Today, we select an exhaust fan with a direct drive electronically commutated motor (ECM) for greater reliability, greater efficiency, and less maintenance, so a combination starter/disconnect is not required. Overloads are no longer needed, as the ECM’s internal rectifier converts the AC current to DC voltage at the motor. A motor starter is not necessary due to the built-in motor protection, but if the control features of the starter are desired, then one can be supplied. The internal rectifier is usually mounted on the end or side of the motor; however, there is an option to mount this hardware remotely, inside the building instead of on the roof. There is also a speed potentiometer at the controller or exhaust fan motor to set or balance the fan to the required airflow rate (in a constant flow application). If variable speed or variable flow is desired, a controller may be added to the end of the motor, and an input speed control from the building management system (BMS) can control the speed. Some ECMs require line voltage for the motor (120, 208, or 277 V) and separate line voltage for the controller (120 or 24 V), in addition to the control (0-10 V, 24 V PWM). If a damper actuator is desired, a transformer may also be mounted with the control board on the end or side of the motor to supply power to the actuator via a built-in auxiliary contact. The transformer is an option for all roof exhaust fan manufacturers but must be specified. Most speed controllers for each ECM brand are proprietary.
It may be recommended that surge protectors be used because of the solid state controls that control the ECM.