PSC Motors vs ECM Motors: A Technician’s Guide

Technicians working in residential and light commercial HVAC (furnaces, air handlers, and package units) should understand both Permanent Split Capacitor (PSC) and Electronically Commutated Motors (ECM). Understanding diagnostics, how to select the correct replacement and installation for both PSC and ECM motors will make your life as a contractor easier.

Permanent Split Capacitor (PSC) Motors

PSC motor technology is over 100 years old and was the largest motor segment until 2019. In July of 2019, the Fan Energy Rating (FER) regulation, which focused on the electrical efficiency of fossil fuel appliances, went into effect. While PSC motors are now rarely being used in OEM parts, they are still the largest segment out in the field today. Contractors in the field can expect to continue working on PSC motors for another 10-15 years while the existing equipment works its way out of existence. PSC motors are primarily built-in fractional horsepower (FHP) ratings for use in residential and light commercial HVAC applications.

Characteristics Of PSC Motors

• Typically constructed with 1-5 speeds
• Always operated with a run capacitor
• Constructed for a 3-wire connection method/designed with one dedicated capacitor wire
• Constructed for a 4-wire connection method/designed with two dedicated capacitor wires
• More efficient than a Shaded Pole Motor (approximately 60-70%)
• Common in most home furnaces and outdoor condenser fans

Click here if you would like to learn more about induction motors.

Click here if you are looking for replacement PSC induction motors.

Electronically Commutated Motors (ECM)

ECM technology was introduced to the residential and light commercial HVAC industry in 1987 when GE®* introduced a constant airflow “variable speed” indoor blower motor. Today, ECMs are found in outdoor condenser fan and indoor blower applications in residential and light commercial HVAC systems.

These direct drive Fractional Horsepower (FHP) motors include constant torque, constant speed and constant airflow. ECMs are the most efficient FHP motor in the market and are built with an electronic control module and motor and do not use capacitors.

ECMs are categorized into the following three types:

Constant Airflow ECMs

Constant Airflow ECMs (also referred to as Variable Speed ECMs) are primarily used for indoor blower motor applications. They are designed with separate line voltage and communication inputs (no speed taps) and are commonly designed with 16-pin or 4-pin communication inputs. Constant Airflow ECMs are programmed by the HVAC system manufacturer. Constant airflow ECMs were introduced to the HVAC industry to provide energy savings and support system capacity related to their ability to maintain airflow when TESP (total external static pressure) increases.

Click below if you’re looking for a constant airflow ECM replacement motor.

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Constant Torque ECMs

Constant Torque ECMs are also primarily used for indoor blower motor applications. However, constant torque ECMs are designed with separate line voltage inputs and low voltage speed taps. They are commonly designed with 5 speed taps and are also programmed by the HVAC system manufacturer. Constant torque ECMs were introduced to the HVAC industry primarily to provide energy savings. This is evident in their adoption into HVAC systems with the 13 SEER energy regulation in 2006 and the FER energy regulation in 2019.

Click below if you’re looking for a constant torque ECM replacement motor.

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Constant Speed ECMs

Constant Speed ECMs are primarily used for outdoor condenser fan motor applications. They are designed with separate line voltage inputs and low voltage speed taps or communication. Constant Speed ECMs are commonly designed with leads out instead of plug connections for weather protection. They are also programmed by the HVAC system manufacturer.

If you’re looking for a constant speed ECM replacement motor, contact the manufacturer of the HVAC system from which it was removed.

ECMs are complex, microprocessor-driven motors that incorporate electronic controls with a mechanical motor. However, from a user point of view, these motors are not that difficult to operate, adjust or diagnose, if you know what they require to operate properly. In fact, these motors can help reduce on-truck inventory, allowing you to complete more calls on the same day without driving to retrieve a part.

Additional Resources

• Installing, servicing, and replacing ECM motors
• Diagnostic information for Genteq^® ECM motors

 

 

* GE is believed to be a trademark and/or trade name of General Electric Company and is not owned or controlled by Regal Rexnord Corporation.