VARIABLE FREQUENCYDRIVE

OPERATION AND APPLICATION OFVARIABLE FREQUENCY DRIVE (VFD) TECHNOLOGY

Carrier CorporationSyracuse, New York

October 2005

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . .2Common VFD Terms

VFD OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . .3

BENEFITS OF VFD . . . . . . . . . . . . . . . . . . . . . . . .4VFD Capacity Control Saves Energy . . . . . . . . . .4Low Inrush Motor Starting . . . . . . . . . . . . . . . . . .5Easy Installation . . . . . . . . . . . . . . . . . . . . . . . . . .5High Power Factor . . . . . . . . . . . . . . . . . . . . . . . .5Low Full Load KVA . . . . . . . . . . . . . . . . . . . . . . .6

HARMONIC DISTORTION AND INDUSTRY STANDARDS . . . . . . . . . . . . . . . . . .6Harmonic Definition . . . . . . . . . . . . . . . . . . . . . . .6What Causes Harmonics? . . . . . . . . . . . . . . . . . . .7Rocks and Ponds . . . . . . . . . . . . . . . . . . . . . . . . . .7Are Harmonics Harmful? . . . . . . . . . . . . . . . . . . .7Understanding IEEE 519 . . . . . . . . . . . . . . . . . . . .7Introduction to Harmonic Terms . . . . . . . . . . . . . .8Mitigating Harmonics . . . . . . . . . . . . . . . . . . . . . .9

TABLE OF CONTENTS

INTRODUCTION

Variable frequency drive (VFD) usage has increaseddramatically in HVAC applications. The VFDs arenow commonly applied to air handlers, pumps,chillers and tower fans. A better understanding ofVFDs will lead to improved application and selec-tion of both equipment and HVAC systems. Thispaper is intended to provide a basic understandingof common VFD terms, VFD operation, and VFDbenefits. In addition this paper will discuss somebasic application guidelines regarding harmonic dis-tortion with respect to industry standards.

Common VFD Terms

There are several terms used to describe devices thatcontrol speed. While the acronyms are often usedinterchangeably, the terms have different meanings.

Variable Frequency Drive (VFD)This device uses power electronics to vary the fre-quency of input power to the motor, thereby con-trolling motor speed.

Variable Speed Drive (VSD)This more generic term applies to devices that con-trol the speed of either the motor or the equipmentdriven by the motor (fan, pump, compressor, etc.).This device can be either electronic or mechanical.

Adjustable Speed Drive (ASD)Again, a more generic term applying to bothmechanical and electrical means of controllingspeed.

This paper will discuss only VFDs.

2

VFD OPERATION

Understanding the basic principles behind VFDoperation requires understanding the three basicsections of the VFD: the rectifier, dc bus, andinverter.

The voltage on an alternating current (ac) powersupply rises and falls in the pattern of a sine wave(see Figure 1). When the voltage is positive, currentflows in one direction; when the voltage is negative,the current flows in the opposite direction. This typeof power system enables large amounts of energy tobe efficiently transmitted over great distances.

The rectifier in a VFD is used to convert incomingac power into direct current (dc) power. One rectifi-er will allow power to pass through only when thevoltage is positive. A second rectifier will allowpower to pass through only when the voltage is neg-ative. Two rectifiers are required for each phase ofpower. Since most large power supplies are threephase, there will be a minimum of 6 rectifiers used(see Figure 2). Appropriately, the term “6 pulse” isused to describe a drive with 6 rectifiers. A VFDmay have multiple rectifier sections, with 6 recti-fiers per section, enabling a VFD to be “12 pulse,”“18 pulse,” or “24 pulse.” The benefit of “multi-pulse” VFDs will be described later in the harmon-ics section.

Rectifiers may utilize diodes, silicon controlled rec-tifiers (SCR), or transistors to rectify power. Diodesare the simplest device and allow power to flow anytime voltage is of the proper polarity. Silicon con-trolled rectifiers include a gate circuit that enables a

microprocessor to control when the power maybegin to flow, making this type of rectifier useful forsolid-state starters as well. Transistors include a gatecircuit that enables a microprocessor to open orclose at any time, making the transistor the mostuseful device of the three. A VFD using transistorsin the rectifier section is said to have an “activefront end.”

