Understanding Projection Welding and Its Power Requirements

Projection welding is a resistance welding process that uses raised sections—called projections—on one or both of the workpieces to concentrate the welding current into a small area. When the parts are pressed together and current is applied, the projections collapse, generating the heat needed to create a strong, localized weld nugget. This method is widely used in high‑volume manufacturing for automotive components (nuts, brackets, studs), electrical contacts, and appliance parts because of its speed, repeatability, and ability to weld multiple points in a single operation.

The quality of the weld depends heavily on the power supply’s ability to deliver precise, consistent current and force control. Unlike spot welding, where the electrode geometry determines the current path, projection welding relies on the shape and size of the projections to dictate the current density. A poorly selected power supply can lead to inconsistent nugget formation, excessive electrode wear, and even part deformation. Selecting the right unit requires a thorough understanding of the application’s electrical, mechanical, and thermal demands.

Core Electrical Parameters That Drive Power Supply Selection

Welding Current Capacity

Current is the primary variable that governs heating in resistance welding. The required current is determined by the material type, thickness, and the number of projections being welded simultaneously. For example, welding a 6‑mm‑thick steel bracket with multiple projections may require 20,000–40,000 amperes, while a thin electronic contact may need only 5,000–10,000 amperes. Manufacturers typically provide current‑range guidelines for different projection geometries. Always select a power supply that can deliver at least 20% more current than your calculated peak demand to allow for process upsets and electrode wear.

Voltage and Secondary Circuit Design

The secondary open‑circuit voltage (typically 2–20 VAC) must match the impedance of the weld circuit. Higher voltages are needed when welding materials with high electrical resistivity (such as stainless steel) or when the secondary cable length is long. A mismatch can cause arcing or insufficient heating. Modern power supplies allow voltage adjustment via tap changers or inverter frequency control, which helps optimize the energy transfer to the weld zone.

Duty Cycle and Thermal Management

The duty cycle is defined as the percentage of time the power supply can operate at its rated current within a specified time window (usually 1 minute). A 50% duty cycle means the unit can weld for 30 seconds and must rest for 30 seconds. For high‑production environments (>100 welds per hour), a duty cycle of 50% or higher is recommended. Inverter‑based supplies often achieve 70–80% duty cycles due to their efficient cooling systems. Ignoring duty cycle limitations leads to thermal overload, premature component failure, and unscheduled downtime.

Types of Power Supplies for Projection Welding

Transformer‑Based (AC) Power Supplies

Traditional AC power supplies use a step‑down transformer to produce high‑current, low‑voltage output. They are robust, relatively inexpensive, and well‑suited for simple, low‑frequency applications. However, they offer limited control over the current waveform (typically 50 or 60 Hz), which can make fine‑tuning difficult for sensitive materials. AC supplies are still common in projection welding of mild steel parts where process tolerance is wide.

Inverter‑Based Power Supplies

Inverter power supplies convert incoming AC to DC and then use high‑frequency switching (1,000–2,000 Hz) to precisely shape the welding current. This technology enables closed‑loop feedback, constant current or constant voltage modes, and very short weld times (as low as 0.5 ms). Benefits include:

  • Higher control accuracy – better for welding dissimilar metals or coated materials.
  • Reduced energy consumption – up to 30% lower than AC supplies.
  • Smaller footprint – transformer size is drastically reduced due to high‑frequency operation.
  • Improved weld consistency – the ability to compensate for voltage fluctuations and electrode degradation.

Most new projection welding stations in automotive and electronics industries now use inverter technology.

Hybrid and Adaptive Power Supplies

Some advanced systems combine a transformer with an inverter stage or add adaptive features such as:
- Pre‑weld checks – measure part resistance before firing to adjust energy.
- Dynamic current steering – balance current among multiple projections.
- Weld‑time modulation – change pulse shape to control heat‑affected zone.

These systems are ideal for complex assemblies (e.g., welding two nuts and a bracket in one cycle) where consistent quality across all joints is critical.

Control Features That Make a Difference

Current and Time Programmability

Look for a power supply that allows you to store at least 10–20 weld schedules. Each schedule should independently control weld current, pulse time, squeeze time, hold time, and off time. Programmable controllers let operators switch between different projection types without manual recalibration.

Feedback Modes – Constant Current vs. Constant Voltage

Constant current mode maintains the preset current regardless of changes in workpiece resistance (within limits). It is the preferred mode for projection welding because the projection collapse alters resistance significantly. Constant voltage mode can be useful when welding materials with stable resistance but is less common in projection applications.

