Variable speed fan motor curves: Background and impact

By Brett C. Ramirez and Jay D. Harmon, Iowa State University | Dec 06, 2022

As we begin to settle into winter and realize the cold weather is not going away anytime soon, this article aims to discuss the importance and impact of selecting the correct motor curve in your controller for operating variable speed fans. With high propane and electricity costs looming for producers, it is imperative to revisit one of the main culprits of an expensive energy bill – improper ventilation rate.

Ventilation accounts for 80% to 90% percent of the heat lost from swine housing during the winter. Air exchange is critical to providing a healthy environment by reducing humidity and noxious gases. Since under-ventilation creates an unhealthy environment and over-ventilation wastes valuable heating energy, finding the right balance is the key to energy savings and efficiency.

Most controllers control fan speed by changing voltage. Newer technologies using variable frequency drives or electrically commutated motors change fan speed using an analog voltage, which can offer energy savings and simpler management, but for this article, we'll focus on traditional variable speed fan control. A frequent mistake is thinking that the percentage readout on the controller is the percentage of full air flow that the fan will deliver. This is not the case.

The percentage shown on the controller does change linearly with temperature, but the voltage associated with that percentage and the response of the motor are not linear. The controller produces a voltage that changes with the percentage readout shown on the controller.). The motor uses this voltage to operate at a corresponding RPM (speed). For example, for a controller set to 70%, voltage output is 113 V (MC #1), 130 V (MC #2), 156 V (MC #4), 189 V (MC #5) and 156 V (MC #6). This speed produces an airflow rate (CFM) that the fan will deliver. This interaction is important for proper controller management, yet is complex and not easily understood.

Ideally, the percent of full-scale output by a fan would smoothly (linearly) increase with the variable speed readout on your controller. If the proper motor curve is selected in a controller, a relatively linear increase will occur with an increase in the readout in the controller. That is, the fan output (CFM) is nearly linear with fan speed (rpm) for fans. However, there is a threshold point where this relationship starts. Below this point the fan will turn, but won’t deliver air. Motor curves are used to describe relationship between the voltage supplied to the motor and the resulting RPM. Motor curves vary with motor make and size as well as there are different motor curves for different motors. It is imperative that the correct motor curve is selected in the controller to make the ventilation rate increase smoothly with temperature change.

Motor curves are entered into a controller associated with variable speed fans. It is normally set once and forgotten unless the fan motor is replaced. If the wrong motor curve is selected, a few negative outcomes can occur such as, a variable speed fan could act like a single speed fan, it may be impossible to get close to the needed minimum ventilation rate causing under or over-ventilation, and/or the motor could burn out prematurely because of low voltage. Table 1 shows the increases for a given fan at different points along it's curve.

In the lowest voltage portion, a 10 V increase yields an increase in 538 cfm. The next 10 V increases the flow rate by 471 cfm; still a large change. This means that 85% of the total increase between 99 and 230 V is in the first 20 V. The last 111 volts of the increase only result in an increase of 181 cfm. So, if the wrong motor curve is selected, small changes in the percentage indicated on the controller could result in very small or very large changes in fan output. Both potentially leading to a unhealthy environment or massive energy wastage.

Table 2 shows the similar information as Table 1 but for a 24 in. fan with corresponding percentage of maximum for supply voltage, motor speed and fan output. A much more linear response between supply voltage and motor speed is observed until about 60% of the supply voltage is reached, then small changes in supply voltage results in larger changes in fan output.

As stated before, when setting minimum ventilation, under-ventilation can lead to a bad environment and over-ventilation can lead to wasted energy. It is estimated that over-ventilating minimum ventilation in a wean-finish barn by 10% can result in approximately 25% more propane usage. Over-ventilation by 40%, which may not be atypical, can double propane usage. This varies by year of course, also based on pig placement timing and weather, but many barns have minimum ventilation rates set improperly and it can lead to excess energy expenditures. Much of this is due to improper settings, improper motor curves or improper minimum fan sizes.

Understand that 50% motor speed does not equal 50% fan output (CFM) and 50% supply voltage does not equal 50% of motor speed (RPM) or fan output (CFM). Also, every motor responds uniquely to supply voltage and motor curves are different for each controller. Finding the correct balance between minimum ventilation rate and the environment is key to successful energy usage and productivity.

Special thanks to Mark Oberreuter with AP for the fan and motor data.

Source: Brett C. Ramirez and Jay D. Harmon, who are solely responsible for the information provided, and wholly owns the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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