PSC motor efficiency drops off dramatically when turned down typically by at least half.
Due to acoustical considerations, the fan motor is usually adjusted to operate at considerably less than full load (where PSC motor efficiencies may be as high as 62%). The widely used three speed fractional horsepower shaded pole and permanent split capacitor (PSC) induction motor in combination with a 3 speed switch or an electronic SCR speed controller is extremely inefficient at typical operating conditions. The major weakness of commercial fan coil units until now, has been their low fan motor efficiency. DC motors are significantly more energy efficient than AC motors and much easier to control.
The ECM (Electronically Commutated Motor) is an ultra high efficiency programmable brushless DC motor utilizing a permanent magnet motor and a built-in inverter. Following two years of research and development and the availability of a new 277 VAC version, Engineered Comfort was first to introduce the ECM motors to the commercial HVAC market as an option for use in commercial fan coil unit applications. Until more recently though, they were only manufactured in 120 and 240 VAC, which precluded their use in commercial applications. These motors have made it possible to achieve SEER ratings of 12 and higher. Since 1985, equipment manufacturers have used ECM motors in residential air conditioners and furnaces.