Monitoring and Data Logging

Modern power supplies include real‑time monitoring of actual current, voltage, and power. Data logging capabilities (to a local display or via serial/ethernet) enable quality assurance teams to trace every weld. Some units automatically flag welds that fall outside user‑defined limits, reducing the need for destructive testing.

Interface and Connectivity

Consider units with a clear touch‑screen interface and support for industrial protocols (Ethernet/IP, Profinet, DeviceNet). Connectivity simplifies integration with robotic workcells or central production monitoring systems.

Mechanical and Integration Considerations

Physical Size and Cable Management

Compact inverter supplies save floor space and can be mounted near the welding station, reducing secondary cable length and associated losses. For systems that use long cables, ensure the power supply has sufficient output voltage to compensate for voltage drop.

Electrode Force System Compatibility

The power supply must synchronize with the press or actuator that applies the welding force. Pneumatic, servo, or hydraulic systems each have different response times. The controller should allow adjustment of squeeze delay (time between force application and current onset) to ensure the projection is properly seated before energy is delivered.

Cooling Requirements

Projection welding power supplies generate significant heat. Most medium‑to‑high current units require water cooling for the transformer, thyristors or IGBTs, and the secondary circuit. Verify that your facility can supply clean, regulated cooling water (typically at 5–15 L/min). Air‑cooled units exist for low‑current applications but are less common in projection welding.

Material‑Specific Selection Guidelines

Different alloys demand different power supply characteristics. Use the table below as a starting point:

  • Low‑carbon steel – AC transformer supplies are often sufficient; inverter supplies improve consistency.
  • Stainless steel – Requires higher voltage and shorter weld times; inverter with fast rise‑time is recommended.
  • Aluminum – Very high thermal and electrical conductivity; needs very high current and short pulse; only inverter or mid‑frequency DC supplies work reliably.
  • Coated materials (galvanized, plated) – Inverter with pre‑pulse and closed‑loop control helps break through coating without splatter.
  • Dissimilar metals – Requires adaptive control to balance heat generated in each part; hybrid supplies with independent current steering are preferred.

Calculating Your Power Supply Requirements – A Practical Example

Assume you need to weld a 5‑mm steel nut with two projections onto a 2‑mm sheet. A typical weld schedule might call for 18,000 A for 60 ms, with a 25% duty cycle expected for a production run of 120 parts per hour. A transformer‑based AC supply rated at 30 kVA with 50% duty cycle would likely suffice. However, if the part is aluminum, you might need 25,000 A with a 15‑ms pulse, requiring an inverter supply capable of 50 kVA peak and 60% duty cycle.

Always consult with the power supply manufacturer to validate your calculations. They can run simulations or provide reference data based on similar applications.

Reliability, Support, and Lifecycle Cost

While upfront cost is important, total cost of ownership includes energy efficiency, maintenance, and spare parts availability. Inverter supplies may cost 20–30% more than equivalent AC units but often pay back within 2 years through lower energy bills and reduced electrode dressings. Choose a brand with a local service network and readily available IGBT modules, capacitors, and control boards. For critical production lines, consider a supplier that offers remote diagnostic support.

Look for power supplies that comply with international safety standards (e.g., IEC 62135 for resistance welding equipment) and have overcurrent, overvoltage, and thermal protection built in. Customer reviews and independent equipment evaluations can provide insight into long‑term reliability.

The next generation of projection welding power supplies will incorporate:

  • Artificial intelligence to adjust welding parameters in real time based on part variation.
  • Predictive maintenance algorithms that alert operators to component degradation before failure.
  • Wireless connectivity for cloud‑based data analytics across multiple plants.

If you are planning a new line, investing in a smart power supply from the outset can provide a competitive advantage in quality tracking and process optimization.

Summary of Selection Steps

  1. Define the material, thickness, number of projections, and required weld time.
  2. Calculate peak current and average power demand.
  3. Determine required duty cycle based on production volume.
  4. Choose between AC, inverter, or hybrid based on precision needs and budget.
  5. Verify control features – programmability, feedback modes, logging, and connectivity.
  6. Assess mechanical integration – size, cooling, cable routing, and force system compatibility.
  7. Evaluate total lifecycle cost and manufacturer support.
  8. Perform a trial with a representative production part to confirm performance.

Proper power supply selection is not a one‑size‑fits‑all exercise. By working through the factors outlined above and engaging with experienced suppliers, you can build a projection welding process that delivers consistent, high‑quality joints with minimal downtime. For deeper technical reference, consult resources like the American Welding Society’s Resistance Welding Manual or the EWI’s technical briefs on projection welding.