After the power flows through the rectifiers it isstored on a dc bus. The dc bus contains capacitorsto accept power from the rectifier, store it, and laterdeliver that power through the inverter section. Thedc bus may also contain inductors, dc links, chokes,or similar items that add inductance, therebysmoothing the incoming power supply to the dc bus. The final section of the VFD is referred to as an“inverter.” The inverter contains transistors thatdeliver power to the motor. The “Insulated GateBipolar Transistor” (IGBT) is a common choice inmodern VFDs. The IGBT can switch on and off sev-eral thousand times per second and precisely controlthe power delivered to the motor. The IGBT uses amethod named “pulse width modulation” (PWM)to simulate a current sine wave at the desired fre-quency to the motor.

Motor speed (rpm) is dependent upon frequency.Varying the frequency output of the VFD controlsmotor speed:

Speed (rpm) = frequency (hertz) x 120 / no. of poles

Example:2-pole motor at different frequencies3600 rpm = 60 hertz x 120 / 2 = 3600 rpm3000 rpm = 50 hertz x 120 / 2 = 3000 rpm2400 rpm = 40 hertz x 120 / 2 = 2400 rpm

3

Fig. 1. AC sine wave

Fig. 2. VFD basics: Existing technology

AC sine wave

-2

-1

0

1

2

0 90 180 270 360

BENEFITS OF VFD

As VFD usage in HVAC applications has increased,fans, pumps, air handlers, and chillers can benefitfrom speed control. Variable frequency drives pro-vide the following advantages:

• energy savings• low motor starting current• reduction of thermal and mechanical

stresses on motors and belts during starts• simple installation• high power factor• lower KVA

Understanding the basis for these benefits will allowengineers and operators to apply VFDs with confi-dence and achieve the greatest operational savings.

VFD Capacity Control Saves Energy

Most applications do not require a constant flow ofa fluid. Equipment is sized for a peak load that mayaccount for only 1% of the hours of operation. Theremaining hours of operation need only a fraction ofthe flow. Traditionally, devices that throttle outputhave been employed to reduce the flow. However,when compared with speed control, these methodsare significantly less efficient.

Mechanical Capacity Control

Throttling valves, vanes, or dampers may beemployed to control capacity of a constant speedpump or fan. These devices increase the head, there-by forcing the fan or pump to ride the curve to apoint where it produces less flow (Figure 3). Powerconsumption is the product of head and flow.Throttling the output increases head, but reducesflow, and provides some energy savings.

4

Pump power ~ flow x head / 39601

Variable Speed Capacity Control

For centrifugal pumps, fans and compressors, theideal fan (affinity) laws describe how speed affectsflow, head and power consumption (Table A).

When using speed to reduce capacity, both the headand flow are reduced, maximizing the energy sav-ings. A comparison of mechanical and speed controlfor capacity reduction (Figure 4) shows that variablespeed is the most efficient means of capacitycontrol.

Fig. 3. Mechanical capacity control

Fig. 4. Comparison of mechanical capacity control and speed capacity control

1 Assumes fluid is fresh water, (specific gravity = 1).

Table AEffects of Changes in Fan Speed

Flow changes linearly with speed Flow Rate2 = Flow Rate1 x (RPM2/RPM1)

Head varies as the speed squared Lift2 = Lift1 x (RPM2/RPM1)2

Power varies as the speed cubed Power2 = Power1 x (RPM2/RPM1)3

ThrottledSystem Curve

1360 GPMUnthrottled

System Curve1700 GPM, 1750 RPM

PumpCurve

00

2020

4040

6060

8080

100100

120120

140140

160160

180180

200200

220220

24024030 40 50 60 70

7578

80

Flow (GPM)Flow (GPM)

To

tal H

ead

(F

T)

To

tal H

ead

(F

T)

200200 300300 400400 500500 600600 700700 800800 900900 10001000 1100110012001200 13001300 14001400 1500150016001600 17001700 18001800

Damper

Vanes

Drive

100%

Ho

rsep

ow

er

0%

0% 20% 100%Flow

Efficiency

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Low Inrush Motor Starting

Motor manufacturers face difficult design choices.Designs optimized for low starting current oftensacrifice efficiency, power factor, size, and cost.With these considerations in mind, it is common forAC induction motors to draw 6 to 8 times their fullload amps when they are started across the line.When large amounts of current are drawn on thetransformers, a voltage drop can occur2, adverselyaffecting other equipment on the same electricalsystem. Some voltage sensitive applications mayeven trip off line. For this reason, many engineersspecify a means of reducing the starting current oflarge AC induction motors.

