Acme Electric DRY-TYPE DISTRIBUTION TRANSFORMERS Frequently Asked Questions

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DRY-TYPE DISTRIBUTION TRANSFORMERS

DRY-TYPE DISTRIBUTION TRANSFORMERS

Transformer Questions & Answers

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
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.

-and-one-half and

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.

4. Can transformers be operated at voltages other than
nameplate voltages?

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.

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In some cases, transformers can be

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8. Can a Single Phase Transformer be used on a
Three Phase source?

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
three phase 4-wire. The transformer output will be single
phase.

9. Can Transformers develop Three Phase power
from a Single Phase source?

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

Yes. Any single phase transformer

-wire or

No. Phase converters or

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load is 40 amperes, such as a motor, and the secondary
voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10
kVA (10,000volt-amperes) transformer is required. ALWAYS
SELECT THE TRANSFORMER LARGER THAN THE ACTUAL
LOAD. This is done for safety purposes and allows for
expansion, in case more load is added at a later date. For 3
phase kVA, multiply rated volts x load amps x 1.73 (square
root of 3) then divide by 1000.

(4) Determine whether taps are required. Taps are
usually specified on larger transformers.
(5) Use the selection charts in Section I.

1 1. What terminations are provided? Primary and

Secondary Terminations are provided on ACME Dry-Type
Transformers as follows:

No lugs—lead type connection on
0-25 kVA single phase
0-15 kVA three phase
Bus-bar terminations
(drilled to NEMA standards)
37 .5-250 kVA single phase
25-500 kVA three phase

1 2. Can 60 Hz transformers be used at higher frequencies?

ACME transformers can be used at frequencies above 60 Hz
up through 400 Hz with no limitations provided nameplate
voltages are not exceeded. However, 60 Hz transformers will
have less voltage regulation at 400 Hz than 60 Hz.

1 3. What is meant by regulation in a transformer?

Voltage regulation in transformers is the difference between
the no load voltage and the full load voltage. This is usually
expressed in terms of percentage. For example: A transformer
delivers 100 volts at no load and the voltage drops to 95
volts at full load, the regulation would be 5%. ACME dry­type distribution transformers generally have regulation
from 2% to 4%, depending on the size and the application
for which they are used.

14. What is temperature rise in a transformer?

Temperature rise in a transformer is the temperature of
the windings and insulation above the existing ambient or
surrounding temperature.

1 5. What is “Class” in insulation? Insulation class

was the original method used to distinguish insulating
materials operating at different temperature levels. Letters
were used for different designations. Letter classifications
have been replaced by insulation system temperatures
in degrees Celsius. The system temperature is the
maximum temperature at the hottest spot in the winding
(coil). Graphical representations of six insulation systems
recognized by Underwriters’ Laboratories, Inc. are shown in
Figure A. These systems are used by Acme for a large part
of the product line.

1 6. Is one insulation system better than another?

Not necessarily. It depends on the application and the
cost benefit to be realized. Higher temperature class
insulation systems cost more and larger transformers are more
expensive to build. Therefore, the more expensive insulation
systems are more likely to be found in the larger kVA units.

Referring to Figure A, small fractional kVA transformers use

DRY-TYPE DISTRIBUTION TRANSFORMERS

DRY-TYPE DISTRIBUTION TRANSFORMERS

insulation class 130°C. Compound filled transformers use
insulation class 180°C. Larger ventilated transformers are
designed to use 220°C insulation. All of these insulation
systems will normally have the same number of years
operating life. A well designed transformer, observing these
temperature limits, will have a life expectancy of 20-25
years.

17. Why should Dry-Type Transformers never be over­loaded?

temperature. This excessive temperature causes
overheating which will result in rapid deterioration of the
insulation and cause complete failure of the transformer coils.

18 . Are temperature rise and actual surface
temperature related?

ordinary light bulb. The filament temperature of a light bulb
can exceed 2000 degrees, yet the surface temperature of
the bulb is low enough to permit touching with bare hands.

19. What is meant by “impedance” in transformers?

Impedance is the current limiting characteristic of a
transformer and is expressed in percentage.

Overloading of a transformer results in excessive

No. This can be compared with an

Total Winding Temperature °C

130

COIL HOT SPOT

DIFFERENTIAL

AV. WINDING

RISE

AMBIENT

20. Why is impedance important? It is used for determining

the interrupting capacity of a circuit breaker or fuse employed
to protect the primary of a transformer. Example: Determine
a minimum circuit breaker trip rating and interrupting capacity
for a 10 kVA single phase transformer with 4% impedance,
to be operated from a 480 volt 60 Hz source.

Calculate as follows:
Normal Full Load Current =

Nameplate Volt Amps
Line Volts 480 V

Maximum Short Circuit Amps =
Full Load Amps 20.8 Amps
4% = 4%
520 Amps

The breaker or fuse would have a minimum interrupting rating
of 520 amps at 480 volts.

105

10
55

40

AGENCY: UL/ANSI 1561 MARCH 1987

10

80

40 40 40

Figure A

10,000 VA

=

20.8 Amperes

180

25

115

=

=

220

30

150

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