Chapter Ac Drive

8/8/2019 Chapter Ac Drive

1/36

OBJECTIVES

At the conclusion of this chapter, you should be able to:

List the reasons for using AC drives to control the speed of AC motors. Explain the methods of controlling the speed of an AC motor and identify which one is

most effective.

Describe the purpose of the following sections of an AC drive: Operator Control Drive Controller AC Motor

Explain the differences among VVI, PWM, and Vector AC Drives. Describe the operation of the following major sections of the VVI drive:

Converter - Intermediate Circuit Inverter :-.-

Determine the V/H ratio when frequency changes and the voltage is constant. Describe the operation of the following VVI circuits:

Control Circuit Phase Control Rectifier Auto BoostBase Driver Overcurrent Protection Chopper Control

Describe the operation of the PWM drive and list its advantages over a VVI drive. Describe the operation of the Flux Vector drive and list its advantages over the VVI and

PWM drives. List the types of parameters that are preset by a programmable drive unit.

8 List the types of functions that are programmed on the control panel of an AC variable speeddrive.

8 Explain the following drive functions of an AC variable speed drive:Acceleration Slip Compensation Power Ride-ThroughDeceleration Critical Frequency Automatic RestartS-Curve Rejection Self-Protection

Identify the classifications of motor braking techniques used for stopping motorsquickly, and describe their operation.

INTRODUCTION

Much of the electricity produced by electric power uti lity companies is consumed by AC motorsthat runs fans, blowers, and pumps. These devices are used in production applications for industry, for the temperature control and ventilation of large buildings, and for furnaces andappliances in the home. A recent United States government report stated that over 50 percent of the electrical energy produced is used by AC motors.

AC induction motors are sized for maximum loads and are operated at a constant full speed, because they are supplied with power from AC lines at a fixed-sinusoidal voltage

CHAPTER

AC 1 Drives


2/36

wrr?

164 SE

I

4 Va riable Speed rives

and f i d frequency. However, a h i h percen tage of pumps, fans, and b lowers haveou t put f lowrequ iremen ts that f luctuate. To vary the f low of liqu ids or gases, compan ies of ten emp loythro ttling and res tr ictive dev ices, such as va lves, dampers, and or if ices. Even though these

mechan ical restr ictors prov ide effec tive con trol methods, up to 30 percen t of the power consumed by the mo tors is no t applied to the work they are mean t to perforrh. The energy that is was ted isconsumed by the f low res tr ictors as fr iction and hea t diffus ion.

Ano ther drawback of us ing mechan ical methods to reduce f low is that it deve lops s tress onthe f low sys tem. Dampers, va lves, fans, pumps, and mo tors wear ou t faster because they arealways f ighting eacho ther. Back pressure that deve lops can cause weak locations in pi pes or duc t work to rup ture, requ ir ing h igh ma intenance cos ts.

By emp loying an A C dr ive that ad justs the speed of the mo tor to vary f lowra te, thro ttling andrestr ictive dev ices can be e liminated. S ince the speed of a mo tor is propor tional to the e lectr icityit consumes, us ing an A C dr ive offers the po tential for s ignif ican t energy sav ings. An examp le of calculated sav ings is shown in Tab le 8-1.

ABLE 8-1

alculated Sa ving s

A cen tr ifuga l pump supp lies a f low of liquid through a p i ping sys tem. The pump runs 24 hours per day, 7 days a week, 52 weeks per year.

(8736 To tal Hours) The mo tor that dr ives the pump has a ra ting of 50 HP, and runs a t full speed. The pump f low is con trolled by a throttle va lve. The e lectr icity cos t to run the mo tor is determined by the fo llowing

calculation:

(50 HP x 0.746 = 37.3 kW) x (8734 Hrs) x (0.07 cen ts/kW-Hr)= $22,810 Annua l Opera ting Cost

Assum ing that a var iable speed dr ive w ill reduce the annua l opera ting cos t by 50 percen t, the sav ings w ill be:

0.5 x $22,810 = $11,405

Because a mo tor may consume as much as twen ty times its acqu isition cos t in electr icityevery year, it is f inanc ially and eco logically advan tageous to improve the eff iciency of the mo tor.

8-1 A rive Fu ndame ntals Un like the DC motor, the speed of an A C motor is no t con trolled exc lusively by vary ing theapplied vo ltage. I t is t rue that reduc ing the AC supp ly vo ltage w ill lower mo tor R PM . However, it will also reduce the torque, so that any changes in torque demand imposed by the mechan ical load w ill cause the mo tor to run a t erratic speeds.

The speed of an A C motor typically is determined by how many cyc les of a lterna ting curren t it rece ives each second. In the Un ited S tates, the frequency app lied is 60 Hz. Speed is alsodetermined by the number of po les. E lectr ic curren ts in the mo tor po le wind ings crea teelectromagne ts. Each magne t must have a nor th and a sou th pole. Therefore, magne tic polesalways eome in pa irs. Motors are common ly w ired to have two, four, s ix, or e ight poles. Thesynchronous speed of a g iven A C motor is der ived from the fo llowing equa tion:

60f P


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4/36

166 SECTION

Var iab le S eed Dr i! es

The objective of most AC drive systems is to control an induction motor so that the systemmeets the following criteria:

1. Provides full-load torque from zero RPM to full speed.2. Prevents torque fluctuations at low speed.3. Maintains a set speed when the load torque varies.

4. Provides a starting torque of at least 150 percent of the rated full-load torque.

5. Controls the rate of acceleration and deceleration.

The most common types of AC adjustable frequency drives used in industry are variable vol t age inver t er (VVI), pulse width m od ula t ed (PWM), and f lux vect or . They differ in theswitching patterns each uses to approximate a sine wave, and in the method each uses for controlling the motor voltage in proportion to frequency.

8-3 Va r iab le Volt a e Inve rte r (VVI) The variable voltage inverter adjustable frequency drive is also known as a six-step voltagesource inverter, or six-pack. Its name is derived from the six distinct output states of the DC to ACinverter during each cycle.

The variable frequency drive (VFD) has four main sections, which are shown in Figure 8-2.

1. Conv e rter

A converter is a rectifier that converts AC line voltage to a pulsating DC voltage. If the drive runsa low horsepower motor, a single-phase bridge circuit can be used for rectification. A three-phaserectifier is used to supply extra current if a high horsepower motor is used. There are two basictypes of rectifiers, con t rolle d an d uncont rolle d .

Converter

Operator control panel

X * *

Intermediatecircuit

Control circuit

Inverter

F IGURE 8-2 Var iab le volt a " e inv erter AC dr ive

line ^voltage

Three-phasemotor stator


5/36

CHAPTER 8 AC Dr ives 167

Uncontrolled Converter A three-phase uncontrolled rectifier is shown in Figure 8-3(a). Itconsists of six diodes. When a three-phase AC voltage is supplied, a pulsating DC voltage is

produced. The average value of the pulsating DC voltage is 1.35 x AC linevoltage. Since diodesare used, it is not possible to control conduction time of the rectifier. Therefore, this type of

converter is referred to as uncon t rolle d, because its output voltage cannot be changed.

Controlled Converter An AC drive varies the speed of a motor by changing the voltage andfrequency of its output; One method of altering the voltage is to change the amplitude produced

by the rectifier of the converter. This action is achieved by the three-phase controlled rectifier shown in Figure 8-3(b). Instead of using diodes, six silicon controlled rectifiers (SCRs) are usedin the full-wave rectifier. However, the SCR does not start conducting until it receives a controlsignal, designated a, at its gate, a is the symbol that represents a time delay stated in degrees. Thedegree value indicates the time delay from zero-crossing of a sine wave up to the point where theSCR starts conducting. By controlling the firing angle, a, somewhere between 0 and 90, therectifier output voltage can be varied. When the gate is activated at or near 0, the output is thesame waveform produced by the uncontrolled rectifier. The waveform produced at the outputwhen the gate fires 30 after zero-crossing is

Volts

wm

Three-phase input

waveforms

Converter output

-*- Time (a)

Sffl ^^ iiWMS is ^f es :.>r: : :

Lj o-

L 2 o- L

3 HH i&$ tm$ % y# Time

(b) (c)

FIGU RE 8-3 Conv erter sec tion: ( a) Un controll ed; (b) Controll ed; (c) Controll ed c onv erter w ave for ms

mm

Firing

Converter output

HI im - I Mi m


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168 SE#

$

I%

& 4 Va riable Speed ' rives

shown in F igure 8-3(c)'. The average va lue of this con trolled waveform is de termined by thecalculation:

1.35 x A C line vo ltage x cos a (30)

2 . ( he I nte r med iate)

ir cu it -The i 0 1 2 3 m 2 di 4 1 2 5 i 3 5 ui 1 transforms the pu lsating DC signal from the conver ter to a smoo th DC waveform. The ou t put of the intermed iate circuit is called the DC bus li 6 7 . There are three typical circuit conf igura tions that provide the f ilter ing ac tion requ ired by this sec tion. The conf igura tionused depends on the type of conver ter and inver ter to wh ich it is interfaced.

