ASHRAE-D-AJ10AprET-20100331.pdf

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6 0 A S H R A E J o u r n a l a s h r a e . o r g A p r i l 2 0 1 0

Variable Frequency Drives,Part 1: The Technology

Does Energy Savings Trump Costs?

By John Dieckmann, Member ASHRAE; Kurtis McKenney;

and James Brodrick, Ph.D., Member ASHRAE

A variable frequency drive (VFD) is a power electronic device that drives

the common “squirrel cage” induction motor 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. Over the years,

improvements have been made to VFDs’ effiency and reliabilty. They are now

viable alternatives to other motor technologies.

Induction motors operate at a speedthat is proportional to the frequency ofthe input power (minus a small amountof slip that varies with the torque load onthe motor). In HVAC&R, the commonmotor loads are refrigerant compressors,fans, blowers and pumps. By varying thespeed of a motor and its driven load, thecapacity can be varied to meet the real-time cooling, heating or ventilation load.As a result, significant energy savingscan be realized, along with better comfort

control. Other advantages include quieteroperation and longer equipment life as aresult of reduced average speed and softstarting of motors, which reduces thein-rush current and impact loading onequipment at startup.

VFDs are alternately called variablespeed drives, inverters, adjustable speeddrives, or adjustable frequency drives. Thefocus of this article is on VFDs for induc-tion motors. However, it is important tonote that brushless dc motors, sometimes

called permanent magnet rotor motors or

electronically commutated motors, areanother important class of electronicallydriven variable speed motors, used inapplications such as blower motors andrefrigerant compressors.Figure 1 is a simplified block diagram

of a typical VFD. It has four basic sub-systems: an ac/dc converter, a dc bus(also called the dc link), an inverter, and acontrol system. Alternating current fromthe electric utility is converted to dc inthe ac/dc converter, generally a full wave

rectifier bridge. The dc link maintains asteady dc voltage level using a capacitorupstream of the inverter. The inverterconverts the dc back to ac at the frequencyand voltage level needed to drive the mo-tor at the desired speed.

In most instances of VFD-driven mo-tors, the ac output is three-phase to drivea three-phase motor. The control systemmanages the inverter so that it producesthe desired voltage and frequency, andgenerally includes fault monitoring

features.

Figure 2 shows the elements of theinverter in detail. The diode bridge shownis for three-phase ac input. The invertersection consists of six pairs of powertransistors and freewheeling diodes.

While there are various ways to syn-thesize a three-phase ac output from a dcsource, the most commonly used methodis pulse width modulation (PWM).

As shown in Figure 3 , the transistorsare switched on and off rapidly (pulsed),at a carrier frequency that is much higherthan the desired output frequency. The on-time of each pulse is varied to

generate an approximation of a sinusoi-dal wave form. During the off-time ofthe pulse, the inductance of the motor

winding draws current flow through thefreewheeling diode such that the currentflow is continuous and close to beingsinusoidal.

Applications for VFDs are sometimesclassified as “variable torque” or “con-stant torque.” Variable torque does notmean randomly variable torque and con-stant torque does not mean that the torqueis fixed under all conditions.

Variable torque refers to applicationswhere the maximum torque load on the

motor decreases as the speed decreasesfrom the maximum speed to lowerspeeds. Centrifugal pumps, axial fansand centrifugal blowers are examples ofvariable torque applications in HVAC.

Constant torque refers to applicationswhere the maximum required torque doesnot fall off appreciably as the speed decreas-es from the maximum. Positive displacementcompressors and pumps are examples ofconstant torque applications in HVAC.

As a general rule, a given VFD will

have a higher motor power rating for

This five-part series will cover variable frequency drives and their applications in air condition-

ing and refrigeration in residential and commercial buildings. The first article will cover general

variable frequency drive technology. The remaining articles will cover major applications.

This article was published in ASHRAE Journal, April 2010. Copyright 2010 American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission

of ASHRAE. For more information about ASHRAE Journal, visitwww.ashrae.org.



A p r i l 2 0 1 0 A SHR A E J our n a l 6 1

a variable torque application than for a constant torqueapplication. There are numerous technical considerations and potential

pitfalls in applying VFDs, including damage from reflected

voltage waves, higher peak voltages within the motor windings,motor noise, increased motor heating, unwanted harmonics onthe ac input line, and induced currents in the motor bearings.

ASHRAE TC 1.11, Motors and Motor Control, maintains achapter called “Motors, Motor Controls, and Variable-SpeedDrives” in the HVAC Systems and Equipment volume of theASHRAE Handbook (Chapter 44 in the 2008 edition). Technicalconsiderations of VFDs are discussed in this chapter.

