Acme Electric Buck-BoostTransformers Catalog
8
Buck-BoostTransformers
Section
A simple and economical way to correct
offstandard voltages... from 95 to 500 volts;
single and three phase, in sizes up to 360
kVA. Simplified buck-boost rating charts make
proper transformer selection easy, accurate.
Description and Applications
Questions and Answers
Selection Charts - Single Phase
Selection Charts - Three Phase
Specifications
Wiring Diagrams
Three Phase Buck Boost
118-119
120-126
127-129
130-132
133-135
135-137
137
Buck-Boost Transformers Buck-Boost Transformers
Where are buck-boost transformers used?
A typical buck-boost application is 120 volts in, 12 volts out for low voltage lighting or control circuitry. In most applications, this low
voltage transformer is field connected as an autotransformer. (See question 2 for the definition of an autotransformer). Buck-boost
transformers provide tremendous capabilities and flexibility in kVA sizes and input/output voltage combinations. Basically you get
75 different transformers... all in one convenient package.
Other buck-boost applications are, where (A) low supply voltage exists because equipment is installed at the end of a bus system;
(B) the supply system is operating at or over its design capacity; and (C) where overall consumer demands may be so high the
utility cuts back the supply voltage to the consumer causing a “brownout.”
Why use buck-boost instead of another type transformer ?
Take a look at the advantages and disadvantages of using a buck-boost transformer (autotransformer) compared to a standard
isolation transformer of the proper size and voltage combination.
Proper voltage is critical
With nearly two-thirds of all electrical loads being A.C. motor loads, maintenance of the proper voltage to that motor is very
important. If the supply line voltage is not maintained, motor winding current is increased causing reduced motor torque and
escalating motor temperature, all of which results in the rapid loss of insulation life expectancy.
In addition to motor loads, the detrimental effects of low voltage on both resistive heating loads and incandescent lighting output
is illustrated in the chart.
Anytime you have a lower than standard voltage, equipment damage and failure can result.
Buck-boost transformers are an economical way to correct this potentially very serious problem. Anytime a line voltage change
in the 5-20% range is required, a buck-boost transformer should be considered as your first line of defense.
Advantages Disadvantages
More efficient No circuit isolation
Smaller & lighter Cannot create a neutral
5-10 times increase in kVA
Versatile, many applications
Lower cost
Application voltages and kVA
don‘t match the nameplate
voltages and kVA
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T211688
T111683
Encapsulated
Single Phase, .05 to 10.0 kVA
Features
n
UL listed, CSA certified and UL 3R enclosure, meets or exceeds all listing criteria, including NEMA, ANSI, and
OSHA standards
n
Flexibility, can be used in single phase and three pase configurations
n
Reduce (buck) or raise (boost) line voltage from 5 - 20%
n
All copper lead wire terminations
n
Long Life, 80° C rise up to 0.15 kVA, and 115° C rise above 0.25 kVA
n
Can be used in Three Phase applications
Encapsulated
Three Phase, 3.0 to 150 kVA
Features
n
UL listed, CSA certified and UL 3R enclosure, meets or exceeds all listing criteria, including NEMA, ANSI,
and OSHA standards
n
One unit, instead of multiple for 3 phase applications
n
Time and installation cost savings as units come pre-wired from the factory
n
Smaller footprint compared to using three individual single phase units
n
Long Life, UL class 180° C insulation system, 115° C rise
119
Buck-Boost Transformers Buck-Boost Transformers
Buck-Boost Transformers Buck-Boost Transformers
HOW LOW VOLTAGE AFFECTS VARIOUS EQUIPMENT OPERATIONS AND FUNCTIONS
150%
140%
130%
120%
110%
100%
90%
80%
70%
60%
50%
5% Low Voltage
=
10% Low Voltage
=
=
15% Low Voltage
Increase of Motor Winding
Current Requirements in
Induction Motor Loads
90.5%
81%
72%
Decrease of Heat Output in
Resistive Heating Loads
105%
111%
117%
84%
70%
Decrease of
Incandescent
Lighting Output
Increase of Motor Temperature and Corresponding
Insulation Life Expectancy
Loss
123%
111%
57%
QUESTIONS AND ANSWERS ABOUT BUCK-BOOST TRANSFORMERS
138%
90.5%
81%
Decrease in Motor Torque
Output
72%
1. What is a buck-boost transformer?
Buck-boost transformers are small single phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 -20%.
The most common example is boosting 208 volts to 230 volts, usually to operate a 230 volt motor such as an air-conditioner
compressor, from a 208 volt supply line.
Buck-boosts are a standard type of single phase distribution transformers, with primary voltages of 120, 240 or 480 volts
and secondaries typically of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 volt amperes to
10 kilo-volt amperes.
Buck-boost transformers are shipped ready to be connected for a number of possible voltage combinations.
2. How does a buck-boost transformer differ from an insulating transformer?
A buck-boost transformer IS an insulating type transformer when it is shipped from the factory. When it is connected at the job
site, a lead wire on the primary is connected to a lead wire on the secondary – thereby changing the transformer’s electrical
characteristics to those of an autotransformer. The primary and secondary windings are no longer “insulated” and secondary
windings are no longer “insulated” and its kVA capacity is greatly increased. Refer to figures 1, 2 and 3.