The s implest type of intermed iate circuit is shown in Figure 8-4(a). I t cons ists of a s inglelarge co il that provides f ilter ing ac tion. Th is type of intermed iate circuit is used w ith a con trolledconver ter. Toge ther, they supp ly the bus line with a var iable DC voltage. They are a lso capab le of transferr ing energy back into the AC line when regenera tion occurs due to a cond ition where theiner tia of the load overcomes the torque of a mo tor (ca lled 8 9 7 @ h A uli 6 B ), or when dece leration of amotor is too rap id.

Induc tor DC bus line

Conver ter out put

Induc tor -O DC bus line

Conver ter out put Capac itor

FI C D E E 8-4 I nte r med iate sect ion f ilter cir cu its: F a G Induct ive f ilter; F b G L

H

f ilter

The intermed iate circuit can a lso cons ist of a co il and capac itor, as shown in F igure 8-4(b).These componen ts can be used w ith con trolled or uncon trolled rec tif iers. Th is low-pass f ilter smoo ths the pu lsating DC at any vo ltage. If it is connec ted to an uncon trolled f ilter, a cons tan t DC voltage is presen t at the bus line. A var iable DC voltage is produced a t the bus line if a con trolledrectif ier is used.

Instead of us ing a con trolled rec tif ier to vary the D C voltage of the bus line, an intermed iatechopper c ircu it connec ted to an uncon trolled rec tif ier will provide the same func tion. Th is type of circu it uses a trans istor that is p laced in fron t of the LC f ilter, as shown in F igure 8-5(a). Thefunc tion of the trans istor is to alterna tely switch the rec tif ied D C voltage on and off. By chang ing

the du ty cyc le of the trans istor ou t put, the average vo ltage supp lied to the f ilter will be var ied. Thedu ty cyc le is the ra tio of time that a trans istor is on to the time that it is off. The waveform inFigure 8-5(b) shows the trans istor turned off for a longer time per iod than when it is turned on.The average vo ltage is lower than the ou t put produced when the trans istor is on for a longer per iodthan when it is turned off, as shown in F igure 8-5(c).

The L C f ilter c ircuit smoo ths the squarewave vo ltage of the chopper. When the trans istor turns off, the magne tic f ield a t the co il collapses very qu ick ly. As it cuts into the co il, it pro ducesa high vo ltage sp ike that can damage the trans istor. The d iode p laced across the trans istor pro tectsit from damage by prov iding an a lterna te d ischarge pa th.


7/36

CH API

E P 8 AC Q rives 169

Coil DC bus

line

Fixed conver ter out pu t

Time O ---------------- i ------------------------ 1------------------ O

.- ii Var iable DC out put

-*- T ime

(a)

V V

Average- voltage

Average- voltage

-* t -- t l

R ff

(b) (c)

FIS T

RE 8-5 Intermed ia te sect ion: U a V With a ch oppe r tr ans ist or that varies the W C voltage ; (b) X h e duty cyc le tha t p rodu c es a low Y ` voltage ; a c b

c

he duty cycle that p roduces a h igh d C voltag e

3. Inve r te r The fu e f t ig e of the ie h i p ter is to " iq r s t t " the D C bus line vo ltage back to s imulated A C power a t thedesired frequency. An inver ter cons ists of s ix electronic switching dev ices used to deve lop an approx imate

three-phase A C signal for the mo tor. In F igure 8-6, sw itching trans istors are used. The vo ltage across thetrans istor ne twork is supp lied from the DC bus line of the intermed iate circuit. The trans istors turn on andoff in the proper sequence to crea te three se ts of vo ltage waveforms be tween A- B, B-C, and C-A.

The D C voltage from the DC bus line is either cons tant or var iable. When the inver ter rece ives avar iable voltage, it must vary the frequency on ly. However, when the vo ltage is cons tant, the inver ter mus t control both the frequency and the amp litude. These types of in ver ters use S CR s instead of trans istors. Thevoltage is var ied by chang ing the f ir ing ang le at which the ga te rece ives its pulse.

Intermed iatevoltage Inver ter

Var iable

Constant

-*- t

-*- t

t ii ii ii

DC busline

I ii ii ii -o A-o B -OC

FIu

v

RE 8- w Inver te r to variable or consta nt inte r med iate c ir cu it voltage


8/36

170 SE CT Ix

y 4 Va riable Speed rives

The d isadvan tage of us ing S CR s in the inver ter sec tion is that they canno t change from aconduc tive to a nonconduc tive cond ition af ter they turn on un til curren t through them reacheszero. For mos t curren t dr ives manufac tured today, the sw itch ing speed prov ided by S CR s is no t fast enough. S ince a trans istor is independen t of the zero-cross ing of the curren t, it can be changedfrom a conduc ting to a nonconduc ting cond ition a t any time. Th is character istic enab lestrans istors to be sw itched a t a very fas t rate. The upper leve l of the sw itchi ng frequency of sometypes of trans istors is presen tly severa l hundred k iloher tz.

4. C on trol C ir cu it The con trol circu it is the four th ma in b lock of the A C var iable speed dr ive. It performs severa l func tions:

It rece ives the opera ting commands from the con trol pane l or other programm ing dev ice.These commands are used to de termine con trolled dr ive var iables such as the ou t put frequency, ou t put voltage, and mo tor d irection.

It genera tes command pu lses, wh ich turn the sem iconduc tors on and off in the inver ter. I t also prov ides the f ir ing s ignals for dr ives that use a chopper or have a con trolled

conver ter. It produces ou t puts for the con trol pane l display to indicate the dr ive speed, opera ting

status, and fau lt cond itions. It performs an order ly shu tdown procedure of the dr ive when an abnorma l cond ition

ar ises.

For many years con trol circu it opera tion was based on ana log techno logy. Today, m icroprocessors and d igital electron ics dev ices are used to prov ide more soph isticated con trol.

T he I nve r te r pe r at i on The VVI dr ive inver ter con trols no thing excep t frequency. The opera tion of the inver ter sec tioncan be illustrated by us ing the sw itch mode l in Figure 8-7(a). Three SPDT sw itches are connec tedin para llel across a D C supp ly. Depend ing on the sw itch pos ition, e ither a pos itive vo ltage or a

zero-vo lt ground connec ts to the sw ivel terminal. The vo ltage is pos itive in the up pos ition andzero in the down pos ition. The w ires connec ted to each sw ivel terminal are labe led A, B, and C, toiden tify each of the three-phase inver ter ou t put lines. F igure 8-7(b) shows how the se ttings of switches A and B crea te a resu ltant AC squarewave line vo ltage. Cons ider ou t put line B as thereference and compare ou t put line A to it. Dur ing time per iod1, both lines are the same ground po tential, so the line vo ltage A B is zero. Dur ing time per iod2, reference line B is zero vo lts and A is pos itive, wh ich produces a pos itive line vo ltage. Dur ingtime per iod 3 bo th lines are the same pos itive po tential, so the line vo ltage is zero. Dur in g time

per iod 4 reference line B is pos itive and A is zero vo lts, wh ich produces a nega tive line vo ltage.The trans istors in the inver ter perform the func tion of SPDT sw itches. For examp le, compare

switch A of the sw itch mode l to the lef t para llel branch of the inver ter sec tion in F igure 8-2. Theswivel terminal of the sw itch represen ts the junc tion be tween trans istors T j and T 4. Whentrans istor T t is on and T 4 is off, the c ircuit opera tes as if the SPDT sw itch is in the up pos ition.Therefore, a pos itive vo ltage ex ists a t junc tion A. If trans istor T x is off and T 4 is on, the c ircuit

opera tes as if the SPDT sw itch is in the down pos ition and zero vo ltage is felt at junc tion A. Whenall six sw itching trans istors are sequenced proper ly, they deve lop squarewave s igna ls to simulatethree-phase vo ltage pa tterns. The line vo ltages form across terminals A B, BC , and A C, and are120 degrees ou t of phase w ith one ano ther. The frequency is con trolled by the speed a t which theinver ter trans istors deve lop the s ix-s tep waveform.

