Selecting a Replacement Motor – Short #296

In this short podcast episode, Bryan covers the points to consider when selecting a replacement motor, particularly blower motors and condensing fan motors in residential and light commercial applications.

Horsepower and amperage both matter for matching, but they are not the only items to consider. We also have to consider phase (three-phase vs. single-phase), voltage, and RPM; those must match exactly (barring VFDs and ECMs with variable RPMs). Frame size, shaft size, mounting, rotation, capacitor (for PSC motors), bearing type, temperature rating, enclosure (or Air Over Motor rating), and operation all matter. The failed motor is also NOT the only source of information; if it is an aftermarket motor, it might not have the correct information, so you are better off using an OEM motor or checking the manufacturer’s data.

Single-phase motors are common for residential condensing fan motors and will need a run capacitor. Three-phase motors tend to be more common in commercial settings, and these will not have a run capacitor. Voltages must be appropriate for the application, and if the RPM is fixed, it must match the same general range because it represents the motor speed and is based on the number of poles. However, RPM may be variable in several applications.

Horsepower is the mechanical output rating (ability to do work); we CANNOT use a replacement motor with less horsepower than the OEM motor (and be careful with fractions; 1/3 is greater than 1/4). Amperage is NOT horsepower; amps are electrical input current. We want the amp rating to be close to the original motor, but it won’t always match, particularly in the case of ECMs. If it doesn’t match, we want to know why that is; if it’s an ECM, then it’s likely not a problem because ECMs can do the same work as a PSC with fewer watts. Power factor indicates motor efficiency for PSC motors, but the rules aren’t quite the same for ECMs, which will have a much lower power factor (or lower wattages with higher amperages). However, if the amperage is off because we have the wrong motor, then that is a problem.

The physical aspects matter for fitting the motor properly and delivering the right airflow. Rotation should match (though most universal motors can run in either direction). Airflow is especially important for blower motors, so factory replacements or ECM replacements are the way to go in those cases (it’s also good to confirm airflow with a TrueFlow grid and by checking static pressure). The speed taps also need to be correct. Noise is also a factor; ball bearing motors tend to be louder than sleeve bearing motors. Temperature ratings also matter because heat can significantly reduce the lifespan of motors, and we need motors that can handle the operating conditions.

Here is the podcast episode about ECM blower amps and watts with Steve Rogers: https://www.hvacrschool.com/podcasts/what-is-up-with-blower-amps-watts-w-steve-rogers/

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