In many instances in HVAC equipment, the variable fre-quency drive is applied in a packaged system (e.g., a variableair volume air handling unit, a variable air-volume packagedrooftop unitary air conditioning system, or a large chiller with

a variable speed compressor), and the manufacturer of thesystem has engineered the drive-motor-load-control system sothat these pitfalls are avoided.

Energy Saving Potential

According to the DOE Buildings Energy Data Book1 spacecooling, ventilation, and refrigeration in residential and com-mercial buildings consumed 8.2 quadrillion Btus (quads) ofprimary electric energy in 2006, the bulk of which is consumedby refrigerant compressors, fans, blowers, and pumps, andtherefore can be reduced by implementing VFD technology.

Additionally, space heating in buildings consumed 1.7 quads

of primary electric energy in 2006, a percentage of which (with

the notable exception of electric resistance heating) can bereduced by applying VFDs as well.

In the next four articles, covering the major potential applica-tions of VFDs in HVAC&R, we will break down the potential

for energy savings in each of these areas. Here, we provide abrief overview of three of the major ways that efficient, vari-able speed operation of motors can save substantial amountsof energy in HVAC&R applications.

• The speed cubed fan and pump power law dictates that forpropeller fans, centrifugal blowers and centrifugal pumps insystems with fixed-flow resistance, the air or water flow ratewill vary with the rotational speed (RPM) while the powerwill vary with the cube of the speed, as shown in Figure 4 .For example at 50% of maximum speed and flow, the powerinput drops to only 1/8 of the power at maximum speed. HVACsystems operate at part load most of the time, so modulating

the flow, rather operating cyclically at full flow, can save sig-nificant amounts of energy.

• Continuous operation of cooling equipment at reducedcapacity, instead of on-off operation at full capacity, results inless temperature lift, and hence increased compressor COP, asshown inFigure 5 .

• Continuous operation of cooling equipment at reducedcapacity, instead of on-off operation at full capacity, eliminateson-off cycling losses.

Market Factors

Acceptance of variable frequency drives was slow initially

(going back 20 or 30 years) because of three basic factors:

Figure 1: Bl ock diagram of a variable frequency drive motor, M. Figure 2: Diagram of a variable frequency drive.

Figure 3: Pul se width modulation to generate a sinusoidal waveform.

a c

S u p p l y

C o n v

e r t e r a c / d c

d

c B u s

Inverter

PWM Inverter

Induction Motor

dc Rectifier

ac Source

M

High Speed

Low Speed

M

Fi gure 4: Speed-cubed fan power relationship.

0 10 20 30 40 50 60 70 80 90 100

100

90

80

70

60

50

40

30

20

10

0

HP

Torque

Flow

% H P

a n d T o r q u e

% Speed



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high cost, questionable reliability, and limited experience inproperly applying VFDs.

Over time, all of these factors have been mitigated sub-stantially. Costs of full-featured, general-purpose drives

and of drives tailored for specific applications have fallensignificantly. The reliability of VFDs has improved significantly at the

component, system, and application levels, and OEMs havebecome more expert in applying drives cost-effectively, whilestaying within reliability parameters.

As with any energy-saving technology, the added cost mustbe offset by energy cost savings in an acceptable period of timeto be attractive in the market. With simple payback periodsoften less than a few years, VFDs are now in widespread usefor variable speed blower drives, for compressor drives inlarge chillers, and for chiller auxiliaries. The next column will discuss the application of VFDs to blowers

in commercial building air-conditioning and ventilation systems.

References

1. EERE. 2009. “Buildings Energy Data Book.” U.S. Department ofEnergy, Office of Energy Efficiency and Renewable Energy, Building

Technologies Program. http://buildingsdatabook.eere.energy.gov/.

2. 2008 ASHRAE Handbook—HVAC Systems and Equipment,Chap. 44.

John Dieckmann is a director in the Mechanical Systems

Group of TIAX LLC, Cambri dge, Mass. Kur tis McKenney is

a senior technologist wi th TIAX, and James Brodr ick, Ph.D.,

is a proj ect manager with the Bui lding Technologies Program,

U.S. Department of Energy, Washington, D.C.

Figure 5: Continuous operation vs. on-off operation of cooling equipment.

Condenser (On-Off)

Evaporator (On-Off)

Condenser (Continuous at Half Capacity)

Evaporator (Continuous at Half Capacity)

0 0.5 1 1.5 2

100

90

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