To fur ther illustrate the opera tion, the inpu t and ou t put waveforms are shown in Figure8-7(c). The s ignals represen ted by the inpu t waveforms are app lied to the NPN trans istors. A

pos itive vo ltage app lied to the base of a trans istor turns it on and causes it to opera te like a c losedswitch. A zero-vo lt potential turns the trans istor off and causes it to act as an open sw itch.


9/36


During the first step, which lasts a sixth of a cycle, transistors T,, T 5, and T 6 turn on. Duringthe second step (second sixth of a cycle) transistors T, and T 6 remain on, but T 5 turns off and T 2 turns on. During the third step, T 6 turns off and T 3 turns on, while T^ and T 2 remain on. Eachsubsequent sixth of a cycle performs in a similar manner; one transistor turns on while another

transistor turns off. During the six-step cycle, each transistor turns on and off once. After the sixthstep is completed, the cycle is back to its starting point and is ready to begin a new cycle.The diodes D[ through D 6 are placed across the collector and emitter leads of each switching

transistor. Their function is to protect each transistor from being damaged by a current surge.When a transistor turns off, the magnetic field of the stator coil to which it

(a)

(b)

+v A

u v +V B OV

c OV +v Phase voltage AB

s s / s

/ / / / \S \

/ -V _

13H ! O 01

\ a c?3. 3o a CL

// /


10/36

. p e r i o d 2

T i

m e

' - ' en a- a - 3 o S o. 4^

FI

RE 8- (a Inve r te r s witch m odel ; (b Inv er te r wavef or ms de veloped by the

s witch m odel ; (c

Input a nd output t iming diagr am ; (d

utput of on e phase du ring on e cycle


11/36

172 SE CT I


(c

ITl

j T2

-

? T3

-

U T4

^ ^ -- ^^ A'O ii^ '^- |:: :' On '""A On

I - :O n. : ; -;

On -i^^HwQn, ^ vifetaA

On'

I - A& tJ &V M;. "v " ' On " ': - :"K 1 1 1 1 1 1

+Vdc - VAB - - vd -0

u + vd . T

P VBC -

? - v, . + vd

- VC A --Vdc

-

.0 it 2 JC3 3

n An I 3

7t 2n 7 TC 8 T I 3I 3 3

i t

- ;.;.;.to' 5 .:.-

-

(d)

+V

Line to

line vo ltage

-V

FI E 8-7 (continued)

supp lies curren t will collapse. The resu lt is that an induced curren t is fed to the co llector lead.Instead of f lowing through the trans istor to the em itter, curren t f lows through the forward-b iasedshor ting d iode and back to the bus line. In add ition to perm itting the re turn of energy to the busline from the reac tive load, the d iode a lso prov ides a pa th for regenera tive energy fed back by theload.

Figure 8-7(d) shows the ou t put of one phase dur ing one cyc le. The smoo thing of the rec-tangu lar wave is a resu lt of the mo tor induc tance. The A C frequency of the inver ter ou t put isdirectly propor tional to the time dura tion of one comp lete cyc le. The inpu t signals to the power trans istors that cause them to turn on or off or iginate in the con tr ol cir cuit of the dr ive.

C on trol C ir cu it pe r at ion The s ix squarewave s ignals fed into the inver ter or iginate in the con trol circuit, which is shown inthe b lock d iagram in F igure 8-8(a).

The speed con trol rheos tat at the opera tor con trol pane l feeds an inpu t signal to the con trol circu it inpu t terminal. Th is terminal cou ld be the inpu t of an as table mu ltivi bra tor. I ts func tion isto genera te a con tinuous squarewave s ignal. The ou t put of the mu ltivi bra tor is called a c lock signal. It is app lied to a d igital r ing coun ter that cons ists of s ix f li p-f lops (FFs).


12/36

CH AP T E 8 AC rives 17 3

This type of d igital circuit is ac tually a sh if t register that f irst moves b it patterns to the r ight andthen rec ircu lates them from the lef t.

Observe the waveform d iagram in F igure 8-8(b). I t is iden tical to the inpu t waveforms of Figure 8-7(c). Dur ing the f irst sixth of a cyc le, FF l, FF5, and FF6 con tain a 1 s tate at their Qout puts. The o ther f li p-f lops have 0 s tates. The b it pattern is 100011. As a c lock pu lse from themultivi bra tor occurs, the bits in FF l through FF5 are sh if ted one pos ition to the r ight. Also, the bit from FF6 is rec irculated by sh if ting to FF l. The b it pattern is now 110001. When the nex t clock

pulse arr ives, the b its are sh if ted ano ther p lace to the r ight and the FF6 b it is rec ircu lated to FF l.The b it pattern is now 111000. The b its are sh if ted one pos ition every one-s ixth of a cyc le, as eachadd itional clock pu lse occurs.

Inver ter

T, T2 T3 T4 T5 T6

t t t t t t Base d rive r

II iL Vcc

(a)I --- 1

555 J Q

b|>FF l K

Q

J QrOt>FF2

K Q

J Q rO(>FF3 K

Q Ring

counter

J Q rOt>FF4 K

Q

J Q r4>FF5 K

Q

J Q rO|>FF6 K

Q

Astablemulti vi bra tor

Control circuit

Speedcontrol 0

6 +10V

Fli p-f lop Qout pu ts FFl

-FF2

-FF3

-FF4

1 0 1 0 0 F1

0 0 1

0 0 1

0

1 1 ' 0

,1 1 i i i i i i

JC 2 3

7C

1 3

t 4

7t 3

< K 2 3

71

1st 2nd

JUT. CLK inpu t

Operator con trol pane l

(b)

5th

1/6

6th

1/6

3rd

1/6

4th

1/6

Cycle


13/36

I 1/6 1/6

FIGU R E 8-8 The con trol circuit in the dr ive con troller sec tion


14/36

SE CT I


When the opera tor increases the speed, the w i per arm of the rheos tat goes upward. Byreduc ing the res istance, the RC time cons tant decreases, wh ich causes the ou t put frequency toincrease. The resu lt is that the b its circu late fas ter, wh ich causes the inver ter's three-phase ou t put frequency to increase.

AC dr ives do no t use a mu ltivi bra tor and d iscre te f li p-f lops in the con trol circuit. Ins tead,they u tilize a m icroprocessor or programmab le con troller to form the b it pattern fed to theinver ter.

Base rive r The c ircuitry of f li p-f lops, m icroprocessors, and programmab le con trollers is manufac tured onintegra ted c ircuit (IC) ch i ps. The out put curren t of ICs is relatively sma ll. If the ICs wereconnec ted d irectly to the inver ter power trans istors, there wou ld no t be enough base curren t toturn the trans istors on. Therefore the IC out put leads are connec ted to a b a e d r iv er. This sec tionamp lif ies and isolates con trol circuit signals to make the inver ter trans istors opera te in theappropr iate sw itching scheme.

The c ircuitry of the base dr iver is illustrated in Figure 8-9, wh ich shows one of the dr iver's s ix NPN op to-trans istors. When the ou t put lead of the con trol circuit is in a 0 s tate, the LED is

forward b iased. The LED lights as curren t f lows through it. Light shines on the base of thetrans istor and turns it fully on. Because the trans istor ac ts as a c losed sw itch when it satura tes, the base dr iver ou t put lead tied to the co llector w ill be a low-s tate potential. When the con trol circu it out put lead is high, the LED is no t forward b iased. Therefore the trans istor is in the cu toff modeand ac ts like an open sw itch. A h igh-vo ltage one-s tate po ten tial is then formed a t the co llector out put lead.

