Design and Implementation of Explosive Opening Switches in Inductive Circuits

Self-excitation in pulsed duty alternators is utilized to meet energy and power density system requirements. In the eventually fielded system, a small energy source will be pulsed into the field coil to start the excitation process. At this point, power electronics on the ac output of the generator will control the charging of the field coil and invert the field energy safely back into the rotor inertia at the end of the discharge sequence (or in the event of a fault). A static rectifier was used in the selfexcitation process in the laboratory testing of a subscale prototype compulsator. Without control of this bridge, a simple cost effective method was devised to terminate the excitation process safely at the end of a generator discharge sequence or in the event of a fault. To safely dissipate the field energy an explosive opening switch in parallel with a resistive element was placed in series with the bridge output and field coil. On command, the opening switch would commutate current into the resistive element properly sized to terminate the excitation process and with sufficient thermal mass to dissipate the field coil energy. The test program for the compulsator was designed to increment the self-excitation current in small steps in the commissioning of the system. Near the end of the test program, it was observed that the opening switch was having difficulty commutating the current into the inductance of the dissipation resistor. A shunt capacitor was sized to limit the commutation voltage and allow the opening switch plasma time to extinguish before large voltage developed across the resistive element. This paper presents information on the sizing of the resistor and capacitor and test data on its performance in the compulsator self-excitation circuit. This work is important because future systems under the control of active power converters will still require a fail-safe element to dissipate field energy in the event of a control system fault.