Acme Electric DRY-TYPE DISTRIBUTION TRANSFORMERS Frequently Asked Questions

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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

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Example: Determine the interrupting capacity, in amperes, of
a circuit breaker or fuse required for a 75 kVA, three phase
transformer, with a primary of 480 volts delta and secondary
of 208Y/120 volts. The transformer impedance (Z) = 5%. If the
secondary is short circuited (faulted), the following capacities
are required:

Normal Full Load Current =
Volt Amps 75,000 VA
√ 3 x Line Volts √ 3 x 480 V
90 Amps

Maximum Short Circuit Line Current =
Full Load Amps = 90 Amps
5% 5%
1,800 Amps

The breaker or fuse would have a minimum interrupting rating of
1,800 amps at 480 volts.

NOTE: The secondary voltage is not used in the calculation.
The reason is the primary circuit of the transformer is the only
winding being interrupted.

21. Can Single Phase Transformers be used for Three Phase
applications?

not readily available whereas single phase transformers can
generally be found in stock. Three single phase transformers can
be used in delta connected primary and wye or delta connected
secondary. They should never be connected wye primary to wye
secondary, since this will result in unstable secondary voltage.
The equivalent three phase capacity when properly connected
of three single phase transformers is three times the nameplate
rating of each single phase transformer. For example: Three
10 kVA single phase transformers will accommodate a 30 kVA
three phase load.

22. Does ACME provide “Zig-Zag” Grounding Transformers?

Yes. Please refer to Page 31 for a special diagram which

can be used to connect standard single phase off-the-shelf
transformers in a three phase zig-zag manner. This system can
be used for either grounding or developing a fourth wire from
a three phase neutral. An example would be to change a 480
V — three phase — three wire system to a 480Y/277 V — three
phase — four wire system.

23. What color are ACME Dry-Type Transformers?

ASA 61 (NEMA) light gray is used on all enclosed transformers
from .050 to 1000 kVA.

24. How do you select a transformer to operate in an ambient
higher than 40° centigrade?

When the ambient exceeds 40°C use the following chart for
de-rating standard transformers.

Maximum Ambient Maximum Percentage
Temperature of Loading

40°C (104°F) 100%
50°C (122°F) 92%
60°C (140°F) 84%

Instead of ordering custom built transformers to operate in
ambients higher than 40°C, it is more economical to use a
standard transformer of a larger kVA rating.

Yes. Three phase transformers are sometimes

25. Can transformers listed in this catalog be reconnected
as autotransformers to increase their kVA rating?

standard single phase transformers listed in this catalog can
be connected as autotransformers. The kVA capacity will
be greatly increased when used as an autotransformer,
in comparison to the nameplate kVA as an insulating
transformer. Examples of autotransformer applications are
changing 600 volts to 480 volts in either single phase or three
phase; changing 480 volts to 240 volts single or three phase
or vice versa; or the developing of a fourth wire (neutral) from
a 480 volt three phase three wire system for obtaining 277
volts single phase. This voltage is normally used for operating
fluorescent lamps or similar devices requiring 277 volts. For
further details showing kVA and voltage combinations for
various autotransformer connections refer to Page 30 and 31
in this catalog.

26. Are ACME Transformers shown in this catalog U.L.
Listed?

listed by Underwriters’ Laboratories and have met their
rigorous requirements. We are also prepared to have
transformers, which are not presently listed, submitted for
listing to Underwriters’ upon the customer’s request. Please
contact the factory for details.

27 . Is CSA certification available for transformers shown in
this catalog?

are certified by Canadian Standards Association. They have
been designed and tested in accordance with the latest
specifications. Please contact the factory if further details
are required.

28. What is BIL and how does it apply to transformers listed
in this catalog?

BIL is an abbreviation for Basic Impulse Level. Impulse tests
are dielectric tests that consist of the application of a high
frequency steep wave front voltage between windings, and
between windings and ground. The Basic Impulse Level of
a transformer is a method of expressing the voltage surge
(lightning, switching surges, etc.) that a transformer will
tolerate without breakdown. All transformers manufactured
in this catalog, 600 volts and below, will withstand the NEMA
standard BIL rating, which is 10 KV. This assures the user
that he will not experience breakdowns when his system is
properly protected with lightning arrestors or similar surge
protection devices.

