YY AUTO 3Ø / NEUTRAL = PRIM YES-SEC NO AUTO TRANSFORMER CONNECTION AND DIGRAM


WHERE USED
For increasing voltage at the end of lines or to step up voltage where line extensions are being added to existing lines, such as from 6900 VAC to 7200 VAC. Cost per kva output is less than a two-winding transformer; losses are low, regulation is good, and exciting current is low. Voltage transformation greater than 3 to 1 is not recommended.

FOR POWER FROM A 3Ø, 4W SYSTEM
When the ratio of transformation from the primary to secondary voltage is small, the most economical way of stepping down the voltage is by using autotransformers as shown. For the application, it is necessary that the neutral of the auto transformer bank be connected to the system neutral. Brand circuits shall not be supplied by autotransformers.

CAUTION
Susceptive to burnouts if the system impedance is not great enough to limit the short-circuit current to 20 to 25 times the transformer-rated current. The primary neutral should be tied firmly to the system neutral; otherwise, excessive voltages may develop on the secondary side.

RATING & FUNCTION
A considerable saving in cost may often be experienced by using autotransformers instead of two-winding transformers. When it is desired to affect a small change in voltage, or where both high and low voltages are low, there is usually no reason why an autotransformer cannot be used as successfully as a two-winding transformer.

Autotransformers should not, except under special conditions, be used where the difference between the high-voltage and low-voltage ratings is great. This is because the occurrence of grounds at certain points will subject the insulation on the low-voltage circuit to the same stress as the high-voltage circuit.

Autotransformers are rated on the basis of output KVA rather than the transformer KVA. Efficiencies, regulation and other electrical characteristics are also based on output rating.


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