SECTION
BUCK-BOOST TRANSFORMERS
ACME ELECTRIC
•
MILWAUKEE, WI
•
800.334.5214
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acmetransformer.com
91
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 = kVA x 1000
Volts
Three phase Amps = kVA x 1000
Volts x 1.73
Single phase kVA = Volts x Amps
1000
Three phase kVA = 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.
14. Should buck-boost transformers be used
to develop a three-phase 4 wire Y circuit from a
three-phase 3 wire delta circuit?
No.
A three phase “wye” buck-boost transformer connection
should be used only on a 4 wire source of supply. A delta to
wye connection does not provide adequate current capacity to
accommodate unbalanced currents flowing in the neutral wire
of the 4 wire circuit.
15. Why isn’t a closed delta buck-boost
connection recommended?
A closed delta buck-boost auto transformer connection
requires more transformer kVA than a “wye” or open delta
connection and phase shifting occurs on the output.
Consequently the closed delta connection is more
expensive and electrically inferior to other three-phase
connections.
Connection and Frequency
16. How does the installer or user know how
to connect a buck-boost transformer?
The connection chart packed with each unit shows how to
make the appropriate connections. These same connection
charts are also shown in this section (page 118).
17. Can 60 Hertz buck-boost transformers
be used on a 50 Hertz service?
No.
Acme buck-boost transformers should be operated only at
the frequencies recommended. However, units recommended
for 50 cycle operation are suitable for 60 cycle operation but
not vice versa.
Selection
18. How do you select a buck-boost
transformer?
Refer to the selection steps on page 101 for easy 4-step
selection, then go to the charts. Also, pages 12 and 13 are
helpful for determining buck-boost kVA when only the H.P.
rating of a motor is available.
Nameplate Data
19. Why are buck-boost transformers
shipped from the factory as insulating
transformers and not preconnected at the factory
as autotransformers?
A four winding buck-boost transformer can be auto connected
eight different ways to provide a multitude of voltage and
kVA output combinations. The proper transformer connection
depends on the user’s supply voltage, load voltage and load
kVA. Consequently, it is more feasible for the manufacturer to
ship the unit as an insulating transformer and allow the user
to connect it on the job site in accordance with the available
supply voltage and requirements of his load.
20. Why is the isolation transformer kVA
rating shown on the nameplate instead of the
autotransformer kVA rating?
The kVA rating of a buck-boost transformer when auto
connected depends on the amount of voltage buck or boost.
Since the amount of voltage buck or boost is different for
each connection, it is physically impossible to show all of the
various voltage combinations and attainable kVA ratings on the
nameplate. A connection chart showing the various attainable
single phase and three-phase connections is packed with
each unit.
Safety
21. Do buck-boost transformers present a safety
hazard usually associated with autotransformers?
No.
Most autotransformers, if they are not of the buck-boost
variety, change voltage from one voltage class to another.
(Example 480V to 240V) In a system where one line is
grounded, the user thinks he has 240V; yet due to the primary
and secondary being tied together, it is possible to have 480V
to ground from the 240V output. A buck-boost transformer only
changes the voltage a small amount, such as 208V to 240V.
This small increase does not represent a safety hazard, as
compared to a buck of 480V to 240V. Refer to Figure on the
following page.
3 PHASE CONNECTIONS
INPUT
(SUPPLY SYSTEM)
DELTA
3 wire
OPEN DELTA
3 wire
WYE
3 or 4 wire
WYE
4 wire
WYE
3 or 4 wire
CLOSED DELTA
3 wire
DESIRED OUTPUT
CONNECTION
WYE
3 or 4 wire
WYE
3 or 4 wire
CLOSED DELTA
3 wire
WYE
3 or 4 wire
OPEN DELTA
3 wire
OPEN DELTA
3 wire
DO NOT USE
DO NOT USE
DO NOT USE
OK
OK
OK
