Constant voltage transformers (CVTs), or ferroresonant transformers, produce a constant secondary (output) voltage for varying primary (input) voltages within a certain range. With CVTs, the saturation of the ferrous core induces ferroresonance, a phenomenon that increases the inductive reactance relative to the capacitance reactance. When the iron cores of CVTs are saturated, the relatively large changes in magnetic current cause only very small changes in magnetic flux. Since the winding current and magnetic flux are proportional to the input voltage and the output voltage, respectively, relatively large changes in input voltage cause only small changes in output voltage.  

Specifications

Depending upon their design, CVTs are used with either single-phase or three-phase power. CVT product specifications include:

• operating frequency range
• maximum primary voltage rating
• maximum secondary voltage rating
• power rating
• operating temperature

Operating frequency range is the range of frequencies over which CVTs meet all guaranteed specifications. The maximum primary voltage is the device’s input voltage range. The maximum secondary voltage is the output range. Power rating is expressed as the sum of the VA (volts x amps) for all of the secondary windings.  

Standards

CVTs differ in terms of approvals and features. In Europe, products may comply with the RoHS and WEEE directives or meet requirements from TUV or VDE. In North America, the American National Standards Institute (ANSI) and the Canadian Standards Association (CSA) publish applicable CVT standards. CVTs that meet IEC requirements or U.S. military standards (MIL-STD) are also available.

Features

In terms of features, some CVTs are waterproof or include current limiting protection. Others have a NEMA enclosure or are designed for outdoor use. 

Selecting CVTs requires an analysis of mounting styles and form factors as well as applications. Some devices are designed for mounting on a chassis, disk, pole, or printed circuit board (PCB). Others mount on concrete pads or incorporate the RJ-45 form factor with magnetics to ensure high common-mode noise immunity and signal integrity. H-frame mounting is used in environments where high vibration or shock is present.

Applications

In terms of applications, most CVTs are used with rectifiers, power supplies, or voltage regulators. Often, ferroresonant transformers that provide voltage regulation are called constant voltage regulators, ferroresonant voltage regulators, or ferros.  

Related Information 

Engineering360—Single-Phase Transformers Information

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Amp-Line Corp.