Transformers
Voltage control transformers are used in electrical circuits that require constant current or constant voltage with a low power or volt-amp (VA) rating. Filtering devices, such as capacitors, are used to minimize the variations in the output and voltage transformers are used to stabilize the brief periods of overload or inrush current. This momentary inrush current occurs when electromagnetic components, such as contactors, relays, timers and solenoids, are energized at start up. When start up occurs, most electromagnetic devices require anywhere from three to ten times their normal operating current for thirty to fifty milliseconds. Voltage stability is required during this time.
Low voltages are typically used in control circuits. These types of control circuits can be found in a control room, on a starter panel door or on the machine itself. It would be a safety hazard to have high voltages used in these applications or to have densely packed control wiring carrying high voltages inside a starter panel. For these reasons, low voltages are used.
Voltage control transformers are used in industrial starter circuits where the main circuit voltage is too high for use in the control circuit and where a separate control circuit feed would not be practical. Power can be tapped off of the main feed and passed through a control transformer which could then supply the lower voltage needed. Heavy electrical machinery run on higher voltages, however, the contactors that start the machinery operate at much lower voltages. Instead of running separate cables or installing extra bus bars, the main circuit voltage can be stepped down using a control transformer to the lower voltage required.
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