A general purpose transformer should no be used for control applications.They are not used for high inrush load.Their use could result in lowering of secondary output voltage during inrush period of 30 to 50 milliseconds.Reduced output voltage can prevent safe and normal component operation and may lead to premature failure of transformers and connected control devices.
SIZING OF CONTROL TRANSFORMERS :
Three characteristics of load circuit must be determined in addition to minimum voltage required to operate the circuit.These are total steady state (sealed VA) ,total inrush VA and inrush load power factor.
A.SEALED VA:Total steady state sealed VA is the volt ampere that the transformer must deliver to the load circuit to the extended period of time
B.INRUSH VA :Total Inrush VA is the total volt ampere that the transformer has to deliver during initial energisation of circuit.Energisation of electromagnetic circuit may take 30-50 milli seconds.
C.INRUSH LOAD POWERFACTOR is difficult to determine without detailed vector analysis of all components.Generally such as analysis is not feasible therefore a safe assumption is 40% powerfactor
1.Determine the supply and load voltage.The supply voltage is available voltage to the control transformer.The load voltage is operating voltage to the device that will be connected to the transformer output
2.Calculate the total sealed VA by adding the VA requirements of all the components that will be energized together(timers,contactors,relays,solenoids ,pilot lamps etc).Sealed VA data is available from control device manufacturer
3.Add the inrush VA of all components that will be energized together.Be sure to include the sealed VA of components that donot have an inrsh (lamps,timers etc)as they present a load to the transformer during maximum inrush current.
4.Calculate selection inrush VA in one of the following two ways :
A.Selection Inrush VA =Square root of ((VA sealed)^2+(VA inrush)^2)
B.VA Sealed + VA Inrush=Selection inrush
Method B will result in a slightly oversized transformer.
5.If your Line voltage varies 10% or more contact technical services for assistance
6.Select the transformer VA needed for your application from the calculation