H4
H3
INPUT
H4
X1
H3
INPUT
H2
H1
Figure 1. Buck-boost transformer connected as a
low voltage insulating transformer (primary and
secondary windings shown series connected).
X2
OUTPUT
X3
X4
H4
X1
H3
INPUT
H2
H1
Figure 2. Same buck-boost transformer
connected as a boosting autotransformer. The
connection from H1 to X4 “converted” the unit to
an autotransformer.
X2
OUTPUT
X3
X4
Figure 3. Illustration No. 2 shown with the primary
and secondary windings “straightened”.
H2
H1
X4
X3
X2
X1
OUTPUT
120
3. What is the difference between a buck-boost transformer and an autotransformer?
When a primary lead wire and secondary lead wire of a buck-boost transformer are connected together electrically, in a recommended voltage bucking or boosting connection, the transformer is in all respects, an auto transformer. However, if the interconnection between the primary and secondary winding is not made, then the unit is an insulating type transformer.
APPLICATIONS
4. Why are they used?
Electrical and electronic equipment is designed to operate on standard supply voltage. When the supply voltage is constantly too
high or too low, (usually more than 55%), the equipment fails to operate at maximum efficiency. A buck and boost transformer is
a simple and ECONOMICAL means of correcting this off-standard voltage.
5. What are the most common applications for buck-boost transformers?
Boosting 208V to 230V or 240V and vice versa for commercial and industrial air conditioning systems; boosting 110V to 120V and
240V to 277V for lighting systems; voltage correction for heating systems and induction motors of all types. Many applications
exist where supply voltages are constantly above or below normal.
6. Can buck-boost transformers be used to power low voltage circuits?
Yes, low voltage control, lighting circuits, or other low voltage applications requiring either 12V, 16V, 24V, 32V or 48V. The unit is
connected as an insulating transformer and the nameplate kVA rating is the transformer’s capacity.
7. Why do buck-boost transformers have 4 windings?
To make them versatile! A four winding buck-boost transformer (2 primary and 2 secondary windings) can be connected eight
different ways to provide a multitude of voltage and kVA outputs. A two winding (1 primary & 1 secondary) buck-boost transformer
can be connected only one way.
8. Will a buck-boost transformer stabilize voltage?
No. The output voltage is a function of the input voltage. If the input voltage varies, then the output voltage will also vary by the
same percentage.
LOAD DATA
9. Are there any restrictions on the type of load that can be operated from a buck-boost transformer?
No, there are no restrictions.
10. Why can a buck-boost transformer operate a kVA load many times larger than the kVA rating on its nameplate?
Since the transformer has been auto-connected in such a fashion that the 22V secondary voltage is added to the 208V
primary voltage, it produces 230V output.
The autotransformer kVA is calculated:
kVA =
kVA =
Output Volts x Secondary Amps
1000
230 V x 41.67 Amps
1000
= 9.58 kVA
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Buck-Boost Transformers Buck-Boost Transformers
The picture to the left illustrates the difference in physical size between
the autotransformer of 1 kVA, capable of handling a 9.58 kVA load, and
an isolation transformer capable of handling a 7.5 kVA load.
To cite an example... a model T111683 buck-boost transformer has a
nameplate kVA rating of 1 kVA, but when it’s connected as an autotransformer boosting 208V to 230V, its kVA capacity increases to 9.58 kVA.
The key to understanding the operation of buck-boost transformers lies
in the fact that the secondary windings are the only parts of the transformer that do the work of transforming voltage and current. In the example above, only 22 volts are being transformed (boosted) — i.e. 208V
+ 22V = 230V. This 22V transformation is carried out by the secondary
windings which are designed to operate at a maximum current of 41.67
amps (determined by wire size of windings).
Maximum Secondary Amps =
Volts x Amps x 1.73
Secondary Volts
(1 kVA) T111683 (7.5 kVA) T2535153S
Maximum Secondary Amps =
1.0 kVA x 1000
=
24V
1000 VA = 41.67 Amps
24V
11. Can buck-boost transformers be used on motor loads?
Yes, either single or three phase. Refer to the motor data charts in Section I for determining kVA and Amps required by NEMA
standard motors.
12. How are single phase and three phase load Amps and load kVA calculated?
Single Phase Amps =
Three Phase Amps =
Single Phase kVA =
Three Phase kVA =
kVA x 1000
Volts
kVA x 1000
Volts x 1.73
Volts x Amps
1000
Volts x Amps x 1.73
1000
THREE-PHASE
13. Can buck-boost transformers be used on three-phase systems as well as single phase systems?
Yes. A single unit is used to buck or boost single phase voltage — two or three units are used to buck or boost three phase
voltage. The number of units to be used in a three - phase installation depends on the number of wires in the supply line. If the
three-phase supply is 4 wire Y, use three buck-boost transformers. If the 3 - phase supply is 3 wire Y (neutral not available),
use two buck-boost transformers. Refer to three-phase selection charts.
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