Electronically Commutated Motors

Electronically commutated motors (ECMs) have quickly become standard in the HVAC industry. They can have unexpected operating characteristics that are very different than their predecessor; the alternating current (AC) permanent split capacitor (PSC) motor. An ECM is a brushless, direct current (DC) motor that operates at a synchronous speed. ECMs utilize integral inverters so they can be powered with AC, but reap the rewards of a DC motor. ECMs can be found in all types of unitary equipment such as fan-powered VAV boxes, fan coils, unit ventilators, furnaces, and water-source heat pumps. Designers, owners, and contractors need to develop different expectations of their performance.

  • Efficiency: The primary reason ECMs have become popular is their efficiency compared to PSC motors. The ECM can be adjusted to operate at a wide range of conditions as if it was designed specifically for that application. A PSC motor usually utilizes a speed controller that operates the motor outside of its ideal range, which results in operating efficiencies of only 10-50%. ECMs operate at efficiencies of 60-80%.
  • No Fan Curve: The ECM has a micro-controller that can change the torque and speed of the motor to obtain, or maintain, a desired airflow within a wide static pressure range. HVAC fans with automatically controlled ECMs can have a fan “curve” that is a straight vertical line (i.e. constant airflow regardless of static pressure in a given static pressure range).
  • Automatic or Manual: Not all ECMs control automatically to maintain constant airflow. Manufacturers are using ECMs because of their inherent high efficiency even to obtain discreet operating points. Even with these “fixed” ECMs, the motor operates like it was optimized for a given piece of equipment or a specific application leading to a higher equipment SEER.
  • Balancing: Testing and balancing contractors need to be familiar with ECMs as it can affect their work. Equipment with ECMs can have numerous jumper settings or proprietary software to select pre-programmed airflow settings. Closing a volume damper may cause a fan with a PSC to “ride its curve”, but an ECM may automatically compensate for the static and reacquire its airflow setting. Amperage readings at the motor can be skewed due to greater harmonic distortions compared to PSC motors.

ECMs were once an additional cost option for HVAC equipment, but are now becoming a standard that you should expect to encounter. If you need assistance in understanding more about ECMs in your existing HVAC systems or for your next project, please do not hesitate to contact Douglas C. Barnhart, PE at 717-845-7654 or dcb@ba-inc.com.

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