SCR Power Controllers

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.