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A control transformer steps down high main circuit voltage to a lower voltage for use in industrial start-up circuits. It provides a stable and efficient power supply for contactors and relays, and allows for the use of lower, safer control circuit voltages.
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 main circuit switching or control components. These devices are commonly used in industrial start-up circuits where the main circuit voltage is not suitable for use in the control circuit and where a separate control circuit power supply would not be practical. For example, in a starter panel designed to start a 500-volt electric motor, the contactors and relays used to turn the motor on or off typically use electromagnetic coils rated for a much lower voltage. To provide this voltage without the need for a separate supply, power is taken from the main 500 volt input supply and passed through a control transformer which would then supply the lower control circuit voltage.
Heavy electrical machinery that starts automatically or remotely generally use contactors that rely on an electromagnetic force to close them and start the machinery. This force is created by an electric coil placed in the center of a rolled steel core. These coils are usually designed to operate at fairly low voltages, ranging from 110 volts down to 12 volts. Since these same machines typically operate on much higher voltages, this creates the need for a separate control voltage supply. Instead of having to run separate wiring or install additional sets of bus bars, it is much easier to take 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 that parts of the control circuit include buttons in a remote control room, on the starter panel door, and on the machine itself. It would be unwise to use high voltages in these applications for obvious safety reasons. It is also undesirable to have tightly packed control cables carrying very high voltages within the starter panel. For these reasons, lower voltages are generally used in control circuits.
Another benefit of using a control transformer is the inherent stability of the voltage provided by a transformer, as well as its ability to handle extreme peaks in demand. When the start button on a motor starter is pressed and the contactor coil energises, there is a very brief (typically 30 to 50 milliseconds) peak in current demand known as the “inrush current”. This peak can exceed 10 times the normal current flow, and transformers handle these peaks much more efficiently than a conventional power supply.
Using a control transformer to supply control power therefore 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 the power supplied by the transformer also make for a more efficient power supply. Finally, the use of lower voltages in a control circuit makes for much safer use by workers using stop and start buttons in hazardous environments.
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