Control Transformers

Control Transformers are transformers which are used in control circuits of motor control circuits, auxilliary relays, etc.  These transformers step down the main power supply to a lower value which can be fed to the relays and contactors in the circuit. 

For instance, a crane whose motors require 440 VAc will have a control circuit which operates at 230V. The 230V power supply can be drawn from a step-down transformer which is connected to the mains power supply.

Control transformers make it simpler by drawing the power for the control circuit from the main power supply instead of having a separate power source.  The voltage for the control circuit can be 230 V or 110 V.  Lower voltages are also possible.  Low voltages may be necessary in hazardous environments. 

Another advantage of the control transformer is the isolation is the excellent transient response it provides.  When heavy contactors or electromagnetically operated valves are switched on, a high inrush current results.  The control transformer is able to withstand such high inrush currents and prevent any instability to the main power circuit. 

The control transformer should be designed to provide the steady state kVA demand of the control circuit.  In addition, it should also be capable of providing the inrush kVA demand which is the initial kVA when all the contactors and other coils may be switched on at the same time.