Technical Article

# “Bucking” Voltage Through a Buck-Boost Transformer

June 30, 2021 by Alex Roderick

## This article highlights how a buck-boost transformer wired in an open delta when the load is relatively low to buck the voltage from 240V to 208V.

There are applications where it may be required to buck a supply voltage for special equipment. Motors are rated to operate at ±10% of the nominal voltage. A piece of equipment may be rated at 208 V in a location where 240 V is available. The voltage difference is enough that the source needs to be reduced in order to prevent an over-current situation.

## Conveyor Motors

In the case of a 208V rated motor turning gear for a conveyor drive, a 240V supply is too high. Since this is a non-fluctuating load, a buck-boost transformer is a good choice to reduce the voltage.

The voltage must be reduced by 32V. A buck-boost transformer with a 7.5:1 winding ratio wired with subtractive polarity can be used to achieve this amount of buck. The buck-boost transformer can be wired in an open delta with only two transformers supplying the load (see Figure 1). The primary is sized for 120 × 240V while the secondary is sized for 16 × 32V. If the motor pulls 24A, the size of the buck-boost transformer can be calculated as follows:

$$P=\frac{I\times E}{1000}=\frac{24\times 32}{1000}=0.768\text{ }kVA$$

The manufacturer can install two 1kVA buck-boost transformers to supply the load. The open delta is popular because only two transformers are needed.

## Bucking 240V to 200V

There are applications where a piece of equipment operates at a nonstandard voltage. This may occur when the equipment is modified locally or when it is purchased from another country.

## Oil Heaters

A manufacturing plant has a 240V service available. A piece of equipment that was built overseas has a built-in oil heater that is rated at 10kW at 200V on 50Hz. The frequency is not a factor since the only opposition to current flow is the resistive heater. If the heater is operated at 240V instead of the nominal 200V, the power is 14.4kW instead of the nominal 10kW. This extra power is well above the design power and could cause overheating and a possible fire.

The 240V source needs to be reduced to a value near the nominal 200V required by the oil heater. A buck-boost transformer with a 7.5:1 winding ratio wired with subtractive polarity can be used to buck the source down to 208V (see Figure 2). The primary is sized for 120 × 240V, while the secondary is sized for 16 × 32V.

##### Figure 2. Voltage can be bucked from 240 V to 208 V to supply a 200 V heater.

At 208V, the heater provides 10.8kW of heat and draws 52A. The size of the buck-boost transformer can be calculated as follows:

$$P=\frac{I\times E}{1000}=\frac{52\times 32}{1000}=1.664\text{ }kVA$$

A buck-boost transformer larger than 1.664kVA and wired in subtractive polarity needs to be installed. This bucks the voltage down to 208V.

## Other applications

There are many applications where a technician may need to determine the proper wiring connections to develop a particular voltage. This requires knowledge of the available source voltage, the desired output voltage, and the type of buck-boost transformer available. There are three very common transformers available for use as buck-boost transformers, all with dual-wound primaries and secondaries.

These common transformers are a 120V × 240V transformer with a 10:1 turns ratio that results in either 12V or 24V across the secondary; a 120V × 240V transformer with a 7.5:1 turns ratio that results in either 16V or 24V across the secondary, and a 240V × 480V transformer with a 10:1 turns ratio that results in either 24V or 48V from the secondary.

##### Figure 3. Buck-Boost Wiring Diagrams - Single Phase

The most common case is where a known source voltage is available, but it is not correct for the particular application. A technician can use the above figure to determine the correct wiring arrangement. 