Saturday, August 27, 2011
Variable Speed Motors
Electronically commutated motors are widely described as variable speed motors. In truth, most blower motors used in air conditioning are variable speed, including shaded pole and PSC motors. Muliple tap shaded pole and PSC motors are designed specifically to vary their speed based on the tap that is powered. All induction motors vary their speed depending on the load. An increase in load will result in decreased motor speed. What makes ECM motors unique is their ability to vary their speed intelligently in response to the load. They are often used to vary motor speed to maintain constant airflow. To do this, the motors are programmed to match the performance of the blower on which they are mounted. When the airflow through the blower is restricted, the motor responds by speeding up enough to keep the blower CFM the same. In truth, a common PSC blower motor also speeds up when the air through the blower is restricted. However, it does not speed up enough to offset the loss of airflow caused by the restriction, so the airflow decreases. The PSC motor is just responding the way any induction motor does to a decreased load. On a blower with a PSC motor, an airflow restriction causes a decrease in airflow and in motor amp draw because it is not under as much load. A blower with a constant CFM electronically commutated motor will still move the same amount of air by increasing the speed enough to overcome the restriction. Because the motor is doing more work, the amp draw will increase. This is why good duct design and clean filters are important to ECM equipment. The blower motor on a new system with an ECM motor may actually draw a higher amp draw than the old PSC blower it replaced if it is connected to a restrictive duct system. The popular X13 motors by Genteq are constant torque ECM motors. They vary their speed to maintain a steady torque output. When the blower airflow is restricted, the X13 ramps up its speed to maintain the same torque output. This will usually not be enough to completely overcome the restriction, but it will not be producing less torque when the system needs more torque, as is the case with a PC blower. Airflow still drops off, but not as much as with a PSC blower motor. A few systems use an external pulse width modulated signal to the motor to create a feedback loop that maintains an external measured system condition, such as static pressure in a duct system.