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B u c k - B o o s t T r a n s f o r m e r s
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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.
3 Phase Connections
Input (Supply System) Desired Output Connection
Delta 3 Wire WYE 3 or 4 Wire Do Not Use
Open Delta 3 Wire WYE 3 or 4 Wire Do Not Use
WYE 3 or 4 Wire Closed Delta 3 Wire Do Not Use
WYE 4 Wire WYE 3 or 4 Wire Ok
WYE 3 or 4 Wire Open Delta 3 Wire Ok
Closed Delta 3 Wire Open Delta 3 Wire Ok
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 135-136).
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 126 for easy 4-step selection, then go to the charts. Also, pages 15 and 17 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.