Soft Starters

Wye-delta, part winding, autotransformer, and solid-state starters are often used to reduce inrush duringmotor starting. All of these starters deliver power tothe motor at a constant frequency and therefore mustlimit the current by controlling the voltage suppliedto the motor. Wye delta, part winding, and auto-transformer starters use special electrical connec-tions to reduce the voltage. Solid-state starters useSCRs to reduce the voltage. The amount of voltagereduction possible is limited because the motorneeds enough voltage to generate torque to acceler-ate. With maximum allowable voltage reduction, themotor will still draw two to four times the full loadamps (FLA) during starting. Additionally, rapidacceleration associated with wye-delta starters canwear belts and other power transmissioncomponents.

VFDs as Starters

A VFD is the ideal soft starter since it provides thelowest inrush of any starter type as shown inTable B. Unlike all other types of starters, the VFDcan use frequency to limit the power and currentdelivered to the motor. The VFD will start the motorby delivering power at a low frequency. At this lowfrequency, the motor does not require a high level ofcurrent. The VFD incrementally increases thefrequency and motor speed until the desired speed is

met. The current level of the motor never exceedsthe full load amp rating of the motor at any timeduring its start or operation. In addition to the bene-fit of low starting current, motor designs can now beoptimized for high efficiency.

Easy Installation

Many pieces of equipment are factory shipped withunit mounted VFDs that arrive pre-programmed andfactory wired. Motor leads, control power for auxil-iaries, and communication lines are all factorywired. The VFD cooling lines on unit-mountedchiller VFDs are also factory installed. Theinstalling contractor needs only to connect the linepower supply to the VFD.

High Power Factor

Power converted to motion, heat, sound, etc. iscalled real power and is measured in kilowatts (kW).Power that charges capacitors or builds magneticfields is called reactive power and is measured inKilovolts Amps Reactive (kVAR). The vector sumof the kW and the kVAR is the Total Power (energy)and is measured in Kilovolt Amperes (KVA)(Figure 5). Power factor is the ratio of kW/KVA.

Motors draw reactive current to support their mag-netic fields in order to cause rotation. Excessivereactive current is undesirable because it createsadditional resistance losses and can require the useof larger transformers and wires. In addition, utilitiesoften penalize owners for low power factor.Decreasing reactive current will increase powerfactor.

2 This is a significant consideration for "soft" systems such as backup generators.

Starter Type Starting Current (% of FLA)

VFD 100%

Wye-Delta Starter 200-275%

Solid State Soft Starter 200%

Autotransformer Starter 400-500%

Part Winding Starter 400-500%

Across the Line Starter 600-800%

Table B Comparison of Starter Types Based on Inrush

kVAR

kW

KVAEnergy used tobuild / decaymagnetic field sin motors,transformers etc.

Total volts xampstransmitted.

Power consu med asheat, sound, worketc.

KVA = √ kW2+ kVAR2

6

Typical AC motors may have a full load power fac-tor ranging from 0.84 to 0.88. As the motor load isreduced, the power factor becomes lower. Utilitiesmay require site power factor values ranging from0.85 to 0.95 and impose penalties to enforce thisrequirement. Power factor correction capacitors canbe added to reduce the reactive current measuredupstream of the capacitors and increase the meas-ured power factor. To prevent damage to the motor,power factor correction capacitors should not exceedthe motor manufacturer’s recommendations. In mostcases, this results in maximum corrected values of0.90 to 0.95.

The VFDs include capacitors in the DC Bus that per-form the same function and maintain high powerfactor on the line side of the VFD. This eliminatesthe need to add power factor correction equipment tothe motor or use expensive capacitor banks. In addi-tion, VFDs often result in higher line side power fac-tor values than constant speed motors equipped withcorrection capacitors.

Low Full Load KVA

Total Power (KVA) is often the limiting factor in theamount of energy that can be transmitted through anelectrical device or system. If the KVA required byequipment can be reduced during periods of peakdemand, it will help alleviate voltage sags, brownouts, and power outages. The unit efficiency andpower factor are equally weighted when calculatingKVA. Therefore, equipment that may be equal orworse in efficiency, but higher in power factor hassignificantly lower KVA (Table C).

