Journal of Scienti fic & Industri a l Research

Vol. 59, December 2000, pp 1002- 10 II

Development of Microprocessor Based Industrial Starter and Slip Regulator of an Induction Motor

A K Mukhopadhyay Department o f Applied Phys ics, Uni vers ity of Cal cult a,

92. A PC Road. Ca lcult a 700009, Indi a

and

Arabinda Das' Elec tri cal Engineering Department , Jadavpur Univers ity, Ca lculta 700 032, Indi a

e- mail: dr araiJind:l@ hotmail .l'olll

Received: 15 May 2000; accepted : 7 Septe mber 2000

In thi s inves ti gat ion, an altempt has been made to deve lop a smart and intelligent di g ital contro ller to regulate the s lip of an inductioll

motor at a desired speed. The overall cont ro l and regulati on have been provided by contro lling the ave rage value of the applied vo ltage.

Besides microprocessor, the hardware consists of three pairs of anti-pa rallel thyri stors connected to three-phase supply lines. An 8085A C PU based microprocessor has been used for sequentia l, symmet ri ca l and accurate firing and fo r trigger pulse generati on at proper instant. To

overcome the loss of synchron ism. to e liminate hardware portion appreciab ly ,lIld for generation of a set of s ix trigger pu lses, inte lli gence of microprocessor has been utili sed to ca lculate the effective tri gge r angle and deliver trigger pul se in suc h a way that a ft e r del ivering a set of six

tr igger pluses, it find s the part icul ar refe rence ph ase wh ich has been used lu r synchroni sati on of contml circuit with power c ircuit. For smooth starting , the so ftware has been so developed that the same contrull e r provides reduced voltage starting. The so ft ware-based starter, referred to

as industri a l starter, eli minates the convention al starte r. A set o f osci Il ograms g iving the speed response of 1110to r has been presented, based on which the deve loped controller can he recommended ove r conventional starte r.

Introduction Though it is regarded that , the induction motor is a

constant speed dri ve, its slip va ries from no- load to full ­load. For example a 3-phase, 4 15 V, 5 A (line), 4 po lar machine suffers fro m the speed fluctuation from 1475 rpm to 1380 rpm when operated from no- load to full­load. Hence, in a wire manufacturing indust ry and so me other industries, where constant speed dri ve is requ ired , such a speed variati on of inducti on motor leads to prob­lems.

The speed of the induction motor may be controlled within a narrow range by controlling the supp ly vo ltage or supp ly frequency' ·· . To control the speed of an induc­tion motor, inverter may be used. Literature survey shows that, for controlling speed by app li ed vo ltage control, a number of modern speed controllers have already been recommended, where electronic or solid-sta te con trol­lers are the main strea ms to fabricate modern contro l­lers. However, the rei iabi I ity of these contro llers is poor and suffers from compl ex circuit. The present trend is to use digital controllers fo r the purpose of speed control of industri al dri ves.

':'Corresponciing au thor

For smooth starting, by li miting starting cun'ent within recommended range, and to adjust acceleration time (that is starting time), a number of electromagnetic starters are commonly used in industrial practice. Electronic starter came into bei ng in the later part of the present century. Use of electromagnetic or electronic starter re­quires addition al space fo r the hardware and additional cost of insta llation , and further needs periodical mainte­nance and repair.

To reduce the conventi onal hardware and electronic circuitry, and to enhance reli ability and flex ibility in COI1-

tro l, use of mi croprocessor along with interfac ing cir­cuitry in electrica l engineering is the present state-or-art in industrial setups. In the present investigati on, an at­tempt has been made to develop a digital controller along with a software package for starting and slip control of an induction motor utilising an 8085A CPU based mi­croprocessor. Due attention has been given to develop intelli gent and powerful software to min imise (or replace part of) conventional hardware, and to elicit qui cker re­sponse in operati on and to furth er enhance control accu­racy and re liabi lity.

MUKHOPADHYAY & DAS : MICROPROCESSO R BASED! DUSTRIAL STARTER lOIn

J-nductiO'YL rrwtor

/---------.,

Fined Fro?"fi. th'Y'ee phase 8upply bUB contro W.ng +---l~ r-\)'_ ---------- 1 \" ___ ) __ .. __ ____ ___ _ -.. -.. -.. -_ FE

elenl.e-nt "----....... --.........----.;>./ '----r

L. ___ ~

Controlle'lA (Mic1A oprocessor)

DAS

Figure 1- Single linc schcmati c hlock diagram. SR => Signal receiving cin:uitry. FE => Feedback element. TC => Triggering circuit ry, DAS => Data acquisi ti on system

Modeling and Control Strategy Figure I represents the schematic block diagram of

theoreti ca l model of the microprocessor-based developed

scheme fo r starting and regulating the slip of an induc­

ti on motor. In the present mode l, a mi croprocessor has

been employed as the workhouse for starting and con­

trol of s li p o f the inducti on moto r through utili sati on of

software. A separate ly exc ited dc generator has been coupled to the moto r to si mul ate the load va riati on.

