A constant torque motor, often marketed as an X13, is a type of high-efficiency blower motor that bridges the gap between basic PSC motors and premium variable-speed ECMs. These motors are programmed to maintain a consistent amount of rotational force, which helps keep airflow steady as duct pressure changes. They are significantly more efficient than standard motors, often helping systems achieve higher SEER2 ratings at a lower price point than full variable-speed models. While they don't have the infinite adjustability of a true ECM, they offer five pre-set speeds that can be selected during installation. This makes them a reliable and popular choice for mid-range furnace and air handler replacements.