In this example, equipment with a higher power fac-tor uses 15% less KVA while performing the same

job. This can lower electrical system cost on newprojects and free up KVA capacity on existing sys-tems.

Backup generators are typically sized to closelymatch the load. Lowering KVA can reduce the sizeof the generator required. When VFDs with activefront ends are used, the generator size can approachan ideal 1:1 ratio of kW/KVA because the powerfactor is near unity (1.0) and the harmonics pro-duced by the VFD are extremely low.

Lower KVA also benefits utilities. When the powerfactor is higher, more power (kW) can be deliveredthrough the same transmission equipment.

HARMONIC DISTORTION AND INDUSTRYSTANDARDS

A discussion of the benefits of VFDs often leads toa question regarding harmonics. When evaluatingVFDs, it is important to understand how harmonicsare provided and the circumstances under whichharmonics are harmful.

Harmonic Definition

In the United States, three-phase AC power typical-ly operates at 60 hertz (60 cycles in one second).This is called the fundamental frequency.

Input Power Amps Volts KVAkW Factor

350.4 .84 502 Nominal 480 417

350.4 .99 426 Nominal 480 354

NOTE: KVA = Volts x Amps x 1.732

Table C Power Factors and Energy Usage

Energy used to build / decaymagnetic fieldsin motors,transformers, etc.

Total volts x ampstransmitted.

Power consumed as heat, sound,work, etc.

Fig. 5. Measuring power

7

When you calculate harmonics you are calculatingthe effect of the harmonics on the fundamental cur-rent wave form in a particular distribution system.There are several programs that can perform esti-mated calculations. All of them take into account theamount of linear loads (loads drawing powerthrough out the entire sine wave) relative to non-linear loads (loads drawing power during only a frac-tion of the sine wave). The higher the ratio of linearloads to non-linear loads, the less effect the non-lin-ear loads will have on the current wave form.

Are Harmonics Harmful?

Harmonics that are multiples of 2 are not harmfulbecause they cancel out. The same is true for 3rd

order harmonics (3rd, 6th, 9th etc.). Because the powersupply is 3 phase, the third order harmonics canceleach other out in each phase 3. This leaves only the5th, 7th, 11th, 13th etc. to discuss. The magnitude of theharmonics produced by a VFD is greatest for thelower order harmonics (5th, 7th and 11th) and dropsquickly as you move into the higher order harmon-ics (13th and greater).

Harmonics can cause some disturbances in electricalsystems. Higher order harmonics can interfere withsensitive electronics and communications systems,while lower order harmonics can cause overheatingof motors, transformers, and conductors. The oppor-tunity for harmonics to be harmful, however, isdependent upon the electrical system in which theyare present and whether or not any harmonic sensi-tive equipment is located on that same electricalsystem.

Understanding IEEE 519

IEEE (Institute of Electrical and ElectronicsEngineers) created a recommendation for evaluatingharmonics. The IEEE-519 standard provides recom-mended limits for harmonic distortion measured atthe point of common coupling. The point of com-mon coupling is the point at which the customer’selectrical system is connected to the utility.

A harmonic is any current form at an integral mul-tiple of the fundamental frequency. For example, for60-hertz power supplies, harmonics would be at120 hertz (2 x fundamental), 180 hertz, 240 hertz,300 hertz, etc.

What Causes Harmonics?

VFDs draw current from the line only when the linevoltage is greater than the DC Bus voltage inside thedrive. This occurs only near the peaks of the sinewave. As a result, all of the current is drawn in shortintervals (i.e., at higher frequencies). Variation inVFD design affects the harmonics produced. Forexample, VFDs equipped with DC link inductorsproduce different levels of harmonics than similarVFDs without DC link inductors. The VFDs withactive front ends utilizing transistors in the rectifiersection have much lower harmonic levels thanVFDs using diodes or silicon controlled rectifiers(SCRs).

Electronic lighting ballasts, uninterruptible powersupplies, computers, office equipment, ozone gener-ators, and other high intensity lighting are alsosources of harmonics.

Rocks and Ponds

Obviously, the magnitude of the contributing waveforms has an effect on the shape of the resultantwave form. If the fundamental wave form (60 Hz)has a very large magnitude (5,000 amps) and theharmonic wave forms are very low (10 amps), thenthe resultant wave form will not be very distortedand total harmonic distortion will be low. If the har-monic wave form current value is high relative tothe fundamental, the effect will be more dramatic.

