Equipment Input Power, Load, and Control Circuits

Troubleshooting electrical problems in HVAC equipment may appear complex. However, electrical troubleshooting can be simplified if the unit’s electrical components are divided into functional groupings based on the operation they perform. Most air conditioning equipment can be divided into the three functional circuit areas shown in Figure SP-7-1. The functional areas are:

• Input power distribution circuits
• Load circuits
• Control circuits

Input Power Distribution Circuits

Input power distribution circuits serve as the power source for the entire unit. They operate at either single-phase or threephase line voltage and distribute the input power to the various loads in the unit. Power circuits usually consist of the field installed power wiring from the main electrical service to a disconnect switch located near the unit, and from the disconnect switch to the unit. The input power and distribution circuits include protective devices such as fuses and/or circuit breakers.

Load Circuits

Loads are devices that convert electrical energy to another form of energy such as heat or mechanical motion. In the process, loads consume power. Compressor motors, fan motors, heater elements, and the primary winding of transformers are all loads normally found in cooling and heating units. Because the input power distribution circuits and the load circuits are both energized and operate at the input voltage level, they are often called the high-voltage circuits.

Control Circuits

Control circuits provide a link between the loads and the input power. Control circuits start, stop, or otherwise control the operation of a load. They usually contain one or more control devices, such as relays, switches, and thermostats that work to apply or remove power from the loads. The more complex the system, the more control devices it will have. When a load such as a compressor motor is not working, you have to determine whether the problem is in the load itself or in the circuits controlling the load.

Control circuits or low-voltage circuits generally operate at 24 volts. This low voltage is obtained by using a control transformer to step down the line voltage. Use of a low voltage to power the control circuits has the following advantages:
• Low-voltage control circuits are safer to operate and to work on.
• Low-voltage control circuits draw less power.
• Low-voltage control circuit components and their related wiring are smaller and lighter.
• Low’voltage control circuit components, such as those in a room thermostat, provide more accurate temperature control.

In some larger systems using three-phase line voltages of 230 volts and higher, a control voltage of 115 volts or higher may be used for some o f the control devices. You should refer to the manufacturer’s literature and/or the unit schematic to determine what specific voltage is used for the unit’s control circuits. Also, it is a good practice to always measure the control circuit voltage. Never assume the control circuit voltage is a low voltage.


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