29. What is polarity, when associated with a transformer?

Polarity is the instantaneous voltage obtained from the primary
winding in relation to the secondary winding. Transformers 600
volts and below are normally connected in additive polarity —
that is, when tested the terminals of the high voltage and low
voltage windings on the left hand side are connect ed together,
refer to diagram below. This leaves one high voltage and

All of the transformers, with few exceptions, are

Most ACME transformers shown in this catalog

H1 H2

240 VOLT

INPUT

ADDITIVE

POLARITY

120 VOLT

OUTPUT

x2 x1

VOLT

METER

Several

360

VOLT

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one low voltage terminal unconnected. When the transformer
is excited, the resultant voltage appearing across a
voltmeter will be the sum of the high and low voltage
windings. This is useful when connecting single phase
transformers in parallel for three phase operations. Polarity is a
term used only with single phase transformers.

30. What is exciting current? Exciting current, when

used in connection with transformers, is the current or amperes
required for excitation. The exciting current on most lighting
and power transformers varies from approximately 10% on
small sizes of about 1 kVA and smaller to approximately .5%
to 4% on larger sizes of 750 kVA. The exciting current is made
up of two components, one of which is a real component and
is in the form of losses or referred to as no load watts; the other
is in the form of reactive power and is referred to as kVAR.

31 . Will a transformer change Three Phase to single phase?

A transformer will not act as a phase changing device when
attempting to change three phase to single phase. There
is no way that a transformer will take three phase in and
deliver single phase out while at the same time presenting
a balanced load to the three phase supply system. There
are, however, circuits available to change three phase to two
phase or vice versa using standard dual wound transformers.
Please contact the factory for two phase applications.

32. Can air cooled transformers be applied to motor loads?

This is an excellent application for air cooled transformers.
Even though the inrush or starting current is five to seven
times normal running current, the resultant lower voltage
caused by this momentary overloading is actually beneficial
in that a cushioning effect on motor starting is the result. The
tables on Pages 11 and 12 illustrate some typical transformer
requirements for use with motor applications.

33. How is an Acme Drive Isolation Transformer (DIT)
different than a General Purpose Tranformer?

DITs, as the name implies, are designed to be used with
motor drives (AC and DC) and to provide isolation from the
service line. They are specifically designed to withstand the
“short circuit like” duty imposed by the firing of the thyristors.
Harmonics generated by drives create added loads on the
transformer. Therefore, it is important that a transformer
of equal or greater kVA to that recommended by the drive
manufacturer be installed for a particular motor application.

34. How are transformers sized to operate Three Phase
induction type squirrel cage motors?

transformer kVA rating required to operate a motor is
calculated as follows:

Minimum Transformer kVA =
Running Load Amperes x 1.73
x Motor Operating Voltage
1000

NOTE: If motor is to be started more than once per hour add
20% additional kVA.

Care should be exercised in sizing a transformer for an
induction type squirrel cage motor as when it is started,
the lock rotor amperage is approximately 5 to 7 times the
running load amperage. This severe starting overload will
result in a drop of the transformer output voltage. When the

DRY-TYPE DISTRIBUTION TRANSFORMERS

DRY-TYPE DISTRIBUTION TRANSFORMERS

The minimum

voltage is low the torque and the horsepower of the motor
will drop proportionately to the square of the voltage. For
example: If the voltage were to drop to 70% of nominal, then
motor horsepower and torque would drop to 70% squared
or 49% of the motor nameplate rating.

If the motor is used for starting a high torque load, the motor
may stay at approximately 50% of normal running speed as
illustrated by the graph below:

100

80
60
40
20

SYNCHROUS SPEED)

SPEED (PERCENT OF

TORQUE (PERCENT OF FULL LOAD TORQUE)

SPEED vs TORQUE FOR A TYPICAL THREE PHASE

INDUCTION TYPE SQUIRREL CAGE MOTOR

The underlying problem is low voltage at the motor terminals.
If the ampere rating of the motor and transformer overcurrent
device falls within the motor’s 50% RPM draw requirements,
a problem is likely to develop. The overcurrent device may not
open under intermediate motor ampere loading conditions.
Overheating of the motor and/or transformer would occur,
possibly causing failure of either component.

This condition is more pronounced when one transformer is
used to power one motor and the running amperes of the
motor is in the vicinity of the full load ampere rating of the
transformer. The following precautions should be followed:

(1) When one transformer is used to operate one motor, the
running amperes of the motor should not exceed 65% of
the transformer’s full load ampere rating.

(2) If several motors are being operated from one transformer,

avoid having all motors start at the same time. If this is
impractical, then size the transformer so that the total
running current does not exceed 65% of the transformer’s
full load ampere rating.