In nature, we see this effect with waves in water. Ifyou continually throw baseball size rocks into theocean, you would not expect to change the shape ofthe waves crashing onto the beach. However, if youthrew those same size rocks into a bathtub, youwould definitely observe the effects. It is similarwith electrical waves and harmonics.

3 The neutral wire sizing should account for 3rd order harmonic current.

8

Although the IEEE standard recommends limits forboth voltage distortion and current distortion, speci-fications that reference a 5% harmonic limitation aregenerally referring to current distortion. In mostcases, if the current distortion falls within IEEE-519requirements, the voltage distortion will also beacceptable.

Determining compliance with IEEE-519 requires anactual measurement of the system during operation.Predicting compliance in advance often requires asystem study that accounts for all electrical equip-ment (transformers, wires, motors, VFDs, etc.) inthe system.

Introduction To Harmonic Terms

Total Harmonic Voltage Distortion - THD (V)

As harmonic currents flow through devices withreactance or resistance, a voltage drop is developed.These harmonic voltages cause voltage distortion ofthe fundamental voltage wave form. The total mag-nitude of the voltage distortion is the THD (V). TheIEEE-519 standard recommends less than 5% THD(V) at the point of common coupling for generalsystems 69 kV and under.

Total Harmonic Current Distortion - THD (I)

This value (sometimes written as THID) representsthe total harmonic current distortion of the waveform at the particular moment when the measure-ment is taken. It is the ratio of the harmonic currentto the fundamental (non-harmonic) current meas-ured for that load point. Note that the denominatorused in this ratio changes with load.

Total Demand Distortion - TDD

Total Demand Distortion (TDD) is the ratio of themeasured harmonic current to the full load funda-mental current. The full load fundamental current isthe total amount of non-harmonic current consumedby all of the loads on the system when the system isat peak demand. The denominator used in this ratiodoes not change with load. Although TDD can bemeasured at any operating point (full or part load),the worst case TDD will occur at full load. If the fullload TDD is acceptable, then the TDD measured at

part load values will also be acceptable. To use ourrock analogy, the full load fundamental current is thesize of our pond and the harmonic current is the sizeof our rock. (See Table D.)

Short Circuit Ratio

Short circuit ratio is the short circuit current value ofthe electrical system divided by its maximum loadcurrent. Standard IEEE-519 Table 10.3 defines dif-ferent acceptance levels of TDD depending on theshort circuit ratio in the system. Systems with smallshort circuit ratios have lower TDD requirementsthan systems with larger short circuit ratios. Thisdifference accounts for the fact that electrical sys-tems with low short circuit ratios tend to have highimpedances, creating larger voltage distortion forequivalent harmonic current levels. (See Table E.)

Mitigating Harmonics

Some utilities now impose penalties for introducingharmonics onto their grid, providing incentives forowners to reduce harmonics. In addition, reducingharmonic levels can prevent potential damage tosensitive equipment residing on the same system.There are many approaches to mitigating harmonics.Several commonly used methods are discussed here.

Line Reactors

Line reactors add reactance and impedance to thecircuit. Reactance and impedance act to lower thecurrent magnitude of harmonics in the system andthereby lower the TDD. Line reactors also protect

Fundamental Harmonic THD(I) TDDCurrent (rms) Current (rms )

1000 50 5% 5%

800 43.8 5.4% 4.4%

600 36.3 6.1% 3.6%

400 29.7 7.4% 3.0%

200 20.0 10% 2%

100 13.4 13.4% 1.3%

Table DComparison of TDD and THD(I)

TDD - Total Demand DistortionTHD(I) - Total Harmonic Current Distortion

9

devices from large current spikes with short risetimes. A line reactor placed between the VFD andthe motor would help protect the motor from currentspikes. A line reactor placed between the supply andVFD would help protect the supply from currentspikes. Line reactors are typically only usedbetween the VFD and the motor when a freestand-ing VFD is mounted more than fifty feet from themotor. This is done to protect the motor windingsfrom voltage peaks with extremely quick rise times.

Passive Filters

Trap Filters are devices that include an electrical cir-cuit consisting of inductors, reactors, and capacitorsdesigned to provide a low impedance path to groundat the targeted frequency. Since current will travelthrough the lowest impedance path, this prevents theharmonic current at the targeted frequency frompropagating through the system. Filters can bemounted inside the drive cabinet or as free standingdevices. Trap filters are typically quoted to meet aTHD(I) value that would result in compliance withIEEE-519 requirements if the system were other-wise already in compliance.

