Variable-speed, Variable-volume Fan and MotorComponent

General Overview

A variable speed fan and motor uses a power-driven rotating impeller to circulate air. Air flow rates fluctuate as required by the plant and system it serves. Outdoor air temperature (OAT) is the independent variable that drives the fan speed. The heating and cooling loads are affected by OAT. Axial and centrifugal fans with a variable speed drive are the most common type of variable speed fans used in buildings.

Table 1 shows the plant and system configurations that may contain variable-speed fans and motors and the most common respective controlling variables.

Evaluation of Energy Consumption

The primary energy source for a variable-speed fan is the electricity used to run the fan motor. Table 2 provides a summary of measurements needed to quantify the annual energy consumption and operating characteristics of the variable-speed fan and motor.

Measurement Strategy

The measurement strategy for a variable-speed fan and its motor is to measure power draw of the fan motor at various known speeds, develop a mathematical relationship between the power and speed, and then collect average hourly motor speed and associated controlling variable (OAT, WBT). Measurement points are generically represented in Figure 1.

Measurement Equipment

Table 3 provides the equipment required to carry out the measurements of this component.

Table 3. Measurement equipment.

Equipment	Description	Measurement (Units)
DENT ELITEproXC Portable Power Data Logger (EXCUNC) DENT 16” RoCoil Flexible Rope Current Transformers (CT-R16-A4-U)	Provides a measurement of true RMS power from voltage and current inputs and records long-term power (kW) and energy (kWh) measurements. Requires ELOG19 software and a USB connection cable for programming and downloading data files.	True RMS Power (kW)
Onset HOBO Temperature/Relative Humidity Weatherproof Data Logger (MX2301)	Records outdoor air temperature and relative humidity using internal sensors. Requires HOBOware software and a USB connection cable for programming and downloading data files.	Outdoor Air Temperature (F) Relative Humidity (%)

Calculation Methodology

The general methodology for quantifying the energy consumption of a variable-speed fan and motor is determined by the true RMS power of the 3-phase power supply either at the main electrical panel of the motor or at the three-phase input to the VFD. To develop a temperature-dependent regression model, these values are regressed against a controlling variable (OAT, WBT). Depending on the variability of operations, daily or weekly models may be developed to better characterize the component. This model is then used with climate normal year data to estimate the typical energy use of the variable-speed fan.

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Fan Motor Energy Consumption Calculation Methodology and CalculatorCalculation

Further Reading

• Boyd, BK.; McMordie Stoughton, KL.; Lewis, T. (2017). “Cooling Tower (Evaporative Cooling System) Measurement and Verification Protocol.” Golden, CO: National Renewable Energy Laboratory. https://www.nrel.gov/docs/fy18osti/70219.pdf.

• Crowther, H.; Furlong, J. (2004). “Optimizing Chillers and Towers.” ASHRAE Journal, Vol. 46, No. 7; pp. 34-40.

• Morrison, F. (2014). “Saving Energy with Cooling Towers.” ASHRAE Journal, Vol. 56, No. 2; pp. 34-40.

• Tom, S. (July 2017). “CHILLED WATER SYSTEM OPTIMIZER.” Cat. No. 11-808-616-01. Farmington, Connecticut: Carrier Corporation.

Footnotes