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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.
SOUND LEVELS
22. Are buck-boost transformers as quiet as standard isolation transformers?
Yes. However, an auto-connected buck-boost transformer will be quieter than an isolation transformer capable of handling
the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and small trans-
formers are quieter than larger ones. (Example) 1 kVA — 40 db; 75 kVA — 50 db. (db is a unit of sound measure).
COST AND LIFE EXPECTANCY
23. How does the cost of a buck-boost transformer compare to that of an insulating transformer — both capable of
handling the same load?
For the most common buck-boost applications, the dollar savings are generally greater than 75% compared to the use of an
insulating type distribution transformer for the same application.
24. What is the life expectancy of a buck boost transformer?
The life expectancy of a buck-boost transformer is the same as the life expectancy of other dry type transformers.
NATIONAL ELECTRICAL CODE
25. Your catalog indicates that a buck-boost transformer is suitable for connecting as an AUTOTRANS-FORMER.
What is the definition of an autotransformer and how does it differ from an isolation transformer?
An autotransformer is a transformer in which the primary (input) and the secondary (output) are electrically connected to
each other. An isolation transformer, also known as an insulating transformer, has complete electrical separation between the
primary (input) and the secondary (output). This is illustrated in the drawing below.
An autotransformer changes or transforms only a portion of the electrical energy it transmits. The rest of the electrical energy
flows directly through the electrical connections between the primary and secondary. An isolation transformer (insulating
transformer) changes or transforms all of the electrical energy it transmits.
Consequently, an autotransformer is smaller, lighter in weight, and less costly than a comparable kVA size insulating transformer.
Please refer to Question 27 for additional information on autotransformers.
Buck-boost transformers are frequently field-connected as autotransformers.
PRIMARY
(INPUT)
PRIMARY
(INPUT)
SECONDARY
(OUTPUT) SECONDARY
(OUTPUT)
AUTOTRANSFORMER
ISOLATION
TRANSFORMER
(ALSO CALLED
“INSULATING TRANSFORMER”)
SHUNTWINDING(S)
B
A
Diagram 450-4