Electrical Terms and Definitions
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When discussing HVAC controls, there are two main circuits that we focus on. The first being the power circuit. This circuit can be either single-phase or three-phase, and power the equipment which is doing the work of either heating, cooling or ventilating.
The second circuit we will focus on is the control circuit. This is the circuit which tells the power circuit when to engage. The control circuit will sometimes operate at an extra-low voltage value of 24 volts relative to the power circuit, which usually operates at a line voltage value of 240 volts or more.
In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.
The simplest electrical circuit. It requires only two lines: one for power to go in and the other is a return path for current. These are often called Line 1 and Line 2, or Line 1 and Neutral. Current only has one path to travel in a single-phase circuit, such as a control circuit.
An electrical circuit that uses three current carrying conductors, called Line 1, Line 2, and Line 3, which have a 120° phase shift in the voltage and current waveforms between them. The Power Circuit of three-phase motors is an example of a three-phase circuit.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
Any voltage not exceeding 30V. Equipment energized at 24 V is commonly referred to as the "low voltage circuit".