Basedr iver

+5V Vcc

Curren t i> limiting < resistor <

Contol circuit j 1 '"^ >*

out pu t lead ~zr

FI E 8-9 Base d river c ir cu itr y

Voltage/ Hz at io As the speed of the mo tor is changed, a cons tant torque is requ ired. To ach ieve this, curren t (I) that f lows through the s tator w indings of the mo tor mus t be kep t at a cons tant level.

It is eviden t that the speed of the mo tor can be changed by s imply vary ing the frequency.However, if the frequency a lone is var ied w ithou t chang ing the mo tor vo ltage, the curren t andresu lting torque w ill change, as is proven by the fo llowing formu las:

I nd uct i j R j k cta nce: X L = 2 jtfL V

Ohm s Law : I = X L


15/36


If the frequency is lowered and the voltage remains constant, the current will rise dra-matically. The result is that the torque will rise by the same proportion. However, the motor mayoverheat due to overexcitation from the excessive current. To maintain a constant current andtorque, the bus voltage must be varied in direct proportion to the inverter output frequency. This

variation is called voltage-to-l

reqm

ency (

n

/Hz) ratio. The V/Hz ratio in a VVI drive is controlled in one of two ways: by a phase-controlledrectifier or by a diode rectifer and chopper configuration.

Po

ase Control A variable voltage source drive with phase control is illustrated in Figure 8-10. Two of the diodesused by the converter's bridge rectifier network are replaced by SCRs. The SCRsor phase con t rol rect i f iers perform two functions. First, they provide full-wave rectification, rectifyingAC power with the diodes. Second, the SCRs control the amount of average DC voltage appliedto the converter output. By turning the SCRs on early in the alternations, the average voltage

produced at the bridge output is high. The later the SCRs fire in the alternations, the smaller thevoltage generated. By controlling the amount of time that elapses before the SCRs are turned on,the output voltage of the converter can be adjusted to any value desired. The instant at which the

SCRs fire during an alternation is controlled by the firing circuit in Figure 8-10. The RC network uses a rheostat to supply resistance. It is mechanically coupled to the speed control potentiometer.

Operator control

To controlcircuit

O ---

AC inputO ----

Converter

To intermediatecircuit -

Phase control bring circuit

F IGURE 8-10 T

e

ase c ontrol cir cui tr y of a VVI AC dr ive

Recall that to maintain a constant V/Hz ratio, the inverter's voltage andfrequency must increase or decrease together. Suppose that the speedcontrol knob is adjusted to a lower speed setting. The ring counter shifts moreslowly, which causes the switching sequence to the inverter to slow down, decreasing the outputfrequency. The reason the output voltage also decreases is that the resistance of rheostat R 2 increases. As it does, the capacitor takes more time to charge, which causes the UJT to fire later.Since the SCRs turn on late during the alternations, the firing delay angle increases. The result isthat the converter's average voltage decreases. The filter components in the intermediate circuitare required for smoothing the pulses.

Overcurrent Protection If the motor current in the output lines of the inverter rises above the ratedvalue of the drive, it is likely that a malfunction has developed. As a precautionary


16/36

176 SECTION

Var iab le S eed Dr ives

measure, the drive is designed to keep the current from exceeding a certain level. An over-currentcondition can result when:

1. The physical load that the motor is driving becomes excessive.

2. The starting inertia of the motor load is excessive and the acceleration time is too short.3. A short circuit exists in the inverter output leads or motor windings.

4. A device in the drive inverter has short-circuited.

A protective circuit in the control section monitors the load current. When it becomes toohigh, the circuit shuts the inverter down by turning off the transistors.

Figure 8-11 shows an overcurrent protection circuit. A current-sensing coil is wound aroundone of the three-phase output lines of the inverter section. The changing lines of flux around thewire induce a voltage into the coil, which is connected to the primary input of a step-uptransformer. The secondary voltage is rectified, filtered, and applied to the inverting input of comparator op amp 1. A reference voltage from the current limit adjust potentiometer is connectedto the noninverting input of op amp 1.

If the output line current is within the rated level, the voltage at the inverting input is less thanthe noninverting input. The output of the op amp goes high. This enables the output signals of the

ring counter FFs to pass through the AND gates to the base driver. Therefore the inverter transistors go through their switching sequence. When the output line current becomes too high,the inverting voltage becomes greater than the noninverting voltage. The op amp output goes lowand disables the AND gates. This blocks the 1 state signals from passing from the ring counter FFsto the base driver. The drive shuts down because the inverter transistors cannot turn on.

F IGURE 8-11 Over cu rr e nt rot ec tion cir cui t

Current limitadjustment

potentiometer


17/36


The current limit potentiometer is used to adjust the circuit to shut down at different currentlevels.

Auto Boost Circuit When a motor is being started or is running at a slow RPM, the drive must

provide enough current to produce sufficient torque to overcome inertia.Figure 8-12 shows an a to boo st circuit that will cause the current to increase when themotor is running at a low RPM. Within the normal operating speed, Q l is biased within its activeregion. As the speed control knob is adjusted to a slower speed setting, the potentiometer voltageat the comparator's inverting input reduces. When it becomes lower than the noninverting inputvoltage, the comparator output goes high. The result is that transistor Qi turns on harder, chargesC] more quickly, and fires the SCRs in the converter earlier. The average voltage increases and

provides additional current for the motor.The auto boost circuit is usually located in the control section of the drive. The auto boost

adjustment potentiometer is used to set the speed at which the added current will be supplied.

Operator control _ Auto boost circuit

f\j AC input

O -----

Converter

To intermediatecircuit .,

F IGURE 8-12 Auto boo s t cir cui tr y

C

o e r Control A VVI with chopper control is shown in Figure 8-13. It uses a single-phase bridge rectifier in theconverter section to produce a pulsating DC voltage. The intermediate circuit has two capacitorsand a chopper switch. The capacitor placed before the chopper smooths out the DC pulsations of the converter output. The chopper switches on and off very rapidly. As it does, the ratio of

on-time to off-time is changed to vary the bus voltage applied to the inverter section. Turning thechopper off for a large portion of the alternation produces a small average voltage at the bus line;turning the chopper on for a large portion of the alternation produces a large average voltage atthe bus line. By varying this ratio, a precise voltage can be produced. A second capacitor is placedafter the chopper for smoothing.

The circuit that controls the chopper function is shown in Figure 8-14. An astable multi-vibrator is used to produce a square wave. Because the current from the555 IC is relatively small,it cannot provide enough base current to control the standard power transistors in the inverter section. The current limitations of the 555 IC can be overcome by connecting its output line to anopto-transistor. When the squarewave output of the IC goes high, it forward biases the LED. Theopto-transistor turns on and acts as a closed switch. A low at the IC output turns the LED off. Theopto-transistor also turns off and acts as an open switch.

To control

Phase control

firing circuit


18/36

178 SECTION

Var iab le S eed Dr ives

Bus line

F IGURE 8-13 VVI wit

ch o er c ontrol

The on-time/off-time ratio of the chopper is controlled by the multivibrator's duty cycle. Theduty cycle is the ratio of time the output terminal is high to the total duration of one cycle. Theduty cycle is determined precisely by the ratio of resistors R A and R B. If the diode was notconnected in parallel with R B, the capacitor would charge up through R A and R B. It would thendischarge only through R B (and the IC internally from pin 1 to pin 7). Therefore the duty cyclewould be less than 50 percent. With the diode connected, R B is bypassed and the capacitor chargesonly through R A. When the capacitor discharges, its current path is

Operator control

+10V

FastSpeed s tcontrol > +knob

Slow

+5V

R 4

To controlcircuit

+5 V Chopper type of f intermediate circuit

OutputOptocoupler

DC bus line

Variable voltagevariable frequency30 power

ifl ss kg SBvietfer/;

555Astable

multivibrator inverter


19/36

FI z { | E 8-14 T he ch oppe r con tro l cir cu itr y of a VVI A C drive


20/36


blocked by the reverse-biased diode, and therefore only flows through R B. Depending on theresistance ratio of R A and R B, this configuration allows the duty cycle to operate over a range of 5

percent to 95 percent.One way to change the resistance ratio is by using a rheostat at R A. When R A is smaller than

the resistance of R B, the duty cycle is less than 50 percent. This means that the I C output is highfor less time than it is low. When R A is larger than the resistance of R B, the duty cycle is more than50 percent.

