What are Transformer Taps and Tap Changers?

Transformer Taps Allow Full Capacity Output as Primary Voltage Deviates From Normal

Several factors may cause the secondary of a transformer to deviate from its rated output voltage. The main reason is the voltage supplied to the transformer primary varying from the normal. This in turn may be due to the distance from the panel or substation. Designers provide taps on the transformer primary as a means to adjust the secondary output voltage to its rated value.

Transformer designers usually provide the taps on the primary or input side. A tap positioned below nominal allows a lower than normal input voltage to be transformed into rated output voltage. Likewise, a tap positioned above the nominal will transform a higher than normal input voltage into rated output voltage.

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A Medium Voltage Transformer

Why Taps Are Necessary

In any transformer, the voltage transformation is a function of its turns ratio. With a turns ratio of 1:1, the output voltage follows the input voltage. To obtain a higher or lower voltage output from the input voltage, the turns ratio must change accordingly. Sometimes, it is desirable to get the rated output voltage when the input voltage is somewhat different from the normal. Placing a tap in the primary winding allows changing the turns ratio to get the rated output voltage. Standard transformers may have from two to six taps for adjusting the output to the actual line voltage.

Standardization demands placement of taps at 2½% or 5% steps. The design of taps ensures full capacity output from a transformer when a connection is set on any one of them. Taps are to be used only for stable input line deviations. They cannot provide a constant voltage where the input line is fluctuating constantly. The manufacturer usually sets the tap connections on a transformer for nominal line voltage. If the site has a different voltage, the tap connection is changed accordingly.

Transformer Specs

Sample Transformer Specifications with Listed Tap Ratings

Tap Positioning

Manufacturers place a tap as a direct connection to a turn on a transformer winding at a voltage other than the normal rated voltage. The lead from the tap is brought out to a terminal on the surface of the coil or on a terminal board of the transformer. Taps on a transformer may be numbered with the lowest number usually representing the highest voltage tap, the middle number representing the nominal voltage tap, and the highest number representing the lowest voltage tap.

Taps require a whole number of turns between them for the tap lead to emerge on the proper side of the transformer. For instance, taps cannot be placed at 2¾ or 7¼ turns. Rather, they must be located at the 3rd or 7th turns. An example will illustrate this requirement.

A 480 V transformer has 960 turns. That means it has 2 turns per volt. Placing a 2½% tap on the transformer will reduce the voltage by 0.025 times 480 V or 12 V. As that makes the input voltage 468 V, the tap has to be placed at 24 turns from the 480 V normal tap.

However, this requirement cannot always be met for large transformers that have few turns. For instance, for a transformer with 2½ turns per volt, the 2½% on the 480 V primary winding calls for 2.4 turns. As the tap cannot be located at less than a whole turn, the tap must be placed at 2 turns. That makes it a 10 V, 2.09% tap rather than a 12 V, 2½% tap.

Tap Capacity

Designers place taps to carry the full capacity of the transformer at voltages above or below the nominal input voltage. Taps placed above the nominal voltage are termed FCAN or Full Capacity Above Normal, while taps placed below the nominal voltage are called FCBN or Full Capacity Below Normal. This is important, as the tap has to handle the full capacity of the transformer, meaning the tap has to handle enough current for the transformer to provide its rated KVA rating at that voltage input.

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