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August 10, 2022

Use The Fan Blade To Size The Motor

Written by US Motors

Using the specifications of the fan blade is the best way to determine the correct horsepower for a replacement condenser fan motor. The diameter, angle and number of fan blades, along with the RPM of the motor, are the pieces of information necessary to make this determination.

Let’s look at the following scenario. You are diagnosing a unit and have determined that it is in need of a new condenser fan motor. The failed motor is 1/5 HP and 1140 RPM. The fan blade seems to be in good shape so you have no plans to replace it. So you should just replace the failed motor with a motor of the same size, right? Well, not exactly. It is always best to check the specs on the fan blade before replacing the condenser fan motor on any unit, and especially on older models. In this case, the original was a four blade 18” fan with a 27 degree angle – but you don’t know that. All you can do is measure the current fan and it is a four blade 18” fan with a 30 degree angle.

By looking at this chart, you can determine the correct horsepower for the replacement motor based on the specification of the fan blade.


The original was a four blade 18” fan with 27 degree angle (1140 RPM motor) and was replaced at some time with the current four blade 18” fan blade having an angle of 30 degrees (1140 RPM motor). Therefore a larger horsepower motor (1/4 HP) is needed.