The rheostat is mechanically coupled to the speed control potentiometer. When the speedcontrol is adjusted to a faster setting, the duty cycle is greater than 50 percent. The result is thatthe chopper supplies a greater average voltage to the DC bus line.

The VVI drive is usually used in industrial applications where low to medium power isrequired. Although the motor operates more effectively when it is powered by a sine wave fromthe supply mains, the squarewave output of a VVI drive is usually satisfactory. However, at lowspeeds, such as below 10 Hz, a stepping or cogging motion of the output shaft may occur astorque pulsations develop each time an output transistor is switched. At higher speeds thecogging disappears, but the pulsations may cause vibration or audible noise problems. Also, thenon-sinusoidal waveforms shown in Figure 8-7(d) may cause extra heating in the motor windings. This situation may require a derating of the motor by up to 25 percent.

8-4 P ulse idth Modu la tion Dr ives The VVI drive has been largely superseded by the p } lse wi d th m odu lation d rive, or PWM. Itscircuitry is shown in Figure 8-15.

Since most drives run high horsepower motors, the single-phase bridge used to describe theVVI's operation may not generate enough current. The PWM drive's converter uses a three-phaserectifier to supply the extra power required by large motors. A three-phase rectifier supplies more

power than a single-phase rectifier because its pulsations are closer together and therefore, thefiltering required is minimized.

A full-wave three-phase rectifier circuit is shown in Figure 8-16. Its six diodes either block or pass current. A wye transformer is the source that supplies both positive and negativealternations of three-phase current to the rectifier.

Three facts about the operation of the rectifier must be known to analyze the circuit:

1. At any given time, two diodes are on and the remaining four are off.2. One odd-numbered diode (Dj, D 3, or D 5) and one even-numbered diode (D 2, D 4, or D 6)

are always on.

Fixed DC bus voltage

S S II

^O W

vx Variable voltagevariable frequency30 power

Three-phase power

21S

A

>y> \C to DCconverter section

Intermediatesection

DGTOAC ::inverter'' 1

F IGURE 8-15 A ~ ulse width modu la ted (P

M) dr ive cir cui t

Motor


21/36

180 SE CT I


D2 D4 D6

~ tZ ~ tZ ik

A

208 V RMS 1

2^B >

D l D3 D5

ii "ir iz

+R L

FI E 8-16 A wye s our ce suppl ies p ower to athr ee-phase full- wave r ect if ie r

3. E lectron curren t f lows ou t of the A, B, or C phase transformer terminals from the mos t nega tive vo ltage, through an odd-numbered d iode, through the load res istor, and throughan even-numbered d iode, to wh ichever phase term inal A, B, or C has the h ighes t

pos itive vo ltage.

Figure 8-17 graph ically illustrates the rec tif ication process in detail. Figure 8-17(a) shows the208 V RM S three-phase vo ltage supp lied from the source terminals of the transformer to therectif ier. The timing char t in F igure 8-17(b) shows interva ls when the correspond ing d iodes areon. F igure 8-17(c) uses c losed sw itches to show wh ich d iodes are forward b iased dur ing 30- or 60-degree interva ls.

Between 0 and 30 degrees, terminal C is mos t positive, wh ich forward b iases d iode D 6, andterminal B is mos t nega tive, wh ich forward b iases D 3. The resu lting 30-degree a lterna tion is

shown in Figure 8-18. From 30 to 90 degrees, terminal A is mos t positive, wh ich turns D 2 on, andterminal B is mos t nega tive, wh ich turns D 3 on. The resu lting vo ltage pu lses form across the loadresistor dur ing this, and every, 60-degree per iod. S ince the magn itude of the pu lses f luctuatesapprox imately 59 vo lts (235 V to 294 V), the rec tif ier ou t put requ ires on ly minimal f ilter ing.

The pu lsating waveform from the conver ter is f iltered by the intermed iate sec tion. Un like theVVI dr ive, wh ich var ies the busline vo ltage w ith a chopper, the PW M bus vo ltage is

A B c +294 V

OV \ '/

i \ i / i \ i

90X 210) ( 330A

_ ?O dV

0 I 60 120 I 180 240l 300 360 30 150 270

FI E 8-17 A gr aph ical illust r at ion of thethr ee-phase r ect if icat ion process

(a)


22/36


Highest positive ___ ------------------------- . -------------------voltage I ~ _______ t______ ' -

" _________________

Highest mrnmm-1

negativevoltage

a m wm

(b)

Intervals 0 30 60 90 120 150 180 210 240 270 300 330 360 II I I I I I I I II I I

Diodes |D6 | D2 | D4 D6turned on I D3 D5 | Dl |D3 |

0 to 30 and330 to 360 30 to 90 90 to 150

i JJ Ii JJ I + D2t > > i +D2t ^ > i + R L A-^B-4 C-4 > R L A-^B-4 C-^


23/36

18 2 SE CT I


Figure 8-19(b) uses SPDT sw itches to show how the trans istors crea te the vo ltage pa t terns.All of the sw itches are connec ted in para llel across a D C supp ly. Depend ing on the sw itch

pos ition, e ither a pos itive vo ltage or a zero-vo lt ground po ten tial connec ts to the sw ivel out put terminal. In F igure 8-19(a), observe the ou t put waveforms a t terminals A and B, and the resu ltant line vo ltage that forms across them. Assume that out put line B is the reference and compare lineA to it:

Dur ing time per iods a, g, i, m, o, and u, bo th sw itches are in the down pos ition. S ince bo thlines are the same ground po tential, line vo ltage A B is zero.

Dur ing time per iods b, d, f, h, and j, sw itch A is up and sw itch B is down. Ou t put A is pos itive in respec t to B. The resu lt is line vo ltage A B becomes pos itive.

Dur ing time per iods c, e, k, q, and s, bo th switches are up. Because ou t puts A and B are thesame pos itive po tential, line vo ltage A B is zero.

Dur ing time per iods 1, n, p, r, and t, sw itch A is down and sw itch B is up. Ou t put A isnega tive with respec t to B. The resu lt is line vo ltage A B becomes nega tive.

The sw itching sequence forms a vo ltage pa ttern that cons ists of f ive pu lses dur ing eachalterna tion. The dashed line is the RM S vo ltage crea ted. The w idths of the individua l pulses aread justed to produce a var iable RM S vo ltage. The longer the pu lse dura tion, the h igher the RM Svoltage. The pu lse groups are timed to prov ide var iable frequency. The way in wh ich the signalsare produced is called mod ul at ion .

In a typical PWM inver ter circu it, more than f ive pu lses are deve loped dur ing an a lterna tion.Figure 8-19(c) shows an ac tual squarewave pa ttern on two inver ter ou t put terminals and theresu ltant line vo ltage waveforms they deve lop. The shape of the pu lses form an RM S voltage that

becomes sinuso idal, which causes a curren t, approx imate to sinuso idal, to f low through themotor. The s ignals that activate the sw itching trans istors or iginate in the con trol section.

There are severa l advan tages of the PW M dr ive over the s ix-step VVI dr ive. As prev i ous lystated, when the VVI dr ive is runn ing at a low frequency, the speed of the mo tor may f luctuate or cog due to the squarewave-shaped ou t put of its inver ter. A PW M dr ive will run a mo tor smoo thlyat any R PM within its speed range because its inver ter can produce a s inuso idal-shapedwaveform by genera ting pu lses.

Ano ther d isadvan tage of the VVI is that its ou t put voltage is con tro lled by the intermed iatecircu it. The resu lt is poor speed con trol because the large f ilter ing capac itor has to be charged or discharged to change the vo ltage. The PW M can vary the ou t put voltage instantly by chang ingthe number of pu lsesand their widthsdur ing each a lterna tion.

