Understanding Common, Start, and Run in HVAC Systems

In the world of HVAC, understanding the basics of common start and run (CS and R) is essential for technicians working with single-phase motors, specifically permanent split capacitor (PSC) motors. Let’s dive into the key concepts and components associated with CS and R.

Single-Phase Motors and PSC Motors

When we talk about common start and run, we’re referring to single-phase motors, which do not utilize three-phase power. The focus is on PSC motors, also known as permanent split capacitor motors, which are commonly found in residential and light commercial HVAC systems.

Components and Terminals

CS and R involves three terminals or connection points to a motor, typically seen in compressors, condenser fans, and older blower motors. These terminals are essential for the operation of the motor and play a crucial role in its functionality.

Understanding the Windings

Within the motor, there are windings in the stator, represented by squiggly lines in diagrams. These windings create the rotating electromagnetic field that powers the motor. The main winding, known as the run winding, is responsible for the majority of the motor’s operational work. Additionally, there is an auxiliary winding, called the start winding, which aids in motor efficiency and directional force.

Start Winding in PSC Motors

In PSC motors, the start winding is always in the circuit, not just during the starting phase. This is a unique characteristic of PSC motors, especially when not using a start capacitor or a hard start kit. Understanding this aspect is crucial for technicians working with PSC motors in HVAC systems.

Importance of Proper Installation and Maintenance

Proper installation and maintenance of CS and R components are vital for ensuring the efficient and reliable operation of single-phase PSC motors. Technicians must be well-versed in the wiring, connections, and functionality of these components to troubleshoot and address any issues that may arise.


In summary, common start and run (CS and R) is a fundamental concept in the realm of HVAC, particularly when dealing with single-phase PSC motors. By understanding the components, terminals, windings, and unique characteristics of PSC motors, technicians can effectively maintain, troubleshoot, and optimize the performance of HVAC systems.

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