Thyristor Fired Coilgun

Description: The thyristor-fired coilgun is a prevalent variant, primarily due to its high voltage and pulsed current capabilities. A diagram of the system designed for these experiments is presented in Figure 1. The thyristor utilized is an International Rectifier device, part number 50RIA100 (50 A / 1000 V). The pulsed current capability is rated at 1200 A for a half-sine pulse duration of 10 ms. This series of experiments investigates the effects of charging voltage and launch delta on projectile velocity and coilgun efficiency. The capacitance values that can be effectively used in this type of coilgun are limited by the characteristic that the thyristor cannot be turned off once it begins conducting current. This necessitates that the current pulse duration be similar to the projectile transit time; otherwise, suckback will significantly reduce muzzle velocity. Consequently, only 13,600 µF and 33,000 µF capacitors are employed. The current sensor is positioned in the capacitor ground return path to monitor capacitor discharge or in the coil current path to record the full coil current pulse. Placing the sensor in the coil current path results in the measuring equipment (PC oscilloscope and laptop) floating to an arbitrary voltage since no part of the system is grounded to earth potential. The charging voltage for these measurements is limited to 80 V. Two switches were considered for commutating the trigger pulse: a mercury-wetted relay and a microswitch. Unlike most mechanical contact switches, the mercury-wetted relay does not exhibit contact bounce, ensuring that the gate current is maintained without interruption during the critical turn-on phase. Figure 2 illustrates the rising edge of the trigger pulse. Figure 3 displays the initial portion of the trigger pulse voltage applied to the gate resistor, while Figure 4 presents the overall pulse. The intermittent trigger pulse produced by contact bounce is likely not problematic when the anode-cathode current pulses have low di/dt, typical of low-power coilguns. However, if the thyristor is operated close to its peak rated di/dt, a detailed examination of the gate pulse is necessary to prevent thermal failure due to excessive current density caused by insufficient gate charge diffusion.

The thyristor-fired coilgun operates by utilizing a thyristor to control the discharge of capacitors into the coil, generating a magnetic field that accelerates the projectile. The design incorporates critical components such as the thyristor, capacitors, a current sensor, and a trigger mechanism. The thyristor, specifically the 50RIA100, is capable of handling high voltage and current, making it suitable for this application. The capacitors, chosen for their effective capacitance values, provide the necessary energy storage for the coilgun operation.

The monitoring of the current is essential for evaluating the performance of the coilgun. By placing the current sensor in the discharge path, real-time data can be gathered to assess the efficiency of the energy transfer from the capacitors to the coil. The design considerations regarding the placement of the sensor are crucial, as floating measurements can introduce inaccuracies, particularly in high-voltage applications.

The use of a mercury-wetted relay for the trigger pulse is a key design feature, as it minimizes the risk of contact bounce, which can disrupt the gate current and negatively impact the thyristor's performance. This is especially important when operating at high di/dt values, where maintaining a stable gate pulse is critical to avoid thermal failure of the thyristor.

Overall, the design and operation of the thyristor-fired coilgun require careful consideration of component specifications, placement, and operational parameters to achieve optimal performance while ensuring reliability and safety.The thyristor fired coilgun is a common variant, due mainly to their high voltage and pulsed current capabilities. A diagram of the system that was designed for these experiments is given in fig 1. The thyristor is an International Rectifier device, part number 50RIA100 (50 A / 1000 V). The pulsed current capability is 1200 A for a half-sine pulse of 10 ms. This series of experiments investigates the effect of charging voltage and launch delta on the projectile velocity and coilgun efficiency. The value of capacitance that can be effectively employed in this type of coilgun is limited due to the fact that the thyristor cannot be turned off once it has begun to conduct current.

This requires that the current pulse be of similar duration to the projectile transit time, otherwise suckback will vastly reduce the muzzle velocity. In view of this only the 13, 600 uF and 33, 000 uF capacitors are used. The current sensor was placed in the capacitor ground return path to monitor the capacitor discharge, or in the coil current path to record the full coil current pulse.

Placing the sensor in the coil current path means that the measuring equipment (PC `scope and laptop) floats up to some arbitrary voltage since no part of the system is grounded to the earth potential. The charging voltage was limited to 80 V for these measurements. Two switches were considered for commutating the trigger pulse; a mercury-wetted relay and a microswitch.

Unlike most mechanical contact switches, the mercury-wetted relay doesn`t exhibit any contact bounce, so the gate current will be maintained without interruption during the critical turn on phase. Figure 2 shows the rising edge of the trigger pulse. Fig 3 shows the initial part of the trigger pulse voltage that is applied to the gate resistor, and fig 4 shows the overall pulse.

The intermittent trigger pulse produced by the contact bounce is probably not a problem when the A-K current pulses have low di/dt, such as those typical of low power coilguns. If, however, the thyristor is going to be operated close to its peak rated di/dt then the nature of the gate pulse needs to be examined in more detail, otherwise the device might fail thermally due to the excessive current density brought about by insufficient gate charge diffusion.