BRIDGES

AC INPUT - AC OR DC OUTPUT
Typical AC to AC High Power Bridge	Medium Power AC to AC Bridge	Large Bridge with Firing Circuit
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Gemini Power bridges convert AC line power to controlled output, either AC or DC.  The technology is SCR, a type of  the general category of switching devices known as Thyristors.  These designs have proven themselves in applications stretching over a 30 year period, and have been used in the Paper, Steel, Plastic, Printing, Passenger Elevator, Conveyor, Machine Tool and many other industries.

Basic Converters come in three phase configurations, which can be combined to form 6 phase, 12 phase, or even 18 phase configurations, where low harmonic requirements dictate.  With the proper firing circuits, also available from Gemini, the converters can be either one or 2 quadrant in operation, with two converters working together for 4 quadrant performance.

Standard 3-Phase Power Converters
All converters rated up to 480volts input, except 12M04-00015 at 240volts.
600volts input available on special order.

AC to AC Converters
The Gemini AC to AC converters are a series of power controllers based on SCR technology. A converter consists of three separate paths, one for each phase, each one containing two SCRís connected in an anti-parallel, or back to back, arrangement. When a path is connected in series with a load, one of the SCRís conducts current in one direction and the other conducts current in the opposite direction. The load can be resistive, inductive, or the primary of a three phase transformer used to decrease the voltage, such as in electroplating use, or to increase the voltage, such as in applications requiring voltages as high as 300-500 Kvolts.

The control of the conduction is by phase control, with the conducting period of each SCR determined by a firing circuit. Gemini has "unit" firing circuits, where one is required for each of the three paths, with the customer providing the wiring from the firing circuits to the SCRís. Gemini also has a three phase firing circuit, where all three paths are controlled with one circuit. When the three phase firing circuit is used, the power controllers are available with the firing circuit mounted and wired to the group of SCRís. This is generally the preferred configuration, since it avoids mistakes in connection by the customer.

For a single bridge use, the three phase lines are connected to the 3 input terminals and the AC load is connected to the 3 output terminals. Synchronization of firing of the SCRís is obtained automatically when the firing circuit is connected to the cathode and gate of the SCRís. Control is by a dc voltage, and when the 3 phase firing circuit is used, a closed loop regulator is included, to be used or not, depending on the customerís requirements. When multiple bridges are used, the phasing and interconnections become more complex, and the factory should be consulted with details of the power arrangement, so that the proper connections can be determined.

Snubbers are advised to keep the possibility of firing the SCRís by excessive dv/dt to a minimum. The controllers with less than 400 amp rating have these snubbers included and wired in place. For the larger bridges, a series arrangement of a capacitor and a resistor is connected by the customer in parallel with each pair of SCRís. These capacitors should be of the "commutating" variety to avoid problems with the nature of the peak currents that result when an SCR is fired. If the customer is technically qualified, he may choose the values himself. If not comfortable with selecting his own values, Gemini recommends the use of 0.1 mfd, rated at 2000 volts for the capacitor and 200 ohms rated at 50 watts for the resistor, as generally acceptable values for any of the large bridges.

FIRING CIRCUITS

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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