Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 633 of 707

finishing plant engineering DC POWER SUPPLIES DYNAPOWER & RAPID POWER CORP., SOUTH BURLINGTON, VT. RECTIFIER OVERVIEW Rectifiers were introduced to the surface-finishing industry over a half century ago to replace rotating DC generators. Rectifiers have a major advantage in that they have few, if any, moving parts, which results in significant decreases in maintenance and downtime. Today, rectifiers are one of the most reliable and effi- cient means of power conversion, and nearly all surface-finishing rotating gen- erators have been replaced. A rectifier can be divided into three major components: a main power trans- former, a regulating device to control the DC output, and a rectifying element to convert the incoming AC to output DC. A rectifier also contains auxiliary com- ponents, such as control electronics and cooling. Main Power Transformer The main power transformer receives line voltage and steps it down to a suit- able but unregulated AC voltage. To produce a transformer of the highest efficiency and reliability, three major design factors must be considered. First, all conductors must consist of electrolytically pure copper. Second, the core lam- inates must be made from low-loss, high-quality transformer steel. Third, extremely high-quality, high-temperature insulating material must be uti- lized. If the quality of any of these areas is compromised, transformer efficiency and longevity will be sacrificed. In a high-quality transformer, electrolytically pure copper is used to wind the transformer coils, with insulating material located between each conductor. Once wound, the coils are vacuum impregnated with a high-temperature varnish, and all terminals are then silver brazed. The coils are then placed onto the core. The transformer core is constructed from low-loss, grain-oriented silicon transformer steel. The steel is cut into the proper lengths and single stack lam- inated to form the core structure. If a great deal of attention has not been paid to the construction of the core, there will be air gaps between the laminations. This will decrease the transformer's ability to handle magnetic flux, resulting in a trans- former with less efficiency. The majority of transformer power losses is the result of excessive temperatures. The only way to avoid this condition is through proper engineering. This includes designing for low-current densities in the windings, low-flux density in the transformer core, and of course, ensuring proper transformer assembly. Quality transformers are manufactured in this manner. Unfortunately, improper transformer design or construction is not always visible to the naked eye. A conservatively designed quality transformer will look physically similar to a lesser quality transformer. Because the differences lie in the design and mate- rials, the effect will only become apparent during operation. A higher quality trans- former will run 10 to 15% cooler. A transformer operating at lower temperatures will have a much higher efficiency and greater longevity. Although the manu- facturing cost is higher on the more efficient unit, the payback for the additional 632

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