Boosting Voltage with a Buck-Boost Transformer

Many small plants in the middle of an industrial park or at the end of a long line suffer from low voltage during peak load periods. A 208Y/120V 3-phase, 4-wire system seems to be especially vulnerable to low power during these peak periods. The phase-to-phase voltage can sometimes go as low as 185V between lines, and the phase-to-neutral voltage can go down to 107V.

We can use a buck-boost transformer to boost the voltage and address this problem. However, with a fluctuating load, as the loads are removed from the adjacent users, the supply voltage rises, and the boosted voltage also rises from line to line and on the single-phase-to-neutral. The single-phase-to-neutral can rise to a level that is a problem for some equipment.

Motor Starting

Low line voltage reduces the available starting torque on 3-phase and single-phase motors, such as water pumps, water coolers, air conditioning compressors in window and rooftop units, and many other applications. Buck-boost transformers can be used to boost the line voltage up to the nominal level.

Starting with a 185V source at times of peak load and a 120 × 240V to 16 × 32V buck-boost transformer, the 120V coils can be put across the lines in parallel and the 16V coils wired in series. This results in 107V per coil with an output of 14V on each coil in the secondary (See Figure 1). The two secondary coils give a boost of 28V, which boosts the line-to-line voltage from 185V to 213V. This gives 123V from phase to neutral, which is very near the nominal 120V expected.

Figure 1. The maximum amount of buck or boost occurs when the secondary is wired in series

 

Note

A buck-boost transformer is shipped from the factory wired as an isolation transformer. When the transformer is installed, the primary and secondary can be connected as an autotransformer to create a buck-boost transformer.

In off-peak times, the input to the buck-boost transformer rises back up to 208V, and the amount of boost rises in direct proportion to the increase in primary voltage. With 120V on each of the primary coils, the output of each secondary coil rises to 16V. With the secondary coils in series, a line-to-line value of 240V is present along with a line-to-neutral value of 138.6V. This is probably too high and can cause problems with electrical equipment designed for 120V.

 

Parallel Secondary

Starting with a 185V source at times of peak load and a 120 × 240V to 16 × 32V buck-boost transformer, the 120V coils can be put across the lines in parallel and the 16V coils wired in parallel. This results in 107.3V per coil with an output of 14.3V on each coil in the secondary. (see Figure 2). The two secondary coils give a boost of 14.3V, which boosts the line-to-line voltage from 185V to 199.3V. This gives 115.06V from phase to neutral, which is very near the nominal 120V expected. 

Figure 2. The minimum amount of buck or boost occurs when the secondary is wired in parallel

In off-peak times, the input to the buck-boost transformer rises back up to 208V, and the amount of boost rises proportionally. With 120V on each of the primary coils, the output of each secondary coil rises again to 16V. With the secondary coils in parallel, a line-to-line value of 224V is present, along with a line-to-neutral value of 129.3V.

The user can select either the series or the parallel secondary, depending on the need. If a change is needed between the two, it is simple to rewire the secondary to provide the solution.

 

Boosting 208V to 230V

There are applications where 208Y/120V power is available, but 230V power is needed. This type of application uses a 230V motor or a 230V air conditioning system powered from a 208V supply power line.

This required boost can be accomplished by wiring both the primary and secondary coils of a buck-boost transformer in series. The coils of the primary in series have 104V across each one. With the turns ratio of the transformer being 7.5:1, the secondary coils have 13.9V across each coil. This gives a boost of 27.8V, giving just under 236V line-to-line.

 

Parts Heater

A small industrial facility is fed with a 208Y/120V service (see Figure 3). Small parts are heated in a small cabinet. The facility is concerned about the amount of time to heat the parts because it slows down production.

Figure 3. Voltage can be boosted from 208V to 236V to supply a 230V parts heater.

 

Note

AC voltage in power systems should be within –10% and +5% of rated voltage.

The cabinet has two 5kW, 230V heating elements. This would give the cabinet 10kW of heat at 230V. Since only 208V is available, the voltage is only 90% of the nominal 230V required by the heaters. Therefore, the power is only 81% of the nominal 10kW or about 8.1kW.

A voltage boost of about 10% is needed for the heaters to work at nominal load. The wiring configuration described above gives a 13.3% boost from 208 V to just under 236 V. This is within the voltage tolerances of most loads. Since this is not a situation where the load is fluctuating, this is a good application for a buck-boost transformer.

 

Extruder Heaters

A manufacturing plant is fed with a 208V, 3-phase service (see Figure 4). The plant uses an extruder to heat plastic material in a 230V machine. The material is forced through a barrel, similar to a cannon, by a large screw located in the bore. The barrel is surrounded by band heaters that provide heat to melt the plastic granules so that the material can be forced through a die to make various products.

Figure 4. Voltage can be boosted from 208V to 226V to supply a 230V extruder band heater.

The heated barrel, combined with the friction of the material being forced through the bore, brings the plastic material up to the desired temperature. Different plastics and different colors require different amounts of heat to melt, so each color has its own heat setting.

A 10kW 230V band heater on 208V only supplies about 8.1kW instead of the design 10kW. Materials that have the highest heat settings are not melting properly because of the low voltage to the heaters. This type of extruder may have eight-band heaters for each extruder. Therefore, the extruder heaters only provide about 65kW of heat instead of the 80kW of heat required.

The load on the extruder is calculated by checking the nameplate for the minimum circuit. In this case, the total load current is 227A. A boost of 10% is enough to raise the 208V source to nearly 230V for the extruder heaters. A buck-boost transformer with a 10:1 winding ratio can be used to achieve this amount of boost. The primary is sized for 120 × 240V while the secondary is sized for 12 × 24V. The buck-boost transformer size for each phase is calculated as follows:

$$P=\frac{I\times E}{1000}=\frac{227\times 24}{1000}=5.448\text{ }kVA$$

The actual voltage rise is only about 18V (10.4 × 1.732 = 18). Therefore, the manufacturing plant should be able to install three 5kVA buck-boost transformers to bring the extruder heaters up to about 226V (208 + 18 = 226). Larger buck-boost transformers may be better to minimize problems with heat buildup but can lower the power factor.