Active Filters

Some devices measure harmonic currents andquickly create opposite current harmonic waveforms. The two wave forms then cancel out, pre-venting harmonic currents from being observedupstream of the filter. These types of filters general-ly have excellent harmonic mitigation characteris-tics. Active filters may reduce generator sizerequirements.

VFDs Using Active Front End Technology (AFE)

Some VFDs are manufactured with IGBT rectifiers.The unique attributes of IGBTs allow the VFD toactively control the power input, thereby loweringharmonics, increasing power factor and making theVFD far more tolerant of supply side disturbances.The AFE VFDs have ultra low harmonics capable ofmeeting IEEE-519 standards without any externalfilters or line reactors. This significantly reducesinstallation cost and generator size requirements.An AFE drive provides the best way to take advan-tage of VFD benefits and minimize harmonics.

Multi-Pulse VFDs (Cancellation)

There are a minimum of six rectifiers for a three-phase AC VFD. There can be more, however.Manufacturers offer 12, 18, 24, and 30 pulse drives.A standard six-pulse drive has six rectifiers, a12-pulse drive has two sets of six rectifiers, an18-pulse drive has three sets of six rectifiers and soon. If the power connected to each set of rectifiers isphase shifted, then some of the harmonics producedby one set of rectifiers will be opposite in polarityfrom the harmonics produced by the other set of rec-tifiers. The two wave forms effectively cancel eachother out. In order to use phase shifting, a specialtransformer with multiple secondary windings mustbe used. For example, with a 12-pulse VFD, aDelta/Delta-Wye transformer with each of the sec-ondary phases shifted by 30 degrees would be used.

ISC/IL <11 11<7<17 17<h<23 23<h<35 35<h TDD

<20 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0

LEGEND: h = harmonic numberISC = maximum short-circuit current at PCC IL = maximum demand load current (fundamental) at PCC

Table ERepresentation of IEEE Table 10.3

NOTE: It is the responsibility of the user to evaluatethe accuracy, completeness or usefulness of anycontent in this paper. Neither Carrier nor its affiliatesmake any representations or warranties regardingthe content contained in this paper. Neither Carriernor its affiliates will be liable to any user or anyoneelse for any inaccuracy, error or omission, regardlessof cause, or for any damages resulting from any use,reliance or reference to the content in this paper.

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REFERENCES

IEEE Standard 519-1992. “IEEE Recommended Practices and Requirements of Harmonic Control in Electrical Power Systems.”

Rockwell Automation. “Dealing with line harmonicsfrom PWM variable frequency drives.”

John F. Hibbard, Michael Z. Lowenstein. “Meeting IEEE 519-1992 Harmonic Limits Using Harmonic Guard Passive Filters” (TRANS-COIL, INC)

Tony Hoevenaars, P. Eng, Kurt LeDoux, P.E., Matt Colosina. 2003. “Interpreting IEEE Std 519 and Meeting its Harmonic Limits inVFD Applications.” (IEEE paper No. PCIC-2003-XX).

Gary Rockis, Glen Mazur, American TechnicalPublishers, Inc. 1997. “Electrical MotorControls.”

Richard H. Smith, P.E., Pure Power. 1999. “Power Quality Vista Looks Good Thanks to IGBTs.”

FURTHER READING FROM CARRIER

Carrier. 1993. Harmonics: A Brief Introduction.

Carrier. 1999. 19XRV Marketing Guide.

Carrier. 2005. Carrier Introduces Rotary Chillerswith Liquiflo2 Variable Speed Drive.

Carrier. 2005. Carrier Variable Speed Screw WhitePaper.

CONCLUSION

• VFDs provide the most energy efficient means of capacity control.

• VFDs have the lowest starting current of any starter type.

• VFDs reduce thermal and mechanical stresses on motors and belts.

• VFD installation is as simple as connecting the power supply to the VFD.

• VFDs with AFE technology can meet even the most stringent harmonic standards and reducebackup generator sizing.

• VFDs provide high power factor, eliminating the need for external power factor correction capacitors.

• VFDs provide lower KVA, helping alleviate voltage sags and power outages.

®

Copyright 2005 Carrier Corporation www.carrier.com Printed in U.S.A 10-05