Power Conditioning for Automation

Applying a Constant Voltage Transformer Correctly to Increase System Reliability

 

Mike Johnson, Product Manager, Sola/Hevi-Duty

Jill Normandin, Director of Marketing & Sales, Sola/Hevi-Duty

 

Electronic 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 a power system. 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, noise or impulses, which may result in lost production and revenue. Severe line disturbances can, also, reduce the life span of, damage, or completely destroy expensive equipment.

 

A power conditioner is commonly used to protect sensitive or expensive electronics from these damaging and disruptive events. There are many different styles of "power conditioners" on the market today, each protecting from a specific disturbance or set of disturbances. It is the purpose of this article to help clarify the term power conditioning and focus on the application one device that protects from a wide range of disturbances, the Constant Voltage Transformer or CVT for short.

 

By IEEE definitions, a power line conditioner is a device that combines one or more "power-enhancement" attributes. Technologies fitting this description include electronic voltage regulators, tap-switching products, buck-boost autotransformers which incorporate surge suppression or noise filters and the constant voltage transformer. The CVT uses ferroresonant technology to provide reliable, comprehensive power protection in one single unit.

 

Figure 1 - Schematic Diagram

A PC, PLC or combination of both control most industrial equipment. These controllers are critical to day-to-day operation so damage and downtime due to power related events cannot be tolerated. The CVT is one of the few devices available that can reliably protect from such a wide range of disturbances. No other power conditioner protects from as many different disturbances. In fact, only a double conversion Uninterruptible Power Supply (UPS) with isolation can offer similar protection at a comparable price. A UPS solution will increase protection from interruptions but provide decreased reliability. If power interruptions are not the predominate concern, the CVT is the better choice.

 

There are several CVT designs on the market but all incorporate the same basic design - a saturating transformer preloaded with a resonant tank circuit. The transformer's reluctance and internal capacitor form this tank circuit, which provides a "pool" of power that supplies constant, clean output voltage free from most types of power disturbances. Since the transformer is always in saturation, input voltage fluctuations have very little effect on the output voltage. At light loads, input variation effects decrease so that at less than 25% load, the input can drop below 60% of nominal continuously with less than a 10% fluctuation in output. Of course, all good things come at a price. In this case, efficiency. Because there is a fixed amount of power required to maintain saturation, the CVT is less efficient at light loads.  At full load, however, the CVT can be over 90% efficient. CVT's are best applied directly before the most sensitive and critical loads in the system, typically the PLC or PC-based controller. At this point, the load is constant and predictable by the designer so the benefit of wide input vs. efficiency is under the designer's control. Most of these devices require a transformer to step-down the voltage from the incoming supply to a lower control voltage. The CVT can be used to provide this step-down voltage change (i.e. 480 to 120 VAC) while avoiding the use of an ‘extra' transformer.

           

The simplistic design of the constant voltage transformer provides high reliability giving the CVT a major advantage over electronic regulators and UPS's, which are sometimes used only for their power conditioning features.  With no fans, switches, batteries or active electronic components, the ferroresonant transformer has a typical operating life of 20 years.  Many CVT manufacturers offer 10-year warranties because of this excellent reliability.

 

Figure 2 - Input vs. output
(at rated load)

CVT's are available with output regulation of +/-3%. Regulation is smooth and continuous over the full input range of the CVT, typically +15 to –20%. The input range can be as high as +25% to –40% if the load can tolerate swings of +5 to –10%. CVT's can also be built with +/-1% output regulation by adding a compensating winding.

 

 Although the CVT is not a back-up device, it will provide hold-up for 3ms. This is often just enough time to keep relays or E-stops from false dropouts due to low voltage or severe input fluctuations. CVT's provide full galvanic isolation adding surge suppression and noise reduction to the protection provided by the CVT. Undervoltage, overvoltage, swells and even some types of sags can be isolated from sensitive loads using a CVT.  

 

In addition to voltage conditioning, the CVT offers harmonic filtering. Output voltage THD (total harmonic distortion) is typically 3%, regardless of input THD. Load current harmonics are also filtered reducing the harmonic current reflected upstream to the mains supply. 

  

The CVT also provides current limiting. The output of the CVT is limited to between 150-200% of rated load. Properly designed CVT's can handle direct short circuits continuously without damage or degradation to the CVT.

 

In choosing a CVT as a power quality device, certain physical factors are important. A CVT is a line frequency transformer, which means it is larger and heavier than a high frequency magnetic device. Heat and airflow are a consideration when choosing the placement of this device.  The CVT will emit heat and should not be placed directly below any sensitive electronics, nor should it be mounted inside a confined space. If mounted inside an enclosure, make sure there is adequate airflow around the unit.

 

The first step in any power quality strategy is developing a complete understanding of your electrical environment, implementing a good power distribution system design and using good grounding techniques.  If you want virtually maintenance free protection from a wide range of power problems, the constant voltage transformer, correctly applied, is still one of the best forms of total power conditioning. 

 

Sola/Hevi-Duty has been a leader of industrial power quality solutions including power supplies, transformers, and power quality equipment. It is a member of the EGS Electrical Group, and is headquartered in Skokie, Illinois. . Visit their website at www.solaheviduty.com.