A buck–boost transformer is a type of autotransformer used to make small adjustments to the voltage applied to alternating current equipment. Buck–boost connections are used in several places such as uninterruptible power supply (UPS) units for computers, electric power distribution, and in the tanning bed industry.
There are two basic types, self adjusting (active) or passive designs. The active types monitor incoming voltages and will adjust the outgoing voltage to be within an acceptable range. This is typically between 115VAC and 225VAC for computer UPS systems. The system will either buck (lower) or boost (raise) the voltage if it senses a variance in the incoming voltage. Several taps are provided on the transformer winding which allow adjustment of the ratio. In an active buck–boost transformer, a control circuit selects which tap to use to maintain the output voltage within the desired range, over a range of input voltages.
Passive transformers are used for larger equipment where the amount of buck or boost is fixed. For example, a fixed boost would be used when connecting equipment rated for 230 VAC to a 208 V power source.
The passive transformers are rated in volt-amperes (or more rarely, amperage) and are rated for a percent of voltage drop or rise. For example, a buck–boost transformer rated at 10% rise at 208VAC will raise incoming voltage of 210VAC to 231VAC. A rating of 5% drop at 240VAC will yield the result of 233VAC if the actual incoming voltage is 245VAC.
All transformers only operate with alternating current. Transformers only change voltage, not frequency. Equipment that uses induction motors will operate at a different speed if operated at other than the design frequency. Some equipment is marked on its nameplate to run at either 50Hz or 60Hz, and would only need the voltage adjusted with a buck–boost transformer.
Most passive transformers come semi-wired, where the installer completes the last internal connections to have the unit perform the amount of buck or boost needed. They have multiple taps on both the primary and secondary coils to achieve this flexibility. They are designed for hard wired installations (no plugs) and allow the same transformer to be used in several different applications. The same transformer can be rewired to raise or lower voltage by 5%, 10% or 15% for either 208VAC or 240VAC applications, depending on the final wiring done by the electrician.
Fixed transformers with around the same cost were introduced primarily for the tanning market. They are prewired, and must be purchased with the exact amount of buck or boost needed for the application. They have factory-installed plugs and receptacles making installation very quick and easy, and reducing the need for hard-wiring small loads.
A typical fixed unit will have a NEMA 6-20 plug for attachment to the prewired 240V wall receptacle, and a receptacle for the load equipment. This eliminates the need for professional installation if the exact incoming voltage can be determined. To make them easier for end-users to select, they are rated in load amps (A) rather than buck–boost volt-amps (kVA). These are used almost exclusively in light to moderate applications that require 40 amps or less.
Not all 240V equipment requires voltage correction. These transformers are used when electrical equipment has a voltage requirement that is slightly out of tolerance with the incoming power supply. This is most common when using 240V equipment in a business with 208V service or vice versa.
Equipment should be labeled with its voltage rating, and may advertise the amount of tolerance it will accept before degraded performance or damage can be expected. A unit that requires 230VAC with a tolerance of 5% will not require a buck–boost transformer if the branch circuit (under load) is between 219VAC and 241VAC. Measurement should be made while the circuit is loaded, as the voltage can drop several volts compared to the open measurement. The transformer must be rated to carry the full load current or it will be damaged.
Operating electrical equipment at other than its designed voltage may result in poor performance, short operating life, or possibly overheating and damage.
For large adjustments in voltage (more than 15% to 20%), usually a two-winding transformer is used with the required voltage ratio, for example 240VAC to 120VAC. These transformers are more costly than buck–boost transformers since both windings must carry the full power delivered to the load, whereas the buck–boost winding must only carry a fraction of the load power.