SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
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MILWAUKEE, WI
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acmetransformer.com
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1. What is a transformer and how does it work?
A transformer is an electrical apparatus designed to convert
alternating current from 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 normal operating conditions, a long
and trouble-free life. It consists, in its simplest form, 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 secondary terminals.
Standard tap arrangements are at two-and-one-half and
five percent of the rated primary voltage for both high and
low voltage conditions. For example, if the transformer 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 “BNFC” (below normal full capacity).
3. What is the difference between “Insulating,”
“Isolating,” and “Shielded Winding” transformers?
Insulating and isolating transformers are identical. These
terms are used to describe the isolation of the primary
and secondary windings, or insulation between the two.
A shielded transformer is designed with a metallic shield
between the primary and secondary windings to attenuate
transient noise. This is especially important in critical
applications such as computers, process controllers and
many other microprocessor controlled devices. All two,
three and four winding transformers are of the insulating or
isolating types. Only autotransformers, whose primary 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 voltages?
In some cases, transformers can be
operated at voltages below the nameplate rated voltage. In
NO
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
kVA capacity is reduced correspondingly. For example, if
a 480 volt primary transformer 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.
5. Can 60 Hz transformers be operated at 50 Hz?
ACME transformers rated below 1 kVA can be used on 50
Hz service. Transformers 1 kVA and larger, rated at 60 Hz,
should not be used on 50 Hz service, due to the higher
losses and resultant heat rise. Special designs are required
for this service. However, any 50 Hz transformer will operate
on a 60 Hz service.
Transformer Questions & Answers
6. Can transformers be used in parallel?
Single phase
transformers can be used in parallel only when their
impedances and voltages are equal. If unequal voltages
are used, a circulating current exists in the closed network
between the two transformers, which will cause excess
heating and result in a shorter life of the transformer. In
addition, impedance values of each transformer must be
within 7.5% of each other. For example: Transformer A has
an impedance of 4%, transformer B which is to be parallel
to A must have an impedance between the limits of 3.7%
and 4.3%. When paralleling three phase transformers,the
same precautions must be observed as listed above, plus
the angular displacement and phasing between the two
transformers must be identical.
7. Can Acme Transformers be reverse connected?
ACME dry-type distribution transformers can be reverse
connected without a loss of kVA rating, but there are
certain limitations. Transformers rated 1 kVA and larger
single phase, 3 kVA and larger three phase can be reverse
connected without any adverse effects or loss in kVA
capacity. The reason for this limitation in kVA size is, the
turns ratio is the same as the voltage ratio. Example: A
transformer with a 480 volt input, 240 volt output— can
have the output connected to a 240 volt source and thereby
become the primary or input to the transformer, then the
original 480 volt primary winding will become the output
or 480 volt secondary. On transformers rated below 1 kVA
single phase, there is a turns ratio compensation on the
low voltage winding. This means the low voltage winding
has a greater voltage than the nameplate voltage indicates
at no load. For example, a small single phase transformer
having a nameplate voltage of 480 volts primary and 240
volts secondary, would actually have a no load voltage
of approximately 250 volts, and a full load voltage of 240
volts. If the 240 volt winding were connected to a 240 volt
source, then the output voltage would consequently be
approximately 460 volts at no load and approximately
442 volts at full load. As the kVA becomes smaller, the
compensation is greater— resulting in lower output voltages.
When one attempts to use these transformers in reverse, the
transformer will not be harmed; however, the output voltage
will be lower than is indicated by the nameplate.
8. Can a Single Phase Transformer be used on a
Three Phase source?
Yes.
Any single phase transformer
can be used on a three phase source by connecting the
primary leads to any two wires of a three phase system,
regardless of whether the source is three phase 3-wire or
three phase 4-wire. The transformer output will be single
phase.
9. Can Transformers develop Three Phase power
from a Single Phase source?
No.
Phase converters or
phase shifting devices such as reactors and capacitors are
required to convert single phase power to three phase.
10. How do you select transformers?
(1)
Determine primary voltage and frequency.
(2)
Determine secondary voltage required.
(3)
Determine the capacity required in volt-amperes.
This is done by multiplying the load current (amperes) by
the load voltage (volts) for single phase. For example: if the
