Understanding HVAC Transformers

A transformer is an inductive stationary device designed to transfer electrical energy from one circuit to another. Each transformer contains a primary and secondary winding. A changing in voltage applied to one of the windings induces a current to flow in the other windings. In this manner, electrical energy is transferred from one circuit to another. Usually the changing voltage is applied to the primary winding and a current is induced in the secondary. The electrical energy may be tranferred at the same voltage (a coupling transformer) or higher voltage (a step-up transformer) or at lower voltage (a step-down transformer).

Transformers that are used in heating and air conditioning equipment are the lower voltage / step-down transformers. They are designed to reduced the higher line volate power to the 24 to 30 volts necessary to run low voltage control circuits found in furnaces or air conditioners. Some models are designed to power thermostats, gas valves and relays in the common HVAC 24-volt systems. In most transformer installations, the room thermostat is poerated by a low-volate circtui. Some gas valves are also operated by a low-voltage circuit.

All wiring connections to tranformers must be done in accordance with the regulations of the National Electrical Code (NEC). A single thin copper wire is used in a low-voltage circuit. It is commonly identified with the standard red and black insulation. High-voltage circuits use a larger-diamter wire that is commonly covered with white or black wire insulation.

Interconnected transformer secondaries are not allowed by the NEC. One method of avoidin th need fo interconnecting tranformers is by using a single tranformer rated to carry both the heating and cooling load. Using a thermostat and subbase combination with isolated heating and cooling circuits is also an option. You may also use an isolating relay to isolate the heating and cooling power supplies.

How to Size A Transformer

Transformers are not 100 percent efficiencies, you are always going to lose some energy between your primary and secondary coils. Knowing how to size your transformer correctly will ensure you have enough energy remaining to drive the load connection to your secondary coil.

Usually when your HVAC equipment is not adequately powered, you should check your transformer primary and secondary coil voltages. If you test the voltage and you are within 10 percent of the rated voltage and you are sure there is no issue with the wiring, the transformer may not be large enough for your equipment. A transformer too small for a system can lead to more serious matters. As a result, contacts or motor starters will not operate properly, and later your compressor may suffer damage.

When you need to replace your transformer, always select on e that is the same size or larger than the one being replaced. For new installations, follow the equipment manafacturers' recommendations. If you do not feel comfortable installing a transformer, turn to a local HVAC company or contractor for help.

The capabilities of a transformer are described by its electrical rating. This information will include the primary voltage and frequency, the open-circuit secondary voltage and the load rating in voltage amperes (VA).

The class 2 transformers used in low-voltage control circuits have a maximum load rating of 1000 VA and a maximum open-circuit secondary voltage of 30 volts. The secondary current must also be limited. This can be accomplished by using an energ-limiting transformer or adding a 3.2 ampere fuse in the secondary.

Installing Transformers

Always closely follow the transformer manufacturer's installation instructions, because they will vary depending on the model and the specific application. The following guidelines apply to most transformers:

  1. Disconnect the power supply to prevent equipment damage or electric shock before attempting to remove or install a transformer.
  2. Separate and tape each exposed, unused lead wire
  3. Do NOT short the transformer secondary terminal or you may burn out th eoverload protection
  4. Check the specification section of the trasnformer for lead wire color-coding
  5. Connect your primary lead wires to the line voltage power supply
  6. Connect the tranformer secondary to the 24-VAC control system. If the transofrmer model has a primary or secondary conduit spud, connect the wires first and then screw the conduit onto the spud.
  7. Check the secondary voltage before connecting the transformer back to the power supply
  8. Turn the power supply back on and operate the system for one or two complete cycles