Article
Industrial automation equipment presents a unique challenge for the 
transformers used to step power down to levels required by their 
electromagnetic control devices. Such solenoids, contactors and timers 
are typically inductive loads, and they can create brief periods of high
 inrush current when they first start up that general-purpose 
transformers aren’t designed to handle.

Managing Industrial Control Equipment with the Right Transformer

Chuck Ross
Industrial automation equipment presents a unique challenge for the transformers used to step power down to levels required by their electromagnetic control devices. Such solenoids, contactors and timers are typically inductive loads, and they can create brief periods of high inrush current when they first start up that general-purpose transformers aren’t designed to handle. In fact, using a general-purpose device in this application could lead to premature failure of both the transformer and the connected controls.

What makes control transformers (which are also called “industrial control transformers”) different from general-purpose devices? To start, they incorporate high-quality insulating materials to protect their internal windings. They also are vacuum impregnated with VT polyester resin and then oven-cured to seal their surfaces and eliminate moisture. These protections allow control transformers to absorb inrush currents three to ten times higher than their nameplate rating.

In selecting the right control transformer for any specific application, specifiers need to know the minimum voltage required to operate the load circuit, along with three specific operating characteristics:

  • Total inrush volt-amperes (VA). This is the inrush current the transformer needs to support during the 30- to 50-milliseconds it takes to energize connected control loads.
  • Total steady-state (or “sealed”) volt-amperes. This is the volt-amperes needed to maintain ongoing operation of the connected controls over an extgended period of time.
  • Inrush load power factor. Because this figure is difficult to determine without a detailed analysis, a power factor of 40% is generally accepted as a safe assumption.
Photo courtesy of SolaHD
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