Buck-boost transformer
From Wikipedia, the free encyclopedia
The term buck-boost (often spelled different ways) refers to a transformer that makes small adjustments to the incoming voltage for equipment that is in use. These are used in several industries, including in more advanced uninterruptible power supply units (UPS) for computers and in the tanning bed industry.
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[edit] Types
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.
Passive transformers are used for larger equipment where the amount of buck or boost is fixed. This is commonly used when someone wants to use a piece of equipment that is made for European power (220VAC to 230VAC) in the United States, which has both 208V and 240V service available.
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. The ratings of 208VAC for business class three phase power, and 240VAC for residential class electrical power are approximate, and will vary by several volts depending on the location and even the time of day and local demand.
[edit] Hertz
Buck/boost transformers only adjust the voltage, not the frequency, or cycles of the electricity, so you can't use one to adjust equipment that requires 50Hz in the US. All US power plants us a 60Hz system. Some equipment is designed to run at 50Hz or 60Hz, and would work fine.
[edit] Alternating Current
Buck-boost transformers, like all transformers, are designed to work with alternating current (AC) only, and do not convert the electricity to direct current (DC). It is a bridge rectifier (diode bridge) that converts AC to DC. Many AC adapters are regular transformers with rectifiers built in, and very different than a buck-boost transformer.
[edit] Configuration
Most passive transformers come semi-wired, where you complete 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.
In the tanning industry (and for use in other industries), there are new fixed transformers that cost about the same but are configured differently. They are already prewired, so you must purchase them with the exact amount of buck or boost you need for your application. Rather than being hard wired units, however, they have plugs and receptacles making installation very quick and easy.
A typical unit will have a NEMA 6-20 cord a plug to plug into the prewired 240V circuit, and a receptacle on the unit to plug in the tanning bed or other equipment into. This elliminates the need for an electrician if you can determine your exact incoming voltage. To futher make them easier to use, they offer ratings in amps rather than volt amps, which makes it easier to match up the right transformer for the task. These are used almost exclusively in light to moderate applications that require 240VAC 40 amp or less.
[edit] Purpose
Not all 240V equipment needs a transformer to buck or boost the voltage. These types of transformers are used when a piece of electrical equipment has an electrical 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. It is also common for electronics that are made in a different country than where they are being used.
Each piece of equipment should be labeled with the voltage requirements and often with the amount of tolerance it will accept before less than optimum performance can be expected. A unit that requires 230VAC with a tolerance of 5% will not require a buck-boost transformer if the incoming power (under load) is between 218.50VAC and 241VAC. The measurement should be made while the circuit is under the full load as the voltage can drop as much as 3VAC compared to the circuit at no load.
Often, equipment will instead be rated with a voltage range, such as 220VAC to 230VAC, requiring a buck-boost transformer if the incoming power is not within this range. It is important that you use a buck-boost transformer that is rated equal to or greater than the rated load of the equipment you are adjusting the voltage for, or you can damage (or melt) the transformer and do damage to the equipment.
If a piece of equipment needs a buck-boost transformer but one is not used, it can cause damage to the equipment. Running the equipment at a voltage that is lower than the rated amount can cause the amperage load to rise, in order to meet the total wattage requirement of the equipment. This can result in damage, including melting of wires or parts. Running at a voltage too high can cause other types of damage. The type of damage that can happen depends on the type of equipment and its sensitivity to improper voltages, but in most circumstances, it will at least greatly reduce the service life of the equipment and make it more prone to failure.
When the amount of rise or drop in the incoming voltage is great (more than 15% to 20%) then this is usually out of the boundries of what a buck-boost transformer is designed for, and a line transformer is required. Line transformers are basically the same, but with different tap points to provide greater rise or drop in voltage, such as 240VAC to 120VAC.
