13-09-2012, 05:31 PM
Transformer Questions and Answers
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1 . What is a transformer and how
does it work?
A transformer is an electrical apparatus
designed to convert alternating curre n t
f rom one voltage to another. It can be
designed to “step up” or “step down”
voltages and works on the magnetic
induction principle. A transformer has no
moving parts and is a completely static
solid state device, which insures, under
n o rmal operating conditions, a long and
t ro u b l e - f ree life. It consists, in its simplest
f o rm, of two or more coils of insulated wire
wound on a laminated steel core. When
voltage is introduced to one coil, called
the primary, it magnetizes the iron core .
A voltage is then induced in the other
coil, called the secondary or output coil.
The change of voltage (or voltage ratio)
between the primary and secondary
depends on the turns ratio of the two coils.
2 .What are taps and when are
they used?
Taps are provided on some transformers on
the high voltage winding to correct for high
or low voltage conditions, and still deliver
full rated output voltages at the s e c o n d a ry
t e rminals. Standard tap arr a n g e m e n t s a re
at two-and-one -half and five percent of
the rated primary voltage for both high and
l o w voltage conditions. For example, if the
t r a n s f o rmer has a 480 volt primary and the
available line voltage is running at 504 volts,
the primary should be connected to the 5%
tap above normal in order that the secondary
voltage be maintained at the proper rating.
The standard ASA and NEMA designation
for taps are “ANFC” (above normal full
capacity) and “B N FC” (below norm a l
full capacity).
3 . What is the diff e rence between
“ I n s u l a t i n g,” “I s o l a t i n g,” and
“Shielded Winding” transform e r s ?
Insulating and isolating transformers are
identical. These terms are used to describe
the isolation of the primary and seco n d a ry
windings, or insulation between the two.
A shielded transformer is designed with
a metallic shield between the primary and
s e c o n d a ry windings to attenuate transient
noise. This is especially important in critical
applications such as computers, pro c e s s
c o n t rollers and many other micro p ro c e s s o r
c o n t rolled devices. All two, three and four
winding transformers are of the insulating
or isolating types. Only autotransform e r s ,
w h o s ep r i m a ry and secondary are connected
to each other electrically, are not of the
insulating or isolating variety.
4. Can transformers be operated at
voltages other than nameplate
v o l t a g e s ?
In some cases, transformers can be operated
at voltages below the nameplate rated voltage.
In N O case should a transformer be operated
at a voltage in excess of its nameplate rating
unless taps are provided for this purpose.
When operating below the rated voltage, the
K VA capacity is reduced corre s p o n d i n g l y.
For example, if a 480 volt primary transform e r
with a 240 volt secondary is operated at 240
volts, the secondary voltage is reduced to
120 volts. If the transformer was originally
rated 10 KVA, the reduced rating would
be 5 KVA, or in direct proportion to the
applied voltage.