35. Why are Small Distribution Transformers not used for
Industrial Control Applications?

Industrial control equipment demands a momentary overload
capacity of three to eight times normal capacity. This is most
prevalent in solenoid or magnetic contactor applications where
inrush currents can be three to eight times as high as normal
sealed or holding currents but still maintain normal voltage at
this momentary overloaded condition. Distribution transformers
are designed for good regulation up to 100 percent loading,
but their output voltage will drop rapidly on momentary
overloads of this type making them unsuitable for high inrush
applications.

Industrial control transformers are designed especially
for maintaining a high degree of regulation even at eight
times normal load. This results in a larger and generally more
expensive transformer. For a complete listing of ACME
industrial control transformers, refer to Section V.

100 150 200 250

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36. Can 4-Winding Single Phase Transformer be auto­connected?

Yes. There are occasions where 480 volts single

phase can be stepped down to 240 volts single phase by
autoconnecting a standard 4-winding isolating transformer
as shown in Figure 1. If connected in this manner, the
nameplate kVA is doubled. For example: A 10 kVA load can
be applied to a 5 kVA 4-winding transformer if connected per
Figure 1.

480V

240V

H1

H3 H2 H4 X1 X3 X2 X4

Figure 1

37. What about balanced loading on Three Phases?

Each phase of a three phase transformer must be considered
as a single phase transformer when determining loading. For
example: A 45 kVA three phase transformer with a 208Y/120
volt secondary is to service 4 loads at 120 volts single phase
each. These loads are 10 kVA, 5 kVA, 8 kVA,and 4 kVA.

NOTE: that maximum loading on any phase does not exceed
10 kVA. Each phase has a 15 kVA capacity.

38. What is meant by “Balanced Loading” on Single Phase
Transformer applications?

Since most single phase

transformers have a secondary voltage of 120/240, they will
be operated as a three wire system. Care must be taken
in properly distributing the load as the transformer secondary
consists of 2 separate 120 volt windings. Each 120 volt
winding is rated at one-half the nameplate kVA rating. For
example: A 10 kVA transformer, 120/240 volt secondary is to
service an 8 kVA load at 240 volts and two 1 kVA loads at
120 volts each.

B

A

C

B

A

C

10 KVA

(NEUTRAL)

8 KVA 4 KVA5 KVA

CORRECT WAY:

10 KVA

(NEUTRAL)

5 KVA 4 KVA

8 KVA

45 kVA
3 phase

= 15 kVA per phase

If incorrect method is used, phase B will have an 18 kVA
load which is 3 kVA above its normal capacity of 15 kVA and
failure will result even though we only have a total load of 27
kVA on a 45 kVA transformer.

Enclosure Definitions

Type 1 Enclosures — are intended for indoor use,

primarily to provide a degree of protection against
contact with the enclosed equipment.

Type 2 Enclosures — are intended for indoor use,
primarily to provide a degree of protection against
limited amounts of falling water and dirt.

Type 3R Enclosures — are intended for outdoor use,
primarily to provide a degree of protection against fall­ing rain, sleet and external ice formation.

Definitions Pertaining to Enclosures

Ventilated — means constructed to provide for

circulation of external air through the enclosure to
remove excess heat, fumes or vapors.

Non-Ventilated — means constructed to provide no
intentional circulation of external air through the
enclosure.

Indoor Locations — are those areas protected from
exposure to the weather.

Outdoor Locations — are those areas exposed to
the weather.

Hazardous (Classified) Locations — are those
areas, which may contain hazardous (classified)
materials in sufficient quantity to create an explosion.
See Article 500 of The National Electrical Code.

INCORRECT WAY:

If t he incorrect method is used, winding A will be loaded
at 6 kVA, and winding B will be loaded at 4 kVA. These
do total 10 kVA but, since each winding is only rated at
5 kVA (1/2 of nameplate rating), we have an overloaded
transformer and a certain failure.

1 KVA

1 KVA

1 KVA 1 KVA

240V

240V

8 KVA

8 KVA

A

120V

B

120V

INCORRECT WAY:

120V

120V

CORRECT WAY:

39. What are typical applications for transfomers?

ACME transformers should be specified to:

(1) Distribute power at high voltage.
(2) Eliminate double wiring.
(3) Operate 120 volt equipment from power circuits.
(4) Insulate circuits/establish separately derived circuits.
(5) Provide 3- wire secondary circuits.
(6) Buck and Boost (See Section VII).
(7) Provide electrostatic shielding for transient noise protection.

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