Buck-boost transformers, sometimes used as push-pull transformers, are a type of transformer that is used to supply power to electrical equipment in cases when the voltage requirements of that equipment are different from the available line or supply voltage. The need to raise the supply voltage might result from a drop in line voltage due to equipment demand on the power distribution system, or from line losses. Should the supply voltage to equipment be unstable, the performance of that equipment may be impacted, causing it to not operate at peak efficiency, or in extreme cases, premature equipment failure may result. For example, a motor that is operated at a level of voltage which is significantly below its rated value may run constantly on its starter windings, resulting in overheating and possible burnout.

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The 2008 National Electric Code (NEC) Handbook Section 210.9 defines a buck-boost transformer as one that “…provides a means of raising (boosting) or lowering (bucking) a supply line voltage by a small amount (usually no more than 20 percent). While standard transformers alter the input voltage to a value of output voltage that may be substantially different from the input, buck-boost transformers are designed to make more modest changes to voltage levels, usually less than +/- 30 percent.

Buck-boost transformers consist of two primary windings and two secondary windings. In standard transformer designs, the primary and secondary windings are typically isolated from each other electrically, meaning that they are only coupled magnetically through mutual induction. With buck-boost transformers, however, the design configuration is altered to one in which the windings are connected to allow the input or supply-side voltage to be altered as needed to suit the specific application. Using this approach, the output voltage of the transformer can be a value that has been bucked (lowered) or boosted (raised) from that of the supply voltage.

For the operation of equipment that is running on single-phase AC power, one buck-boost transformer may be used. Several units are needed to adjust the input voltage for three-phase AC powered equipment, depending on the type of three-phase connection employed, open delta or wye. Four-wire wye configurations will require three buck-boost transformers; 3-wire wye configurations will need two buck-boost transformers.

Single-phase buck-boost transformer connection diagram

Figure 1 below shows an example of an electrical connection diagram that illustrates the connection configuration for single-phase power to boost and buck a supply voltage. To obtain a boost in output over the supply voltage, the input voltage is applied across two of the four windings and the output is drawn from terminals that are across all four windings. The opposite is done when the goal is to buck a supply voltage down to a lower voltage output.

Figure 1 – Single-phase buck-boost transformer connection diagram for increasing or decreasing output

Image credit: https://cdn.automationdirect.com/static/specs/buckboosttransformerspecs.pdf

Buck-boost transformers are built as isolation transformers, meaning they have separate primary and secondary coils. Once the unit is ready to be installed on-site, the installation team or end-user can connect the primary to the secondary to change the device’s electrical characteristics. By wiring together the primary and secondary windings, the buck-boost transformer functions as a single-winding autotransformer. Using additive and subtractive polarity, small changes to voltage can be achieved in an electrical distribution circuit. Manufacturers provide specific connection diagrams applicable to their different transformer models to achieve the desired output voltage buck or boost. The use of four windings in buck-boost transformers permits the device to be wired in eight different ways or configurations, making buck-boost transformers flexible enough to meet a variety of application conditions.

Buck-boost transformer applications

Common uses of buck-boost transformers are as follows:

to boost 110 VAC to 120 VACto boost 240 VAC to 277 VAC for lighting applicationsto provide low voltage outputs of 12, 16, 24, 32, or 48 VAC from a high voltage input

Buck-boost transformers are used in applications such as powering:

Industrial lighting systemsTelecommunications applicationsUninterruptable Power Supply (UPS) for computer systems

Buck-boost transformers are generally highly efficient, have a smaller footprint, are lighter, weigh less, and are less expensive than other transformer solutions such as a distribution transformer. A few of the limitations of these devices are that they do not offer circuit isolation, they cannot provide a neutral, and they cannot be used with a closed delta three-phase wiring configuration, It should be noted as well that buck-boost transformers do not provide any voltage stabilization, so if the supply side voltage fluctuates, the output voltage will change by the same percentage.

Key specifications & selection process

Buck-boost transformers are available in many standard catalog items with defined low and high voltage levels and kVA ratings. Manufacturers and suppliers provide selection charts that can be used to choose an appropriate buck-boost transformer to suit specific application needs and conditions. The general process for the selection of a buck-boost transformer begins with defining the application conditions. This includes specifying the following parameters:

The system phase – single or three-phase operation, which must be the same for the load as for the supply.The system frequency – the frequency of the operating load which must be the same as that of the supply or line, e.g. 50 Hz, 60 Hz.The line or supply voltage – the measured value of supply voltage which is desired to be bucked (decrease) or boosted (increased)The load voltage – the voltage level that the equipment being powered is designed to operate.The electrical configuration – delta or wye.The load kVA, load current, or horsepower – only one of these values is needed and can usually be found on the nameplate of the equipment being operated.

Buck-boost transformers typically consist of series-multiple windings, meaning that there are two similar coils in each of the windings that can be connected either in series or in parallel. Units that have series-multiple windings will show their ratings in terms of two values separated by a slash (e.g. 120/240 VAC Primary, 12/24 VAC Secondary).

Once the use conditions are specified, the appropriate device can be selected from the manufacturer’s selection chart and the reference wiring diagram can also be accessed to define the configuration and the terminal connections for the installation of the transformer.

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This article presented a basic summary of buck-boost transformers, including what they are, their applications, key specifications, and the selection process. For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories.