ASHRAE Journal - May 2010 - 58

VAV Energy Savings in All Operating Modes

Variable Frequency Drives, Part 2: VFDs for Blowers
This second column in a five-part series covers application of variable frequency drives (VFDs) to blowers in commercial building air-conditioning and ventilation systems.

By John Dieckmann, Member ASHRAE; Kurtis McKenney; and James Brodrick, Ph.D., Member ASHRAE

A

s discussed in last month’s column, a variable frequency drive (VFD) is a power electronic device that drives common squirrel-cage induction motors

over a range of speeds by converting standard frequency and voltage ac power from the electric utility to variable frequency, variable voltage power to energize the motor. In HVAC, the common motor loads are refrigerant compressors, fans, blowers and pumps. By varying the speed of a motor and its driven load, the capacity can be varied to meet the real-time cooling, heating or ventilation load. Because of the speed cubed blower power law, the application of VFDs to blowers for movement of indoor air and ventilation makeup air in commercial building HVAC systems saves significant energy and energy costs, all while providing better comfort control. Figure 1, repeated from last month’s column, illustrates the basic speed cubed fan power law, which dictates that the airflow rate will vary with the rotational speed (rpm) while the power will vary with the cube of the speed. Blowers are a variable torque VFD application (where the maximum torque load on the motor decreases as the speed decreases from the maximum speed to lower speeds, as shown in Figure 1), a relatively simple application for VFDs. The basis of the fan power law is that the pressure rise through a centrifugal blower is proportional to the square of the speed, while at the same time, the pressure loss through a system of fixed-flow
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resistances tends to be proportional to the square of the flow. As a result, the flow ends up being proportional to the blower speed. Since power is proportional to flow times pressure, power is proportional to the speed cubed. Cooling loads generally vary from the design load down to zero over a cooling season, averaging somewhere around 50%. Therefore, in a capacity modulated system, the average conditioned airflow rate can be around 50% of the maximum airflow needed at the design load, nominally reducing blower power to 1/8 of the power at maximum flow. At reduced speed and torque load on the motor, the combination of drive losses and reduced motor efficiency result in somewhat more power than 1/8, but the savings are still substantial. The blower energy savings that result depend on how the indoor air system would otherwise operate. If it were to operate in an always-on, constant volume
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airflow mode, savings greater than 50% would be realized. If the indoor air system were to run in operating modes such as on-off flow or variable flow with dampers or inlet guide vanes, the energy savings would be less, but still substantial. In a simple VAV application, where the air-distribution system presents a fixed airflow resistance to the blower, the speed cubed power law applies, albeit with the caveat mentioned earlier that with the low motor loads at lower speeds, the motor may be operating below its peak efficiency point. In many VAV applications, air is supplied to the individual air diffusers at a constant pressure to maintain sufficient air velocity to penetrate a distance into the space, and promote mixing with the room air. In this case, a variable area diffuser is used to control the airflow into the zone to the level needed to maintain the zone temperature. As illustrated in Figure 2, 100% of the speed cubed power law energy savings are not realized, but the blower energy is reduced significantly. When outdoor ventilation makeup air is provided by a VAV air-handling system, control measures are necessary to ensure that the minimum required outdoor airflow is supplied as the total airflow rate is reduced. While the focus of this series of columns is on VFDs for induction motors, it is important to note that brushless dc motors, sometimes called permanent magnet rotor motors or electronically commutated motors, are another important class of electronically driven variable speed motors that are used for blower motor applications and provide essentially the same energy saving benefits.
	 May	 2010
ASHRAE Journal - May 2010

Table of Contents for the Digital Edition of ASHRAE Journal - May 2010
