Following parameters shall be considered for selecting a transformer for industrial control applications:
INRUSH VA: Is the product of load voltage V and the current A that is required during circuit start-up. It is calculated by adding the inrush VA requirements of all devices (contactors, timers, relays, pilot lights, solenoids, etc.), which will be energized together.
SEALED VA: Is the product of load voltage V and the current A that is required to operate the circuit after initial start-up or under normal operating conditions. It is calculated by adding the sealed VA requirements of all electrical components of the circuit that will be energized at any given time.
PRIMARY VOLATGE: Is the voltage available from the electrical distribution system and its operational frequency, which is connected to the transformer supply voltage terminals.
SECONDARY VOLTAGE: Is the voltage required for load operation which is connected to the transformer load voltage terminals.
APPLICATION INRUSH VA: Determine the application inrush from the formula-
APPLICATION INRUSH VA = √(INRUSH VA)^2 + (SEALED VA)^2
REGULATION DATA CHART: Refer to the regulation data chart. If the primary voltage is basically stable and do not vary more than 5% from the nominal, the 90% secondary column should be used. If the primary voltage varies between 5 and 10% from the nominal, the 95% of the secondary voltage column should be used. Moreover the value of the secondary voltage in no case shall be less than the Application Inrush VA.
TRANSFORMER VA RATING: It shall always be equal to or greater than the Sealed VA