| I'll like to know the difference between a Dry Type Distribution Transformer
| and a Voltage converter please. For instance, if I have a 10KVA dry type
| distribution transformer at a frequency of 60Hz, will this work the same as
| a 10KVA voltage converter? There are Voltage converters on the market to
| convert 220/240V to 110/120V.
Voltage converters can use a variety of different means to convert from
one voltage to another. Most do use a transformer. However, they are
typically wired in what is known as an "autotransformer" configuration,
as opposed to the isolation configuration a distribution transformer
would generally be wired as.
A normal isolation transformer has 2 separate windings. Many transformers
have more than 2 windings where they can be grouped together in different
ways for differnet voltages. If a transformer has 4 windings rated to 120
volts each, two of them can be wired in series for 240 volts and the other
two wired in parallel for 120 volts supporting the extra amps needed. A
typicaldistribution transformer for single phase would have two windings
for 120 volts plus two windings for 240 volts. The two for 240 can be
wired in parallel if the incoming power is 240 volts, or in series if the
incoming power is 480 volts. The two for 120 would usually be wired in
series to provide a 240/120 volt split phase system typically seen in USA,
or wired in parallel if there is a special need for a lot of amps in one
120 volt circuit.
This is called an isolation transformer because there is no metallic
connection between the primary and secondary. Usually, the conductor
of the secondary designated as the grounded conductor, is connected to
the grounding wire coming in with the power.
An autotransformer reduces the number of windings needed to achieve the
same amount of power conversion. When isolation is not needed or is not
required by electrical codes, an autotransformer is the more economical
way to achieve the voltage conversion. A typical autotransformer would
have just 2 windings each rated for 120 volts, and connected in series
with a tap at that connection between them. The 120 volt side would be
wired between one end of the series pair, and the middle interconnect.
The 240 volts side would be wired between both far ends.
The power rating of the autotransformer is still the sum of the power
rating of both windings. If both windings are rated for 120 volts and
15 amps, you can still get 30 amps of 120 volts even though it seems
the 120 volt circuit is powered from just one winding. It is actually
getting power from the other winding, as well, if it is correctly wired.
What is different about the autotransformer is that the wire coming in
that is the grounded conductor (neutral) must be wired in common with
the output grounded conductor (neutral), or the wiring could be unsafe.
When converting power between USA and European type electrical systems,
this is basically simple once the correct neutral wire is identified.
One end of the winding series pair is designated as the neutral and both
neutrals (in and out) connect there. The hot wires are then connected
at the low voltage (center) or high voltage (farthest from neutral) taps.
Conversion from USA style 240 volts to 120 volts with an autotransformer
cannot be done safely. An isolation transformer is required in this case.
However, conversion the other way, from 120 volts to USA style 240 volts
can be done safely with an autotransformer.
DO NOT attempt to use transformers intended to convert European voltage
to USA voltage (or visa versa) to convert USA 240 volt systems, as they
are probably autotransformers and this can be unsafe.
There are voltage converters that use methods other than transformers, but
they are often just for special needs, including uninterruptible power
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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