windings. In an ECM, the control module uses electronics to energize and de-energize each winding in sequence to control the speed and torque of the motor. This electronic control allows for a wide variation in motor speeds. ECMs are programmed by their manufacturer for the specific type of equipment in the system. When replacing an ECM motor, the replacement motor must also be programmed for the specific equipment. ECM motors can be programmed to provide constant air volume, constant torque, or constant speed. For example, an ECM powering an indoor blower is often programmed to maintain constant air volume. As it is operating, the ECM monitors motor speed and torque, and adjusts the motor speed based on changes in static pressure. For example, if a filter becomes dirty, static pressure increases and the volume of airflow decreases. This increases the torque on the motor. The ECM recognizes this change and increases the motor speed to maintain the design airflow. ECMs provide several benefits when compared to similar AC motors: • • ECMs can adjust to changes in the system and maintain a constant volume of airflow, improving system performance. • • ECMs on blowers can be programmed to run at 50% for an initial few minutes to remove humidity more effectively, improving the performance of the system. • • ECMs use significantly less energy than comparable AC motors. ECMs have soft start capability, which eliminates a large inrush of starting current. In low-load conditions, an ECM can reduce motor speed and use less energy. • • ECMs make less noise than similar AC motors during operating. • • ECMs generally have ball bearings and require less maintenance than AC motors with sleeve bearings. ECM motors also have some disadvantages relative to comparable AC motors. First, ECMs are much more expensive than AC motors. Second, servicing an ECM can be more difficult than servicing an AC motor and may require special equipment. Finally, obtaining a replace- ment ECM programmed for the specific equip- ment can be more difficult than obtaining a more generic AC motor. 25.11 Electronically Commutated Motors (ECMs) Electronically commutated motors (ECMs) are brushless DC motors with integrated elec- tronic controls. ECMs provide more precise control of the motor speed and torque when compared with AC motors. ECMs are also more responsive to changes in the system. An ECM consists of two parts: a motor and a control module. See Figure 25-61. On many ECMs, the motor and control module can be separated. When troubleshooting an ECM, a tech- nician can disconnect the control module and then check the motor operation using an ECM testing tool. ECMs are powered by standard AC voltage. Some ECMs can be configured to use either 120 V, 240 V, or 277 V. The electronics in the control module convert the AC voltage to DC voltage using an internal rectifier. The DC voltage is then supplied to the motor. The motor component of an ECM is a brush- less DC motor. The rotor consists of permanent magnets. The stator consists of several windings, often six. Most DC motors use brushes and a commu- tator to change the polarity of the current in the Control connector Control module Motor DiversiTech Corporation Figure 25‑61. An ECM has two components: a control module and a motor. This ECM could be used for an indoor blower on a residential or light commercial sys- tem. Chapter 25 Motors 517 Copyright Goodheart-Willcox Co., Inc.