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Ferroresonant Transformers

Ferroresonant transformers are a special type of laminated transformer which provides a regulated output. These are sometimes known simply as "ferros", or "CVTs" (constant voltage transformers). Using a special magnetic structure and a capacitor, ferroresonant transformers can supply a well regulated output voltage which remains constant despite changes in input voltage and load. In some specialized applications, these devices can be designed to supply a constant current output, rather than constant voltage. Current limiting is an inherent benefit of ferroresonant transformers. If an overload, or even a short circuit is applied, secondary current is limited to a safe level and primary current is actually lower than at normal full load. Another inherent benefit is that the ferro serves as a low-pass filter, supplying to load with power, but effectively removing transient and surge voltages. Energy stored in the transformer structure even allows the output to "ride through" brief input power interruptions.

Read more here: http://www.generaltransformer.com/transformer/ferroresonant-transformers.htm

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