Buck-Boost Transformers

Shop buck-boost transformers used to make relatively small adjustments to an existing AC supply voltage. They are commonly applied when equipment voltage differs slightly from the available electrical source, such as raising or lowering voltage for motors, HVAC equipment, lighting and industrial loads.

Compare buck-boost transformers by supply voltage, required load voltage, phase, frequency, transformer VA rating and connected-load current. The required transformer size depends on the amount of voltage correction and the load—not solely on the total load kVA.

How to Select a Buck-Boost Transformer

A buck-boost transformer adjusts an existing AC voltage by adding to or subtracting from the supply voltage. It is commonly used when the available voltage is close to—but not within the preferred operating range of—the connected equipment.

Buck-boost applications require the correct transformer and connection diagram for the source voltage, desired output voltage, phase and load current. The same transformer may support different voltage corrections depending on how its windings are connected.

Common Applications

• Correcting low or high supply voltage at equipment
• Adapting voltage for motors and HVAC equipment
• Adjusting voltage for lighting systems
• Compensating for voltage drop in long electrical runs
• Matching imported or replacement equipment to an available supply

Important Selection Factors

• Measured supply voltage: Use the actual voltage available at the equipment under representative operating conditions.
• Required load voltage: Confirm the equipment’s acceptable input-voltage range.
• Load current: Determine the connected equipment’s full-load current.
• Phase: Verify whether the application is single phase or three phase.
• Frequency: Match the transformer and equipment to the available frequency.
• Required correction: Calculate the amount of voltage that must be added or subtracted.
• Connection diagram: Use the manufacturer’s diagram for the exact voltage and phase combination.
• Enclosure and environment: Confirm suitability for the installation location.

Buck-Boost Transformer Sizing

In a typical buck-boost connection, the transformer handles only the portion of power associated with the voltage correction. This means its nameplate VA rating may be much smaller than the total connected load. Proper sizing requires the source voltage, target voltage, phase and full-load current.

Buck-boost transformers generally do not provide the same electrical isolation as a separately connected isolation transformer when wired as an autotransformer. Confirm whether isolation is required before selecting this product type.

Replacement Checklist

• Manufacturer and complete transformer part number
• Actual measured supply voltage
• Required equipment voltage
• Single-phase or three-phase application
• Connected-load current or equipment horsepower
• Frequency
• Existing winding and connection diagram
• Required enclosure and mounting configuration