The pu lses genera ted from the con trol circuit or iginate in a modu lator logic board. The boardcons ists of low-power s igna l-genera ting c ircuits and m icroprocessor-based LSI logic circu its,wh ich con trol the performance charac ter istics of the dr ive. Pu lse w idth is decreased for lower RM S vo ltages and increased for h igher vo ltages. Lower frequenc ies have a grea ter number of

pulses in one a lterna tion. As the frequency increases, the c ircuits on the logic board se lect thenumber of pu lses per cyc le.

Because the sem iconduc tors in the inver ter mus t switch at extreme ly high speeds to producea PWM out put, insu lated ga te bi polar trans istors (IG BT) are frequen tly used. The IG BT trans istor is a comb ination of the b i polar trans istor and the MOS-FET trans istor. I t utilizes the des irablecharac ter istics of each one, including h igh power capac ity, good conduc tivity, and s imple ga tecontrol, to prov ide a h igh sw itching frequency.

The s tandard induc tion mo tor is des igned for a f ixed speed de termined by its rated opera tingfrequencyusua lly 60 Hzand its number of po les. However, the AC dr ive is capab le of ach ieving mo tor performance beyond its namep late ra ting. Therefore, to preven t motor damage,it is necessary to take precau tions when its velocity is above or be low its ra ted speed. Norma llythe mo tor is coo led by a fan wh ich is attached to the ro tor shaf t. When the dr ive causes the mo tor to run a t a velocity slower than its base speed, the fan w ill not produce suff icient cooling. As thetempera ture r ises, the insu lation tempera ture ra tings are exceeded, wh ich can shor ten the life of the mo tor's insu lation or cause the mo tor to fail. To pro tect the


24/36

CHAPTER 8 ACDr ives

183

motor from overheating, a thermal protection device can be connected to the motor windingto stop the motor if its temperature gets too high. Another solution is to add a constant speed,separately-powered blower to the motor to ensure adequate cooling at all speeds. Preventingvibration becomes especially important in higher speed motors. If the motor is expectedto run at speeds greater than 3000 RPM, accurate coupling and shaft alignment are important

(a)

+ A 0 =; Line voltage 0AB

a b c d ef

l h i j

k lm n < 3 P 1 r

5 t U

ii ii U U

j__ H __ H __j ; ; ; ; n r~ u

M te MM:* * y "- Jrr '

-

L__ H

* -

(b)

(c) Waveform at inverter output line A

Waveform at inverter output line B

n- - +V

OV

------ +v

J --------- OV

Voltage between output lines A and B

FIGU RE -19 Inverter sec tion o ut ut wa vefor ms of a P

M dr ive


25/36

184 SE CT I


to reduce v i bration, and the ro tor mus t be prec ision ba lanced. Spec ial bear ings mus t also be f ittedif the mo tor speed exceeds 7500 R PM .

The mo tor tempera ture may a lso become very h igh when a dr ive opera tes a t full speed andfull load. Some of the power app lied to the mo tor is lost because it genera tes hea t when opera ted

by the dr ive's s imulated waveform, wh ich is no t an exac t dup licate of a s ine wave. By us ing amotor w ith a dera ted power fac tor, the excess ive hea t that is genera ted w ill be reduced.

8-5 Flux Vect or rives The opera tion of VVI and s tandard PW M dr ives is adequa te for s teady-s tate cond itions, or inapplications where a s low response speed change is adequa te when load var iations areencoun tered. However, there are some spec ialized app lications where abrup t load, speed, and

pos ition changes are likely to occur. A flux ve c tor d r iv e is better su ited to hand le thesecond itions.

The vec tor dr ive is one of the mos t recen t and s ignif ican t deve lopmen ts in dr ive techno logy.It is capab le of ach ieving con trol over an asynchronous squ irrel cage mo tor so that its

performance is s imilar to that of a D C motor and advanced D C dr ive. For examp le, max i mumtorque is ava ilable at all speeds and the dynam ic response is qu ick, thereby prec isely maintaininga cons tant velocity under vary ing load cond itions. In these respec ts a vec tor dr ive far exceedsother A C dr ives.

The term v ect or is der ived from the me thod in wh ich the dr ive con trols the magne tic f luxvector. A vec tor in ma thema tics indicates amp litude and phase ang le. The vec tor dr ive con trolsthe mo tor by vary ing the magn itude of the s tator f lux and ad justs the ang le of the s tator f lux inreference to the ro tor.

The pr imary reason for a D C motor's effec tive opera ting charac ter istics is its phys ical cons truction. The way in wh ich the brushes make con tact witji the commu tator causes theresu ltant f lux of the ro tor to be perpend icular to the s tator f ield. Because the arma ture's magne ticf ield and the s tator f ield are a lways quadra ture (90 degrees to one ano ther), max imum torque is

always produced. The drawback of a D C motor is that it is a h igh-ma intenance mach ine; the brushes requ ire frequen t replacemen t due to their phys ical con tact with the commu tator. The A C asynchronous mo tor is preferrab le to the DC motor for mos t indus tr ial applications because of thelow cap ital and ma intenance cos t, h igh eff iciency, and long- term dependab ility.

The f ixed 90-degree re lationsh i p between the ro tor and s tator in a D C motor does no t natura lly occur in an A C induc tion mo tor. The induc tion mo tor ro tor is bas ically two r ings w ith

bars p laced be tween them like a whee l for a squ irrel, hence the name squ irrel cage. Un like the DC motor, ro tor f lux is produced by hav ing a ro tating s tator f ield induce curren t into the bars of thecage. As the s tator and ro tor f ields interac t, torque is produced, wh ich causes the ro tor to turn.However, the ro tor lags beh ind, and ro tates s lower than, the s tator f ield. Th is difference inrotational speed is referred to as sli . Sli p is measured as an angu lar ve locity expressed as afrequency. Under norma l opera ting cond itions, the (s li p) frequency is abou t 3 Hz. When a grea ter load is placed on the ro tor, s li p increases, more curren t is induced into the arma ture, and torqueincreases. The max imum mo tor torque is produced when the ro tor f lux re lative to the s tator f ieldis at 90 degrees. The drawback of the A C induc tion mo tor is that it responds s lowly to loadchanges. I t is accep table for dr iving such app lications as pumps, fans, and compressors. However,when rap id dynam ic con trol of speed, torque, and response to a change in load is necessary, itsopera tion is too crude.

The fundamen tal prob lem w ith con trolling an A C induc tion mo tor is that the stator and ro tor f lux canno t be regu lated separa tely. Ins tead, curren t induced into the ro tor is on ly respons ive tothe var iation of the frequency and curren t amplitude supp lied to the s tator. R ecen t deve lopmen tsin d igital signa l process ing (DSP) have overcome the d iff iculty of


26/36

CHAPTER 8 ACDr ives

185

providing accurate control of the rotor, by manipulating the stator flux. The control scheme of digital signal processing is to constantly monitor and control the amplitude of the stator MMF flux, as well as its positionrelative to the rotor. A high-performance microprocessor repeats 2000 or more computations per second byusing a look-up table to make readjustments if they are required. This method is referred to as f lux vect or con t rol .

