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Selecting and Applying Buck-Boost Transformers

October 22, 2007
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HVAC equipment manufacturers build units that can be applied with various voltages. Some applications utilize an input or supply voltage of 277-1-60. This power supply comes from a 460-3-60 (4-wire) system (the neutral leg and the line side opposite the neutral leg).

While some components are available for direct application of 277-1-60 power, most manufacturers, including United CoolAir, find that the use of a buck-boost transformer to lower the voltage is more cost effective. The units are fabricated using components suitable for 208/230-1-60 power supply. The control voltage transformer is wired on the 230-volt tap to provide the 24-volt control voltage.

Table 1. Available buck-boost transformers. (Click on the table for an enlarged view.)

SELECTING A BUCK-BOOST TRANSFORMER

The following information should be confirmed for the application:

Line Voltage - Confirm that the supply/input voltage is 277.

Frequency - Input power supply frequency must match the equipment - either 50 or 60 Hertz (Hz). This article is based on only applications of 60 Hz.

Phase - The supply/input power phasing must be the same as the equipment requirements. This article is based on single-phase applications.

Load KVA or Load Amps - You only need to know one or the other. This information usually can be found on the unit nameplate(s). On United CoolAir equipment the minimum circuit ampacity (MCA) should be used as the load amps.

The transformer is used to buck (decrease) the incoming power supply from 277 volts down to the 230 volts required for the unit. Table 1 lists the buck-boost transformers available from United CoolAir.

Read down the column until you see a KVA or amp value that is just higher than the equipment to be operated.

Figure 1. Transformer connection diagram. (Click on the diagram for an enlarged view.)

Connect the transformer according to the diagram shown in Figure 1. Only one transformer is required in the wiring diagram for each power supply to the equipment.

CAUTION: Do not make connections other than those shown. The transformer must be as large (KVA) as the load it must operate. Never exceed the nameplate rating as this could result in overheating, reduced life expectancy, or worst case, fire.

Notes:
1. The symbol “O” used in this connection diagram illustrates where to field install an overcurrent protective device (typically a fuse or circuit breaker) when one input conductor is grounded and the other input conductor is ungrounded.

2. When both input conductors are ungrounded an overcurrent protection device is required to be installed in series with each input conductor.

3. For additional information refer to the National Electrical Code, Article 450-4.

General
The installation, operation, and maintenance of dry-type transformers should be performed by an electrician or other qualified personnel who are familiar with international, national, or local electrical codes. Familiarity with the potential shock hazards associated with electrical equipment is also necessary.

Encapsulated units are NEMA 3R enclosures suitable for either indoor use in harsh environments or for outdoor use. CE marked units have a protection index of IP23.

The transformer must be installed per the National Electrical Code (NEC) and local code requirements. CE marked transformers must be installed per EN 60742.

Inspection and Handling
The transformer should be inspected carefully upon receipt to check for any visible or concealed damage that may have occurred during shipment. If damage is found, a claim should be filed immediately with the carrier.

Transformers 55 pounds or larger are provided with lifting ears. Incorrect handling can bend the enclosure, cause other damage, or result in personal injury.

Installation
When installing a buck-boost transformer, always follow all applicable national and local codes. Outside installation of the transformer should have the wiring in a conduit or as required by national and local codes.

Encapsulated transformers can be installed indoors or outdoors. When installed outdoors, these units should be installed with the wiring compartment down to prevent the entrance of moisture. Some encapsulated units have a top-entry wiring compartment and can be installed vertically (wiring compartment up).

For indoor floor mounting of an encapsulated unit that has a bottom-entry wiring compartment, the unit can be installed horizontally (on its back side) for ease of making wire connections.

WARNING: Danger of electric shock. Turn power off supplying device before installing. Do not remove parts or make connections while transformer is energized.

Electrical Connections
Refer to the transformer nameplate label or enclosed wiring diagram for primary and secondary voltage combinations, frequency, and number of phases. Tap connections and voltage combinations are also listed on the diagram or nameplate.

Proper assembly of the field wiring to the transformer leads is extremely important. Make certain that the connector or terminal is sized for the cable. Space and insulate connections or terminals per the NEC.

Grounding
All dry-type transformers have a ground stud in the enclosure. The transformer enclosure should be solidly grounded to protect personnel. The customer-supplied grounding conductor should have a current-carrying capacity to meet international, national, and/or local requirements.

Maintenance
Non-ventilated encapsulated styles only require periodic wiping of dust and dirt from the outside of the case under normal conditions and environments. Adverse conditions may require more frequent inspections.

CAUTION: Never perform internal maintenance while the unit is energized.

If a dry-type transformer accidentally gets wet, it must be cleaned and thoroughly dried before energizing. Otherwise, complete failure could result.

Storage
Encapsulated transformers should be stored in a clean, dry area. Care shoud be taken to prevent moisture or condensation from entering the transformer. If stored outside, the transformer must be covered and protected from water, dust, and other airborne contaminants.

NOTICE: These instructions are general in nature and may not cover all variations in transformer design or conditions of installation, operation, and maintenance in enough detail to meet customer needs. Additional instructions may be included with the transformer. If you need further information or should a problem arise, contact the manufacturer.

Reprinted with permission from the United CoolAir Corp. TechTips sheet “Selecting and Applying Buck-Boost Transformers.” For more information, visit www.unitedcoolair.com.

Publication date: 10/22/2007

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