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Frequently Asked Questions

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Question: Taps for Transformers

Answer: Transformers are supplied with taps to adjust the transformer voltage to the correct input or output voltage or to permit selecting various voltages for various purposes, adjustments, etc. On some transformers the primary is equipped with taps to vary the primary approximately 10%. For example: we have a transformer that has a primary of 480 volts with four 2.5% taps below and a secondary of 120/240 volts. If the line voltage or primary is actually down to 432 volts, the output voltage will be only 108/216. By using the lowest tap on the transformer, we can adjust the primary so that it will be suited for 432 volts, and full 120/240 volts will be obtained on the output. Most small and medium size transformers have no-load tap changers. This means that the power must be shut off and the taps changed on the transformer while it is de-energized. On very large size and specially built transformers the tap changing can be done automatically. This is a comparatively expensive method and is not commonly employed on transformers of 1000 KVA and less. Other transformers have taps on the secondary and are used for such purposes as welders, furnaces and other types of loads where it is necessary to change the voltage to match the load or to supply different voltages to various sections of a complex piece of equipment like a motor drive system.

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