
- GMAW/FCAW: 20-40V
- GTAW: 60-80V
Purpose:
- Facilitate arc starting
- Maintain arc stability
- Safety consideration (higher = more dangerous)
Safety:
- Lower OCV safer
- Standards limit OCV (typically <80V)
Arc Voltage
Arc Voltage = Voltage across arc during welding
Typical Values:
- SMAW: 20-30V
- GMAW: 15-35V
- GTAW: 10-20V
- SAW: 25-40V
Factors Affecting Arc Voltage:
- Arc length (longer → higher voltage)
- Shielding gas (affects ionization)
- Electrode type
- Current level
Dynamic Characteristics
Arc Ignition
Requirements:
- High OCV for easy starting
- Rapid current rise
- Stable arc establishment
Inverter Advantage:
- Fast response time
- Better arc starting
- Hot start feature
Arc Stability
Factors:
- Power source response time
- Output ripple
- Inductance/capacitance
Inverter Advantage:
- Low output ripple
- Fast response
- Stable arc
Short Circuit Response (GMAW)
Short Circuit Transfer:
- Wire touches pool → short circuit
- Current rises rapidly
- Wire melts, arc re-establishes
- Cycle repeats (50-200 times/second)
Power Source Requirements:
- Fast current rise during short
- Controlled current to prevent spatter
- Quick arc re-establishment
Inductance Control:
- Limits current rise rate
- Reduces spatter
- Adjustable on modern machines
Polarity
DC Polarity
DCEP (Direct Current Electrode Positive):
- Electrode connected to positive
- Also called "reverse polarity"
- Heat distribution: 70% electrode, 30% workpiece
- Used for: GMAW (most common), FCAW, some GTAW
DCEN (Direct Current Electrode Negative):
- Electrode connected to negative
- Also called "straight polarity"
- Heat distribution: 30% electrode, 70% workpiece
- Used for: GTAW (most common), some SMAW, SAW
AC Polarity
Alternating Current:
- Polarity alternates (50/60 Hz or higher)
- Balanced heating
- Used for: GTAW aluminum (oxide cleaning), SMAW
AC Balance Control (Advanced GTAW):
- Adjust positive/negative time ratio
- Optimize penetration vs cleaning
- Inverter technology enables this
Power Source Selection Criteria
1. Welding Process
- SMAW → CC
- GMAW/FCAW → CV
- GTAW → CC (with AC option for aluminum)
- SAW → CV
2. Material and Thickness
- Thin materials → lower current capacity
- Thick materials → higher current capacity
3. Duty Cycle Requirements
- Production welding → 60-100%
- Maintenance/repair → 40-60%
- Hobby → 20-40%
4. Portability
- Field work → lightweight (inverter)
- Shop use → any type
5. Power Supply
- Single phase (230V) → limited capacity
- Three phase (400V) → higher capacity
6. Budget
- Transformer → lower cost
- Inverter → higher cost, better features
7. Features Needed
- Multi-process capability
- Pulse welding
- Synergic control
- Data logging
Advanced Features (Inverter Technology)
Pulse Welding
- Alternating high/low current
- Better control
- Reduced heat input
- All-position spray transfer (GMAW)
Synergic Control
- Pre-programmed settings
- Select material and thickness
- Machine sets optimal parameters
- Simplified operation
Waveform Control
- Customize current waveform
- Optimize for specific applications
- AC balance, frequency control (GTAW)
Digital Display and Control
- Precise parameter setting
- Real-time monitoring
- Data logging
- Remote control capability
Hot Start (SMAW)
- Increased current at arc start
- Easier electrode starting
- Prevents sticking
Arc Force (SMAW)
- Increases current when arc shortens
- Prevents electrode sticking
- Better penetration
Soft Start
- Gradual current increase
- Reduces spatter
- Smoother arc start
Power Factor and Efficiency
Power Factor:
- Ratio of real power to apparent power
- Transformer: 0.5-0.7
- Inverter: 0.9-0.95
Efficiency:
- Transformer: 50-70%
- Inverter: 85-95%
Implications:
- Inverter: Lower electricity costs
- Inverter: Less heat generation
- Inverter: Smaller input cable size
Safety Considerations
Electrical Safety:
- Proper grounding
- OCV limits
- Insulation
- Overload protection
Thermal Protection:
- Overheat shutdown
- Duty cycle monitoring
- Cooling system
EMC (Electromagnetic Compatibility):
- Inverters generate high-frequency noise
- Proper shielding required
- May interfere with electronics