Power conditioning with a constant voltage regulator

06/05/2002


E lectronic systems are designed to run on clean, well-regulated, distortion free power. Heavy machinery, storms, utility issues, power factor correction capacitor switching, and harmonic generating electronics will add voltage distortion to power systems. A minor power disturbance, lasting only a fraction of a second, can contaminate or completely ruin data that might take days or weeks to replace. An entire process can be shut down due to voltage disturbances or interruptions that may result in lost production and revenue. Severe line disturbances can reduce the life span, damage, or completely destroy expensive equipment. Power conditioners are commonly used to clean-up ac line problems, and protect sensitive or expensive electronics downstream.

In recent years, power conditioning has taken on many meanings. This article focuses on applying of one of the most tried and true forms of power conditioning: the ferro-resonant voltage regulator or constant voltage transformer (CVT).

IEEE defines a power line conditioner as combining one or more power-enhancement attributes to aid in power protection and improve power quality. Applicable technologies include electronic voltage regulators, tap-switching products, buck-boost topologies and CVTs, which are one of the most reliable approaches used in the industrial market. This ferro-resonant technology provides the most complete power protection and quality in a single product.

CVTs are designed with a saturating transformer, which uses a resonant or tank circuit. The transformer's reactance and a capacitor form this tank circuit, which provides a 'pool of power' that supplies constant, clean output voltage free of most types of power disturbances, and without concerns for response times. The saturation part of this transformer gives it the unusual quality of being less efficient at lighter loads. At full loads, CVTs can be more than 90% efficient.

Simplicity aids reliability, protection

The reliability of CVTs is based on simple design. With no mechanical or active components, such as fans or switches, ferro-resonant transformers have been used for decades. Because CVTs are actually transformers, they can be purchased to provide additional functions, such as a step-down voltage change, for example, from 480 to 120 V ac.

CVTs offer superior regulation of input voltage swings from 20-40% or more if the user isn't operating at maximum load. They also provide hold-up for 3 msec, and deliver surge suppression and noise reduction in normal and common-mode forms. Under-voltage, over-voltage, swells and some types of sags can be isolated from sensitive loads. Harmonics are reduced to 3% THD. This maintenance-free box, guaranteed by some manufactures for over 10 years, provides one of the most comprehensive power protection and power quality devices without a battery presently available.

CVTs also act as overload protectors because output to the CVT is limited to between 150-200% of overcurrent. While this provides load protection, correct sizing is important when considering any high inrush or peaks current from those loads.

Choosing wisely

When picking a CVT as a power quality device, many factors must be considered. Usually available in a wall-mounted chassis, CVTs come in floor-mounted configurations for higher-power applications. Heat is also a consideration when deciding how to place these devices. Other selection criteria for CVTs include:

  • Input Voltage: Often this is simple-120 to 120 V ac or 480 to 480 V ac-but CVTs can also provide voltage 'step-down' or step-up to eliminate the need for another control or distribution transformer.

  • Input Frequency: Due to their resonant-based nature, CVTs are not frequency converters, nor do they aid in dramatic frequency changes. This is especially important for users with frequency-sensitive equipment. Most electronic products, such as computers and PLCs, are not sensitive to frequency changes thanks to the deployment of switching power supplies.

  • Load Power: Sizing the correct CVT is critical to its proper application. CVTs can commonly take 150-200% of overload condition before they begin to shut down. The good news is that CVTs usually act as an extremely quick overcurrent device to protect equipment from overloads. However, users must be cautious, and must size both peak power-consider the crest factor in any switch-mode type products, such as power supplies-and steady state power.

As always, good power distribution system design, good grounding techniques, and complete understanding of your electrical environment is this first step in any power quality application. If you want virtually free maintenance for many years, and protection from a wide range of power problems, a correctly applied CVT is still one of the best forms of total power conditioning.

Jill Normandin, director of marketing Sola/Hevi-Duty, Skokie, Ill.




Jim Montague, news editor
jmontague@cahners.com





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