I hear the terms constant speed, constant airflow, and constant torque applied to ECMs. What do they mean?

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I hear the terms constant speed, constant airflow, and constant torque applied to ECMs. What do they mean?

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Electronically Commutated Motors (ECMs or EC Motors) can be designed in ways that better react to load changes in the system as compared to the reaction of shaded pole or permanent split capacitor (PSC) motors in the same application. In HVAC/R direct drive air moving applications, an axial fan (fan blade) or a centrifugal fan (blower) will be attached to the motor shaft to produce airflow.

Axial Fan/Fan Blade Centrifugal Fan/Blower

The load on the motor is impacted dramatically by the static pressure in the system. As the static pressure changes, the load on the motor changes. Static pressure changes as a result of blocked coils, dirty filters, kinks in ducts, and the like. Static pressure changes effect a motor operating a fan blade in just the opposite way that it effects a motor operating a blower. EC motors can be designed to better react to these changes. Please keep in mind, that does not mean that an EC will correct a bad system installation or excessively high static conditions. There are still limits to their performance characteristics.

When used with a fan blade, the EC motor is typically designed as a constant speed. In refrigeration, fan blades are generally used with both the condenser and the evaporator. In air conditioning, fan blades are used with the condenser. A constant speed EC is designed to operate at a specific speed. Common operating speeds are 1550, 1075, and 825 RPM. As the static pressure changes as a result of the coils, grills, and guards becoming dirty, the load on the motor increases. The EC motor will adjust its output torque to the load change in order to maintain the design speed. If the load decreases, the motor would reduce its output torque.

Blowers utilize one of two designs:

A constant airflow ECM is often used with the highest efficiency systems.Unlike constant speed and constant torque ECMs, these are designed as a true variable speed.As static pressure changes in the system the load on the blower changes.If the filters or evaporator coil gets dirty, for example, the static pressure would increase.This would restrict air resulting in less air movement and less load on the motor.The constant airflow motor would increase its speed to make up for the loss of air.
A constant torque ECM performs more like a PSC motor.There are taps from the motor that provide different torques, similar to a three or four speed PSC.When applied to the blower, the different taps will result in different speeds.As static pressure changes, the load to the motor changes.With a constant torque ECM, the output torque of each tap remains the same or constant.As the load changes, however, the actual operating speed of the motor will change resulting in a change in airflow.

There are numerous benefits with the ECMs beyond the obvious higher operating efficiency. One of these benefits is that one ECM rating can usually replace numerous shaded pole and PSC ratings. This can result in the technician needing fewer motors on the truck to cover more applications.