Direct-Drive vs. Gear-Driven: Choosing the Correct Motor for a Large-Diameter Ceiling Fan

Cost. Direct-drive motors tend to cost more than their gear-driven counterparts. First, their design tends to be more complex, incorporating sensors for gathering the high-resolution feedback required for operation. Additionally, because of their fewer revolutions per minute, low-speed versions of direct-drive motors are more “niche,” with less utilization across industries and, thus, less volume.

Installation. While direct-drive motors may be assumed to weigh more than their gear-driven counterparts because of the copper and magnets inherent in their design, significant weight is saved with the lack of a gearbox. The bottom line is the weight of a motor is determined primarily by the manufacturer’s design choices, rather than whether the motor is direct-drive or gear-driven. Ultimately, differences in weight usually are not significant enough to affect the equipment or procedures required for fan installation.

Maintenance. Regular inspection and external cleaning are recommended for both motor types. With a gear-driven motor, however, the gearbox is a point of potential failure. Internal seals can fail. Oil pressure can spike because of conditions such as overheating. A pressure vent may be left closed because of improper installation or, in rare cases, leak oil. How often such failures occur is dependent on the motor/gearbox model as well as installation and operating conditions. However rare they may be, these failures simply do not occur with direct-drive motors because there is no gearbox at risk of failing.

Energy efficiency. In 2021, the U.S. Department of Energy (DOE) published a final rule changing the regulatory metric for LDCF from cubic feet per minute per watt (cfm/W) to ceiling-fan energy index (CFEI). CFEI compares the performance of a LDCF to that of a theoretical reference fan. Manufacturers are required by the DOE to report CFEI at 100-percent speed (CFEI₁₀₀) and 40-percent speed (CFEI₄₀). The higher a fan’s CFEI, the better the fan will perform and the more energy-efficient it is. Note that CFEI ratings cannot be compared meaningfully across diameters. A CFEI₁₀₀ of 1.18 for a 24-ft fan is not necessarily “worse” than a CFEI₁₀₀ of 1.20 for a 20-ft fan. One must stay within a single fan diameter to compare CFEI ratings. See the AMCA white paper “Introducing Ceiling Fan Energy Index (CFEI) and Changes to the U.S. Regulation for Large-Diameter Ceiling Fans” for a more detailed explanation of CFEI.

Calculating Energy Consumption/Cost

Calculating CFEI involves comparing the electrical input power of a fan as measured in a laboratory to the electrical input power of a reference fan. The energy consumption of the reference fan is calculated with a formula. By rearranging terms in that formula, we can find any fan’s energy consumption at a particular speed.

We are going to take a quick detour into the mathematics and equations required to get to that formula. Readers interested in only the practical application may choose to skip to the end of this section.

Note: Many of the following equations are from ANSI/AMCA Standard 208-18, Calculation of the Fan Energy Index (available to download free of charge here). Calculations are performed in inch-pound (I-P) units (cubic feet per minute for airflow, feet for fan diameter, and watts for energy).

We will start with Equation 5.2 for reference-fan electrical input power (FEP_ref) from ANSI/AMCA Standard 208-18:

To get a value for reference-fan electrical input power, we need to introduce additional terms and equations and eventually rewrite the equation for reference-fan electrical input power in terms of only fan airflow and fan diameter. To do that, we use Equation 5.3 for reference-fan shaft power from ANSI/AMCA Standard 208-18:

Combining these substitutions with an assumption of standard air density (which matches CFEI calculation assumptions), we can rewrite the equation for reference-fan shaft power as:

The following equation for fan total pressure is not in ANSI/AMCA Standard 208-18 but is needed to complete our calculations:

We now have all of the tools we need to rearrange and simplify the equation for reference-fan shaft power purely in terms of fan airflow and diameter:

Building on our success, we will use Equation 5.5 for reference transmission efficiency from ANSI/AMCA Standard 208-18:

Similarly to what we did with the equation for reference-fan shaft power, we can simplify the equation for reference transmission efficiency as a function of only fan airflow and diameter:

The final piece of the puzzle involves Equation 5.6 for reference motor output power (H_t,ref) from ANSI/AMCA Standard 208-18:

Using what we have learned to this point, let’s rewrite the equation for reference motor output power:

Next, we need to write Equation 5.7 for reference motor efficiency using Table 5.1 from ANSI/AMCA Standard 208-18 (Table 3):

TABLE 3. ANSI/AMCA Standard 208-18 Table 5.1, Reference Motor Efficiency Coefficients.

