Phase Converters & Power Factor
Phase Converter Efficiency
Installing a Phase Converter
Rotary Phase Converters
Static Phase Converters
VFDs as Phase Converters
     • Harmonic Distortion
Three-Phase Motors
Phase Converters & Voltage Balance
Phase Converter Applications
     • Submersible Pumps
     • Woodworking Equipment
     • Dual Lift Stations
     • Phase Converters & Welders
     • Phase Converters & CNC Machines
     • Phase Converters & Air Compressors
     • Phase Converters & Elevators
     • Phase Converters & Wire EDM
     Phase Converters & HVAC
Phase Converters & Transformers
     • Step-up Transformers
     • Buck-Boost Transformers
     • Isolation Transformers
Phase Converter Experts
Digital Phase Converters
Regenerative Power
Three-Phase Power
     • Delta vs. Wye Configured Power
Motor Starting Currents

Phase Converters and Buck-Boost Transformers


Buck-Boost transformers are single phase, four winding transformers designed for two purposes. The first purpose is as a low voltage isolation transformer for use on 12, 16, 24, 32, or 48 volt circuits. Their second, and more important use, is as a "Buck-Boost" transformer. Buck-Boost means that these transformers are used to buck (lower) or boost (increase) line voltage to match required load voltage.

Buck-Boost transformers are used to correct consistently low or high voltage conditions, where the voltage difference ranges from approximately 5% to 27%. Variation of the input voltage is passed through to the output side of the transformer in the same percentage. This concept is important because the question is frequently asked if Buck-Boost transformers will stabilize variable voltages. They will not!


The most common applications for these transformers are boosting 208 volt lines to 230 or 240 volts, and vice-versa. These applications include

both single and three phase lines and are particularly common in HVAC applications for air conditioning appliances.  Additionally, Buck-Boost transformers are very well suited to motor loads, boosting 240 to 277 for lighting loads and many applications where nominal voltage is above or below 120 volts.

Their use should be considered where supply line voltage is found to be consistently above or below desired nominal values of load voltage.

Buck-Boost transformers are also commonly used on the input line side of phase converters. Most three-phase utility service is 208 volts and single-phase service is 240 volts. Phase converters produce three-phase power equal in voltage to the single-phase input voltage of 240 volts. While most three-phase equipment can operate on 208 or 240 volts, other equipment is designed to operate only on 208 volts and 240 volts will be outside its operating range. Reducing the input voltage to a safer level with a Buck-Boost transformer is a convenient, affordable solution to this problem.