A vector drive is similar in construction to a PWM drive. It consists of a power conversion sectionwhich uses six input diodes that rectify the three-phase power to a fixed pulsating DC voltage. A capacitor filters the pulsating ripple from the rectifier to a smooth DC bus-line voltage. A set of six transistors in theinverter section convert the DC bus-line voltage into a synthesized three-phase PWM voltage to power themotor. The vector drive differs from the PWM drive in using two independent feedback control loops, asshown in Figure 8-20(a). The loop that controls amplitude is referred to as the curren t f eed ba ck . It uses asensor such as a clamp-on ammeter or Hall-effect transducer to detect the magnitude of stator current at eachof the three stator lines. When an increase in load occurs, the vector drive will not allow the slip to increase.Therefore, the current reduces. The vector drive responds by increasing the amplitude of the current to thestator, as shown in Figure 8-20(b). The result i s that the torque increases to meet the demand of the load. Theloop that controls relative positioning is referred to as the spee d f eed ba ck . It uses a tachometer, resolver, or encoder to sense the velocity of the rotor. The vector drive's control technique of positioning

DC bus

Inverter ttf tit

Base driver AAA

PWMamplitudeadjustment

IE Field

orientation

adjustment

Encoder to speedconverter

IESpeed feedback

urnUJf

(b)

Beforeload

After load

Pulse width modulation generator

Velocity loop

Control circuitry

(a)

F IGURE 8-20 Block dia gr am of the vec tor dr ive control

(_j)

feedback

Motor map

calculation


27/36


28/36

CHAPTER 8 AC Dr ives 1 7

A 30 Advance

A 30Delay

A B^-^

-H

c~"- \A - c

180(120)

f, = ) ^

(a) (b)

FIGU RE -22 Aff ec ts of s tator flux s tep cha ng es : (a) Magn etic flux ad vanced 30 de gr ees; (b)Magn etic flux he ld s tation ar y for 30 de gr ees

The feedback signal from the speed loop is used by the DSP operation to cause the stator currents to make an advanced or delayed step change so that the stator and rotor fluxes return

back into quadrature. To cause a rotor to vary its speed, a proportional output frequency changefrom the inverter section is produced. The direction of the motor's rotation is reversed by

producing an inverter output signal with the opposite sequence of instantaneous voltage steps. Asimilar switching scheme is used to apply reverse torque when the motor needs to be stoppedquickly. There is, therefore, no special need for independent braking. To hold a motor in astationary position, the phase frequency is reduced to zero by keeping some of the inverter transistors turned on and the others turned off. The result is that a DC current flows through t hemotor coils. Figure 8-23 illustrates the concept of how the rotor can be held in a fixed position. Anaxe head made of a conducting metal is suspended within the flux lines of an electromagnet. Anymovement results in voltage being induced into the conductor. The current that is produceddevelops a magnetic field, which is opposite to the electromagnet, thus exerting a force to resistthe movement. If the current supplied to the electromagnet is removed, the axe will move morefreely. Therefore, the rotor, which is a conductor, will also oppose movement if it is placed withinthe fixed field of the stator.

8-6 Motor s Dr ive n by AC Dr ives Standard induction motors are designed to be powered by AC supply mains that provide afixed-sinusoidal voltage and fixed frequency. These motors are exposed to voltage spikes from acollapsing magnetic field only when the motor is turned off.

When an induction motor is powered by an AC drive, high frequency pulses are applied tothe stator windings to create simulated sine waves. Each time the rising or falling edge of arectangular pulse occurs, the stator coil induces a voltage spike, as shown in Figure 8-24. Eachspike can range from 900 to 1400 volts. These pulses stress the insulation in the firstfew turns of each winding. The coils beyond the first few turns are not affected because of the inductance they

produce and the capacitance that develops from adjacent windings. Eventually, the voltage spikes

can punch through the insulation.

120(150) 60

30

120(90)

90(60)

FIGU RE

-23 Con duc tor moveme ntr esis ted by induc tion


29/36

188 SECTION 4 Var iab le S peed Dr ives

A

[F IGURE 8-24 S pike volt a g e wavefor m

The insulation of a standard induction motor can be inadequate when it is powered by a drive.As an alternative, specially designed induction motors called inver t er d ut y , or inver t er ra t ed motors have been developed. A thicker layer of insulation is added between adjacent windings.This insulation, referred to as phase paper , int erp hase insula t ion , or phase barrier , is made of cambric, Nomex, Milar, or other insulating film. Slot paper is normally used to separate adjacentcoils within the stator slots.

To suppress the amplitude of the spikes, inductors are often placed at the inverter outputterminal of the drive. For severe spike conditions, L-C filter modules can be added to the driveoutput.

8-7 Control P a ne l Inpu ts a nd Dr ive F unction s AC drives have an operator station called a con t rol panel which is typically located on the front of the drive enclosure. It has adjustable inputs such as knobs and keypads that are used to set up,operate, and control the drive. It also has an.alphanumeric display. The hardware and display can

be used together for programming desired operating parameters during setup. The display also

provides information about operating conditions while the drive runs the motor. This informationincludes showing the RPM of the motor, that the drive is accelerating, or which direction it isturning. LEDs are also used to provide some of this information.

Technological advancements are constantly being made in the design of electronic motor drives. The most advanced drives use microprocessor and EEPROM memory chips. These "smartdrives" are programmable. If proper preset adjustments are made, they are capable of optimizingtheir operation under various load conditions.

Figure 8-25 shows the control panel for the Model 1305 Allen-Bradley AC drive. The panelis referred to as the H uman I nt er f ace M od ule (H I M). All parameters are entered by simply

pressing its keys. The data are shown on the display and stored in its memory. Fault conditions aredisplayed if a malfunction develops. The drive also has terminals for making hardwareconnections to external devices, such as potentiometers to provide the standard 0 to 10 VDC or 4to 20 mA DC analog signals from remote locations. These terminals also provide interfacingconnections to other equipment, such as programmable controllers or computer I/O cards that

process digital signals.There are two sections of the HIM panel, the d ispla y panel , and the con t rol panel . The

different types of input options on each section of the panel and the types of drive functions theycan adjust for are listed as follows:

Inpu t O p tion s

Disp lay P a ne l

Escape When pressed, the ESCape key will cause the programming system to go back one level inthe menu structure.


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CH AP T E 8 AC rives 189

A t s pee d + 10 .00 Hz

HIM display pane l

WuE ;

fl P E HIM

control panel

FI E 8- 2 5 odel 1 305 Allen-Br adley A C drive (C our tesy of Allen-Br adley, a oc well Aut omat ion bus iness

Select Press ing the SELec t key a lterna tely causes the top or bo ttom line of the d isplay to become

active. A f lash ing f irst charac ter ind icates wh ich line is active.

Up/Down Arrows These keys are used to incremen t and decremen t a va lue or to scro ll throughdifferen t groups or parame ters.

En ter When pressed, a group or parame ter w ill be se lected or a parame ter va lue w ill be en teredinto memory. Af ter a parame ter has been en tered into memory, the top line of the d isplay w ill automa tically become ac tive, a llowing ano ther parame ter (or group) to be chosen.

Examples of Inf or mat ion P rovided by the H I isplay

[ [ [ [

Accelerating+10.00 Hz

At Speed+60.00 Hz

Decelerating+30.00 Hz

Stopped+0.00 Hz

] [ ] [ ] [ ]

[

Ch oose ModeProgram

Ch oose ModeDISPLAY

Overvolt FaultF 5

Out put Curren t 0.00 Amps

1 ] ] ]


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C on tro l Pa nel

Star t The s tar t key w ill initiate the dr ive opera tion.

Stop When pressed, a s top sequence w ill be initiated caus ing the mo tor to "Coas t," "R amp, " "Brake, " or perform an "S-Curve " opera tion to stop. The type of s top opera tion depends on wh ich

parame ter se tting is programmed into the dr ive.

Jog When pressed, the dr ive w ill star t and jog to the frequency se t by the "Jog Frequency " parame ter.

R everse Th is key con trols the d irection of the mo tor ro tation. When it is pressed wh ile the mo tor is runn ing in one d irection, the dr ive w ill dece lera te un til it stops, change d irection, and thenaccelerate un til it reaches the se t speed. Th is sequence is called ant i- l ugg in g .


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19 0 SE CT I


Withou t using a dr ive, this func tion is ach ieved by sw itching any two of the three s tator leads of the induc tion mo tor, as shown in F igure 8-26. A dr ive does no t switch two leads of the mo tor tochange its direction. Ins tead, the con trol circuitry causes the inver ter's so lid-state sw itches toreverse the phase sequence.

U VW

U VW

FI E 8- 2 6 T he m otor r ever ses d ir ect ion whe n thephase seque nce is cha ng ed

T

Direction LEDs (Ind icators) Dur ing norma l opera tion, the appropr iate LED w ill illuminatecontinuous ly to ind icate commanded d irection of mo tor ro tation. When the reverse command isinitiated, the f lash ing LED w ill indicate motor ro tation wh ile dece lera ting. The oppos ite LED w ill

be illuminated con tinuous ly, ind icating, a change in commanded d irection.