Combining equations 5.2 and 5.7 and substituting the longer equation for reference motor efficiency, we now have an equation for reference-fan electrical input power as a function of only fan airflow and diameter. The exact algebra is trivial and left as an exercise for a particularly enthusiastic reader. The full equation is omitted for practical purposes, as the substitution of reference motor efficiency in combination with the equations above is exceedingly tedious.

We have a few simple equations left:

We can rearrange this to come up with:

Combining this result with earlier equations, we are able to calculate reference-fan electrical input power using a manufacturer-reported CFEI value (dependent on operating speed). When looking at the energy use of a fan run at 100-percent speed, CFEI₁₀₀ should be used; when looking at the energy use of the same fan run at 40-percent speed, CFEI₄₀ should be used. As a result, we are able to calculate the actual fan electrical input power of an operating point if given the fan’s airflow and diameter and the CFEI of the desired operating point:

Calculating Payback

Determining payback is comparably straightforward once actual fan electrical input power is calculated. Payback periods vary dramatically, depending on geographic location (local electricity prices) and manufacturer costs. In general terms, however, a simple equation can help guide us:

Table 4 shows specifications for a fictional fan with a gear-driven motor and a fictional fan with a direct-drive motor we will use in our calculations.

TABLE 4. Example fan specifications.

Inserting the diameter, airflow, and CFEI₁₀₀ numbers from Table 4 into our formula for ΔFEP_act, we get a difference of 255 W between the two fans. The table also gives us a ΔCost_fan of $1,000.

Running at 100-percent speed 16 hours (two eight-hour shifts) a day 220 days a year, our fictional direct-drive fan would have a just-under-six-year payback period over our fictional gear-driven motor. This, of course, simplifies the financials by assuming a fixed electricity cost, no maintenance/failures, and a calculation purely in today’s dollars without regard to inflation.

Adding a little complexity, a better approach would be to assume 100-percent speed during the hottest five months of the year and 40-percent speed the remainder of the year. Inserting the diameter, airflow, and CFEI₄₀ numbers from Table 4 into our formula for ΔFEP_act, we get a difference of only 39 W between the two fans run at 40-percent speed. The ΔCost_fan remains $1,000.

We now can introduce a weighted average for energy consumption based on expected operating speed:

We have effectively doubled the payback period by running the fan at slower speeds for most of the year. This shows that correct estimation of the real-life utilization of a fan will have a major impact on the accuracy of a payback calculation.

Payback should not be the sole consideration in the choice of motor but, rather, guide the purchaser toward the correct decision in light of the application and organizational priorities. For example, reliability or noise may be weighted more heavily by the customer, or the customer may be striving to meet certain green initiatives regardless of the payback. It is worth noting that the cost of carbon emissions is being omitted here, as it is a much more complicated discussion and outside the scope of this article.

Summary

When selecting a motor for a LDCF, specifiers and purchasers often must prioritize reliability, first cost, and energy efficiency. Often, reliability favors direct-drive motors over gear-driven motors because of the absence of a gearbox, a potential point of failure. Energy efficiency can quickly offset upfront cost and be calculated using methods shared in this article. It is important to have a complete understanding of operating conditions, requirements, and goals when making a decision.

About the Author

A senior mechanical engineer for 4Front Engineered Solutions, Ryan Perkinson has over a decade of HVAC engineering experience, the last several years of which have been focused on LDCF. He has served on the AMCA North America Region Air Movement Advocacy Committee as well as technical committees for ANSI/AMCA Standard 99, Standards Handbook; ANSI/AMCA Standard 210/ASHRAE Standard 51, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating; and ANSI/AMCA Standard 230, Laboratory Methods of Testing Air Circulating Fans for Rating and Certification.