Control Transformer Selection Guide

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Selection Procedure

The following steps will ensure that the secondary voltage delivered by your transformer will be either 85%, 90% or 95% of the nameplate secondary voltage under maximum inrush conditions, at the rated input voltage.

Step 1:

Calculate the total sealed (steady state) VA load of your control circuit. This is done by adding the continuous VA requirements of the maximum number of components that will be energized at any given time, including non-inductive as well as inductive components. For sealed VA data, see the Component VA Chart below.

Step 2:

Calculate the peak inrush VA for your control circuit.

1. Analyze the sequence of operation of all components.
2. Add together the inrush VA ratings of the components that will be energized simultaneously.
3. Determine the peak - or maximum simultaneous - inrush VA load that the transformer will "see." The VA requirements for indicating lights, timers, and other non-inductive components which do not have an inrush VA also should be included since they will present a load to the transformer at the time of maximum inrush.

Step 3:

Calculate the transformer selection inrush VA. Use the following formula:

Selection Inrush VA =  √((VA Sealed)² + (VA Inrush)²)

NOTE: Transformer selection inrush VA also can be determined by adding the inrush VA and sealed VA arithmetically, but this usually results in an oversized transformer.

Step 4:

Determine the correct transformer nameplate VA Rating. Refer to the Inrush/Regulation Data Chart :

• If the line supply to the transformer is fairly stable (does not vary more than 5%), use the 90% secondary voltage column - the 90% column is most commonly used.
• If the line supply voltage varies up to 10%, use the 95% voltage regulation column.

To determine the correct VA transformer rating, go down the column until you arrive at the inrush VA rating closest to, but not less than, the transformer selection inrush VA calculated in Step 3. The left-hand column of the table will give the corresponding transformer nameplate VA rating.

NOTE: When evaluating supply-line stability remember that supply-line voltage drop frequently is associated with motor-starting inrush current. When motors and motor controls are connected to a common feeder, the controls will experience a momentary voltage dip when the motor starts. This reduces the control transformer voltage supplied to the motor starting contactor and may cause the contractor to chatter or drop out.

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