W61 N14280 Taunton Ave. Cedarburg, WI 53012
Gemini's Modular firing circuits are designed to control the conduction periods of two SCR's that are connected to ac voltages which are 180 electrical degrees apart. Synchronization of firing is obtained from the ac voltages that supply the SCR's and their load. One of these unit circuits can be used to provide the SCR equivalent of a triac, by connecting two SCR's in an anti-parallel arrangement. One can also be used with a center-tapped transformer to provide full wave controlled dc output.
Multiple unit firing circuits can be used in a system to accomplish greater complexities of control. For example, two of these circuits can be used in a standard SCR bridge to produce full wave dc output. Three circuits will control a full wave three phase bridge for either ac or dc output. Six circuits can give 12 pulse control in a 6 phase system (or with delta-wye dual secondary transformers), when lower harmonic currents are necessary.
Each unit firing circuit provides a range of conduction that varies from 0 to 180 electrical degrees. On one of the models, the range of control corresponds to the zero crossings of the input line voltage. This circuit is useful for resistive loads or non-regenerative motor loads. With the other model, the range of control lags the zero crossings by approximately 30 electrical degrees. This characteristic is useful for regenerative power circuits and for certain types of loads.
Firing pulses are delivered by isolated pulse transformers, and consist of a train of pulses occupying the entire time from the beginning of the conduction period to the end. This is important, since the voltages in many power circuits will result in reverse polarity to the SCR's during part of the conduction period, and the train of pulses "waits" until the forward voltage appears and the SCR can be switched on. The high energy pulses provide enough power to bring even large SCR's into conduction, without the excess dissipation that would occur if a long duration dc signal of comparable magnitude would be used.
Factory adjustment sets the input voltage for full range of control at 0-6 volts dc. This range can be changed by the customer if a different input range is desired, with the caution that all units in a given system be adjusted to the same value to make sure that the individual phases "track".
A pair of terminals is provided to allow instant shut down of all pulses, in the event of a fault or for any other reason. If relay contacts are used for this purpose, they should be of the "dry circuit" type for reliability of this function.
To utilize these modular circuits, the power engineer should carefully analyze both the phase relationships and the phase sequence to determine the connections
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