Up/Down Arrows Press ing e ither of these keys w ill increase or decrease the ou t put frequency of the dr ive. A v isua l indication of the commanded frequency w ill be d isplayed on the SpeedInd icator LEDs.

Speed Ind icator LEDS Th is d isplay uses LEDs to visua lly ind icate, in steps, the commandedfrequency.

Drive Fu nctions Acce leration Th is con trol is used for s tar ting a load gen tly. Th is func tion is also known asra m in g or so ft start . The acce leration ra te is de termined by an ad justment setting that con trolsthe ra te at wh ich the f ir ing pu lses app lied to the inver ter sw itching trans istors increase. Thecontrol setting for this func tion ad justs for the time dura tion of the acce lera tion per iod. Theminimum amoun t of acce leration time is less than one second. The max imum time is as h igh assevera l minu tes. Minimum se ttings are usua lly for lighter loads and max imum se ttings are for heavy loads.

Withou t the ramp ing fea ture, a mo tor can demand an inrush of curren t that can reach600-70Opercen t of the ra ted curren t value when s tar ting heavy loads. By acce lera ting more s lowly,an excess curren t surge is preven ted, wh ich pro tects the mo tor and inver ter from be ing damaged.Var iable speed dr ives typically limit star ting curren t and torque to approx i-. ma tely 150 percen t of full load. R amping is also des irable in some produc tion app lications, such as preven ting bo ttles ona conveyor be lt from fa lling when its dr ive mo tor s tar ts.


33/36


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19 2 SE CT I

4 Va riable Speed Drives

8-8 I nve r te r Self-P rotect ion Fu nction

Most dr ives have the capab ility to de tect when a fau lt deve lops. A fau lt cond ition occurs whenthere is a defec t in the dr ive c ircuitry, when there is a prob lem w ith the mo tor, when an ex tremeload cond ition ar ises, or when the power supp ly ou t put deviates from its requ ired parame ters. To

pro tect itself, the dr ive w ill stop runn ing and a llow the mo tor to coas t to a -s top. Th is fea ture isreferred to as a fau l t tr i func tion. The fo llowing types of fau lt tr i p func tions pro tect the dr ive:

Ove r cu rr e nt P ro tect ion

This func tion shu ts off the sem iconduc tors in the inver ter if the ra ted mo tor curren t ismomen tar ily exceeded. The fo llowing cond itions cause an overcurren t situation:

Iner tia of the load is excess ively large. A shor t circu it deve lops in the ou t put leads or mo tor w ind ings. A componen t inside the inver ter sec tion has shor t -circu ited.

Ove r load P ro tect ion This func tion shu ts off the inver ter if curren ts in excess of 150 percen t are drawn.

Ove rvo lta ge P ro tect ion

This func tion shu ts off the inver ter if the vo ltage a t its ou t put terminals e levates to a des truc tivelevel. Th is cond ition can deve lop if an overhau ling load occurs or the load is dece lera ted tooquick ly. In e ither s ituation, regenera tion occurs and energy is fed back into the dr ive. The D C

busline vo ltage is mon itored to de termine if this cond ition deve lops.

Un de rvo lta ge P rotect ion

When the incom ing line vo ltage fa lls below a cer tain level 195 vo lts for a 220-vo lt three-phaseinver ter, as an examp le the dr ive stops to preven t a misopera tion cond ition.

Ove r heat ing P ro tect ion

This func tion stops the inver ter opera tion, when the tempera ture of the hea tsink r ises, and reducesthe coo ling effec t to the sw itching trans istors. The fo llowing cond itions can cause overhea ting:

The amb ient tempera ture is too h igh. The hea tsink is dir ty.

The dr ive's coo ling fan has s topped. The ven tilation s lots are obs tructed.

9 otor Br a ing One me thod of qu ick ly stopp ing a mo tor and the load to which it is mechan ically coup led is to usethe dece leration func tion of the dr ive. However, if the iner tia of the load is too large, it will overcome the magne tic f ield streng th of the stator co ils. The resu lt is that it will rotate -fas ter thanthe synchronous speed of the ro tating s tator f ield. Th is action is referred to as over hau lin g .


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CH AP T E 8 AC Drives 193

o ----- 1 Z ~i ' z i

i

z > z

"ir "i zit

] M

O ------ ik " i HI Z

i z

in r i

i n i i: i

i n 12

r _

When the synchronous speed drops be low the ac tual speed of the coas ting load, the mo tor becomes a three-phase genera tor and feeds the vo ltages back into the dr ive. These vo ltagesforward b ias the d iodes p laced across the trans istors of the inver ter sec tion. The curren t that resu lts is fed back into the intermed iate sec tion of the dr ive. The intermed iate circuit voltage can

r ise un til the VFD tr i ps ou t to pro tect its interna l circu itry. As a resu lt, the load w ill slowly coas t to a s top. To prov ide a brak ing ac tion that canno t be ach ieved by the dece leration func tion of thedr ive due to overhau ling, two a lterna tive me thods can be used.

One me thod is to connec t a dynam ic brak ing modu le across the D C bus line. When thevoltage genera ted by the mo tor reaches a se t level, a brak ing res istor is sw itched across the D C

bus, as shown in Figure 8-27(a). The vo ltage sens ing and the sw itching ac tion is done by asolid-s tate circuit. The torque a t the mo tor reverses as its energy is transferred to the res is tor anddissi pated as hea t. Heavy loads can be s lowed down very rap idly using this method. Th is methodis called dynamic bra kin g .

Ano ther way in wh ich the energy can be transferred from the mo tor is to send curren t back tothe AC line. Th is process is ach ieved by connec ting a rec tif ier be tween the DC bus and the power lines, as shown in F igure 8-27(b). Th is three-phase rec tif ier ne twork is p laced in para llel to therectif ier in the conver ter sec tion, and in the oppos ite direction. Th is type of connec tion is referredto as a nt i ara ll el. The d iagram shows that SCR s are used. When the D C bus vo ltage r ises to alevel grea ter than the po tential at the power lines, the S CR s are forward b iased and a f ir ing pu lseturns them on. The power transfer from the mo tor to the supp ly lines causes a good brak ingaction. Th is me thod is ca lled r e g ene rat ive bra kin g .

P roblems

1. T/F One common me thod of s lowing the speed of an A C motor is to reduce the app lied vo ltage.

2. Wh ich of the fo llowing cond itions may cause an A C motor that is con trolled by an A C dr ive to overhea t? ___ a. The R PM are lower than the ra ted speed of the mo tor.

b. A non-sinuso idal six-step ou t put is app lied to themotor.

c. Both a and b.

d. None of the above.3. Determine the R PM of an e ight -pole AC motor when 60 Hz

is app lied.4. AC dr ives change the speed of an A C induc tion mo tor by

wh ich of the fo llowing me thods : __ a. Vary ing the bus vo ltage.

b. Varying the arma ture curren t.c. Vary ing the app lied frequency.d. Chang ing the number of po les.

5. AC dr ives are a lso common ly referred to as _________ .6. List the two ma jor differences be tween the VVI dr ive and

the PW M dr ive.

10.

11.

Match the following names of the VVI sec tion with the appropr iate circuit func tions that they perform : a. Conver ter c. Inver ter

b. Intermed iate Circuit ___ R ectif ies AC into pulsating DC.

__ Filters pu lsating DC signals into a pure D C voltage. ___ Conver ts a DC voltage into a simulating AC

waveform. How many s tep vo ltage levels are

produced by the inver ter section of a VVI A C dr ive? ___ How many times does each transistor in the inver ter sec tionof a VVI dr ive turn on and off dur ing the produc tion of onecyc le? __ a. 1 c. 3

b. 2 d. 6List two types of con trol circuit devices that feed sw itchingsignals to the transistors in the inver ter sec tion of a VVI A C dr ive.List two circuits that maintain a cons tant V/Hz ratio asfrequency is var ied in a VVI dr ive.

(a)

(b)

FI URE 8- 2 7 otor br a ing: (a A r es ist or usedf or dynam ic b r a ing; (b An antipa r allel r ect if ie r network used f or r egene r at ive b r aking


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Chapter Ac Drive

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