Precise Pulse Energy Control and Repetition Rate Improvement of High Voltage Power Supply for Electromagnetic Forming
Electromagnetic forming uses the magnetic pressure from a rapidly discharged current pulse to deform metal workpieces at high velocity. The process can form, join, or cut metals with speeds and characteristics not achievable with conventional methods. The high voltage power supply that provides the pulse energy must enable precise control of the energy and support high repetition rates for production applications.
The electromagnetic forming process stores energy in a capacitor bank and discharges it through a coil near the workpiece. The discharge current generates a magnetic field that induces eddy currents in the conductive workpiece. The interaction between the magnetic field and the induced currents produces a magnetic pressure that accelerates the workpiece. The workpiece can reach velocities of hundreds of meters per second in microseconds.
The pulse energy determines the forming velocity and the deformation achieved. Higher energies produce higher velocities and greater deformation. The energy must be controlled precisely to achieve the desired forming result without over forming or under forming. The energy precision requirement depends on the process sensitivity and the tolerance on the formed dimensions.
The energy stored in the capacitor is proportional to the capacitance and the square of the voltage. For a given capacitor bank, the energy is controlled by the charging voltage. The power supply must charge the capacitor to the precise voltage required for the desired energy. Voltage accuracy directly translates to energy accuracy, with the relationship magnified by the square.
Charging precision requires accurate voltage measurement and control. The voltage measurement must be accurate across the operating range and stable over temperature and time. The control loop must settle to the target voltage without overshoot or oscillation. The final voltage must be maintained until the discharge is triggered.
Repetition rate determines the throughput in production applications. The time between discharges includes the charging time, the forming time, and any handling time. The charging time depends on the energy and the charging power. Higher charging power enables faster charging but requires larger power supply components.
The charging profile affects the charging time and the component stress. Constant current charging provides linear voltage increase with time. Constant power charging provides faster initial charging but decreasing rate as the capacitor voltage increases. Resonant charging can achieve very fast charging with high efficiency. The optimal profile depends on the requirements and constraints.
Thermal management limits the repetition rate. Each discharge dissipates energy in the coil, the workpiece, and the power supply components. The average power dissipation increases with repetition rate. The components must be cooled to maintain acceptable temperatures. The thermal design determines the maximum sustainable repetition rate.
The discharge circuit includes the capacitor, the switch, and the coil. The switch must handle the peak current, which can be tens to hundreds of kiloamperes. Spark gaps, ignitrons, and thyristors have been used for switching. Each technology has advantages and limitations for current handling, switching speed, repetition rate, and life. Solid state switches using multiple devices in parallel can achieve high current capability with good repeatability.
Pulse shaping can optimize the forming process. The current waveform affects the magnetic pressure profile. A single peak may be appropriate for simple forming. Multiple peaks can provide incremental forming with intermediate relaxation. The pulse shape is determined by the circuit parameters including the capacitance, inductance, and resistance. Adjustable parameters enable optimization for different applications.
Process control integrates the power supply with the forming system. The control system sets the charging voltage based on the desired energy, monitors the charging process, triggers the discharge at the correct time, and coordinates with material handling. Programmable logic controllers or computer based systems provide the flexibility for complex process recipes.
Quality control monitors the forming results and correlates them with the process parameters. Dimensional measurement verifies that the formed part meets specifications. Statistical analysis identifies the sensitivity of the dimensions to the energy and other parameters. This information guides the setting of energy tolerances and process control limits.
