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A control transformer is a device used to transform or "step down" a high main circuit voltage to a lower voltage which is then used to operate the control or switching components of the main circuit. These devices are commonly used in industrial starter circuits where the main circuit voltage is not suitable for use in the control circuit and where a separate control circuit feed would not be practical. For example in a starter panel designed to start a 500 volt electric motor, the contactors and relays used to switch the motor on or off would typically use electromagnetic coils rated for a far lower voltage. To supply this voltage without the need for a separate power feed, power is tapped off the main incoming 500 volt feed and passed through a control transformer which would then supply the lower control circuit voltage.
Heavy electrical machinery that starts automatically or remotely generally makes use of contactors which rely on an electromagnetic force to close them to start the machinery. This force is created by an electric coil placed in the center of a laminated steel core. These coils are typically designed to operate at fairly low voltages, ranging from 110 volts to as low as 12 volts. As these machines themselves typically run on far higher voltages, this creates the need for a separate control voltage feed. Instead of having to run separate cables or install extra sets of bus bars, it is far simpler to use the main circuit voltage and step it down with a control transformer to the appropriate control voltage.
Low control circuit voltages are used for various reasons including the fact that parts of the control circuit include push buttons in a remote control room, on the starter panel door, and at the machine itself. It would not be wise to have high voltages used in these applications for obvious safety reasons. It is also undesirable to have densely packed control wiring carrying very high voltages inside the starter panel either. For these reasons, lower voltages are typically used in control circuits.
Another benefit of using a control transformer is the inherent stability of the voltage supplied from a transformer as well as its ability to handle extreme peaks in demand. When the start button on a motor starter is pushed and the contactor coil energizes, there is a very brief (typically 30 to 50 milliseconds) surge in current demand known as an "inrush current". This peak can exceed 10 times the normal current flow, and transformers handle these peaks far more efficiently than a conventional supply.
Using a control transformer to supply control power thus allows a lower, safer and more efficient control circuit voltage to be used in high working voltage applications. The excellent inrush current handling characteristics of transformer supplied power also makes for a more efficient power supply. Lastly, the use of lower voltages in a control circuit make for far safer use by workers using stop and start buttons in hazardous environments.