Variable-speed, Variable-volume Pump and MotorComponent

General Overview

A variable-speed, variable-volume (VSVV) pump and motor circulate liquid (e.g., water or water and glycol solution) through a piping network where the flow rate fluctuates as required by the plant and systems they serve. Outdoor air temperature (OAT) is the independent variable that drives the pump speed since heating and cooling loads are generally affected by OAT. The pump motor is equipped with a variable frequency drive (VFD) which may be integrated in the pump and motor package or installed separately. Centrifugal pumps are the most common type of pumps used in buildings.

Table 1 shows the plant and system configurations that may contain a VSVV pump and motor and the controlling variable.

Evaluation of Energy Consumption

The primary energy source of a VSVV pump is the electricity used to run the pump motor. Table 2 provides a summary of measurements needed to quantify the annual energy consumption and operating characteristics of the VSVV pump and motor.

Measurement Strategy

The measurement strategy for a variable-speed pump and motor is to measure power draw of the pump motor at various known speeds, develop a mathematical relationship between the power and speed, and then collect hourly OAT. 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)
Onset HOBO Motor On/Off Data Logger (UX90-004)	Records when a motor is on and off, as well as runtime. Requires HOBOware software and a USB connection cable for programming and downloading data files.	Motor Runtime (Minutes)

Calculation Methodology

The general methodology for quantifying the energy consumption of a VSVV pump motor is determined by the true RMS power of the three-phase power supply either at the power main for the motor or at the three-phase input to the VFD. These values are regressed against outside air temperature to develop a temperature-dependent regression model. Depending on the variability of operations, daily or weekly models may be developed to better characterize the system. This model is then used with climate normal year data to estimate the typical energy use of the variable-speed pump.

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Pump Motor Energy Consumption Calculation Methodology and CalculatorCalculation

Further Reading

• ​​Chaurette, J. (2003). In PUMP SYSTEM ANALYSIS AND CENTRIFUGAL PUMP SIZING (5th ed.). Retrieved November 2019, from https://www.pumpfundamentals.com/download/book/chapter4.pdf

• Satterfield, Z. (2010). Fundamentals of Hydraulics: Pressure. Tech Brief, 9(4), pp. 1-4. Retrieved November 21, 2019, from https://www.nesc.wvu.edu/files/d/5c5a9fd1-0f8b-48c5-9063-b55d12651b91/fund-of-hydr-pressure.pdf

• Taylor, S. T. (2002, February). Primary-only vs. primary-secondary variable flow systems. ASHRAE Journal, 25-29

• Trane. (2002). Variable-Primary-Flow Systems Revisited. Trane Engineers Newsletter, 31(4). Retrieved November 2019, from https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/waterside-design/adm_apn005_en.pdf

Footnotes