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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