Technical Review
(Issue 2, 2004)

Common Questions about
Control Transformers

by Keri Schieber,
Managing Editor
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What is a control transformer?

A control transformer is an isolation transformer designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also known as 'inrush current'). Control transformers are also known as machine tool transformers, industrial control transformers, or control power transformers.

How do I select a control transformer?

Selecting a control transformer requires that you have a first hand knowledge of the specific application for the transformer, and that you understand some basic terms used in the selection process. By using the following information, you will be able to select the correct control transformer for your application.

To select the proper transformer, you must first determine three characteristics of the load circuit. They are total steady-state (sealed VA), total inrush VA, and inrush load power factor.

Total steady-state "sealed" VA is the total amount of VA that the transformer must supply to the load circuit for an extended length of time. Calculate by adding the total steady-state VA of all devices in your control circuit. (The operating VA data for the devices should be available from the manufacturers.)

The inrush VA is the amount of VA that the transformer must supply for all components in the control circuit that are energized together. Consideration for the start-up sequence may be required. (Inrush VA data should be obtained from the device manufacturers.)

The inrush load power factor is difficult to determine without detailed vector analysis of all the control components. In the absence of such information, we recommend that a 40% power factor be utilized.

Once the three circuit variables have been determined, follow these six steps to select the proper transformer:

1. Determine your primary (supply) and secondary (output) voltage requirements, as well as the required frequency (i.e. 60 HZ).

2. Calculate the total sealed VA of your circuit.

3. Calculate the inrush VA by adding the inrush VA of all components being energized together.

4. Calculate the total inrush VA using one of two methods:



5. If the nominal supply voltage does not fluctuate more than 5%, then reference the 90% secondary column in the manufacturer's Regulation Data Table for the correct VA rating. If the supply voltage varies up to 10%, the 95% secondary voltage column should be used to size the transformer.

6. Using the manufacturer's regulation data table, select the appropriate VA rated transformer:

a. With a continuous VA rating that is equal to or greater than the value in Step 3.

b. With a maximum inrush VA equal to or greater than the value obtained in Step 5.


Frequently asked questions

Q. When you calculate the VA requirements of a trans- former, do you use the primary or secondary voltage?
A. The secondary voltage is used in calculating the VA.

Q. Can the control transformer be used in reverse?
A. Yes, a control transformer can be used in reverse. However, keep in mind the output voltage will be less than its rating, due to the compensation factor of the windings.

Q. Can a control transformer regulate the output voltage?
A. No, a control transformer will not regulate voltage. Output voltage is a function of the coil's turn ratio times the input voltage.

Q. Can you explain the "VA" or "Volt Ampere Output" rating?
A. The VA or volt ampere output rating designates the output which a transformer can deliver for a specified time at its rated secondary voltage and rated frequency, without exceeding it's specified temperature rise.

For additional information on control transformers, visit the Technical Support Applications and FAQ section of the AutomationDirect web site. Be sure to read the application note on control transformers: "Fuse sizing for primary and secondary windings."
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