The s ignal receiving c ircuitry (SR) receives voltage

signal fro m power line and converts it into microproces­

sor-compatible vo ltage signal. The microprocessor, upon

receipt o f the sig na l, synchroni ses itself w ith power line,

and de li vers tri gger pul ses at correct in stants th rough triggering c ircuitry (TC) to trigger the controlling e le­

ments. The feedback e lement (FE) prov ides vo ltage signal

corresponding to the in stantaneous speed of a motor. The mi croprocessor receives instantaneous dy nami c s ignal

th rough the data acquisit ion system WAS). The control strategy has been so deve loped that , on

receipt of the signal from the s igna l recei vel'. micropro­cessor synchroni ses itse lf with power line and generates a set of s ix tri gger pul ses fo r three sets of anti -paralle l

thyri stors. Tn thi s way, the thyri stors conduct at a par­ticular trigger ang le and the cont ro lled vo ltage is im­

pressed on the stato r of the inducti on moto r. For smooth starting, the mi croprocessor ini ti a ll y generates tri gger pulses at a max imum possib le tri gger ang le and then

continues the process for a numbe r of cyc les to stab ili se the motor. Then, the microprocessor success ive ly de­

c reases the trigger angle, and fo ll ow ing the same cyclic

operation gradua ll y inc reases the impressed vo ltage and

hence the speed. At every in stant , the microprocessor

rece ives feedback on the in stantaneous speed s ig na l

through DAS and compares it w ith the prev iously stored set va lue. When the in stantaneous speed matches the set speed , the microprocessor fi xes the tri aO'er an(yle and

00 0

continues with the same speed cyc lic operati on.

The slip may fluctuate with flu ctu ati on of load, sup­

ply vo ltage and / or the suppl y frequency. S ince the speed is proportional to the suppl y vo ltage, it is poss ible to

recover the speed by adjust ing the suppl y vo ltage corre­

sponding to the speed fluc tuati on. On the basis of infor­mation throu!!h FE based on data accessed throu ah DAS

~ 0 '

the microprocessor computes required tri gger ang le in steps and de li vers the tri !!!!er pul ses throuah tri aaerin a

~~ 0 00 0

c ircuitry.

Hardware Organisation ami System Functioning Based on the proposed scheme, a prototype mode l

has been developed using a microprocessor and the in­te rfac ing c ircuitry. The simplified block diagram has heen

presented in F igure 2 . For easy and quick in stall ation and maintenance, the developed hardware porti on has been secti onali sed into four part s, viz .. i) Power c ircuit modu le, ii ) Signa l rece iving c ircuitry, iii) Cont ro l mod­ule, and iv) Feedback module.

1004 J SCI IN D RES VOL. 59 DECEMB ER 2000

F1"01'n.. 81.l..pply bt.M'

B

y ----------.-----~ R ------.------~

Ant-i.-pa1"u.ll.el. Thyri.stOTs

~:~--~~~roceS80r . {7 --

-~ I/O

K r,y board

J)isplay [ Memory

Figure 2 - Schematic block diagram of the proposed sc heille

FTorn S'l.l PP [y bus

Figure: :1- Power ci rcuit Illod ule

In power ci rcuit module (Figure 3), the three-ph ase

su pply is connected to the induc ti on motor through three sets of anti-paralle l thyri sto rs, with trigger pul ses for thy­risto rs being generated and contro ll ed by the micropro­

cesso r. In s ig nal receiving c ircui try (Figure 4), a potenti a l

transformer (PT) is connected between R- phase (refer­ence phase) and neutra l fo r rece iving vo ltage sig na l. T he PT sca les down the load bus vo ltage. T he s inusoida l vo lt-

age signa l is fed to an iso lated ZeD (ze ro cross ing de­tector), which produces a train of rec tangu lar pul ses dur­in g each pos iti ve half-cyc le from ris ing to fa lling edge of ae wavefo rm ;.lIld su ppli es the same to the mic ropro­cessor.

In contro l c ircui t module , the m icroprocessor. used as speed compensator and trigger ang le contro ller, senses

through an I/O port the zero cross ing instant at the ri s ing edge (i.e ., sta rting in sta nt of posi ti ve ha lf-cycle) of the

MUKHOPADHYAY & DAS : MICROPROCESSOR BASED INDUSTRIAL STARTER 1005

Frorn supply B

bus

y To rnotor R N

>

zeD

To rnicroprooessoT ~

1 Figure 4 - Signal receiving circuitry

100 t •

From. MicroprOC€SBO'l"

2

3

6

5 MCT-2E

10 K

+ 5 V

To }--..£VVV'----I'")I-.--.---'> Gate

To Cathode

Figure 5 - Tri ggering circuitry.

reference vo ltage waveform obta ined from ZCD, and emits a set of s ix tri gger pul ses th rough the 1/0 port se­quentiall y to tri gger the six th yri stors at a parti cular tri g­ger angle. In a c losed-loop system, the mic roprocessor, on receiving the feedback signa l through the IIO port, dec ides the fur ther course of ac tion.

As the microprocesso r output is utili sed to control app li ed vo ltage of inducti on motor co nn ected to the power line, it is necessary to iso late power circuit from the logic c ircui t. Therefore, an opto-i solator circu it based on MCT-2E with an ampl ifier c ircuitry was used , which

e lectricall y isolates microprocessor and switches from the transistor. The detai Is of the c i rcu it diagram for tri g­gering c ircuitry are shown in Figure 5. When the output is high, it causes current to fl ow through LED of MCT-2E, energising the light sensing di ode and the vo ltage connected ex terna lly with pin-S of MCT-2E is impressed on the 10K res istor through the transistor SL- i 00 and the same vo ltage is impressed on the gate of the thyri stor and the th yri stor conducts.

In feedback c ircuitry mod ul e . th e co nvent ional tachogenerator (TG) connected to the motor shaft is used

100(, J SCI IND RES VOL. 59 DECEMBER 2000

Initialise Stack Pointer, Port A as input port and Ports Band

C as out ut ports

Set starting delay number 8(s), upper limit o(ul), lower limit 8(11) and set value corresponding to

set speed E(set)

Set subprogram Sync for synchonistion and detect the rising edge instant of the R­

phase

Call subprogram Cycle to calculate the effective trigger angle (op) and to emit a set of six trigger pulses at 60 degree

interval with N-cyctes

Input instantaneous tacho signal E(in) through Ch.1 of ADC and compare E(inl with

E(set)

Is Yes E(in) > E(set)

?

Input instantaneous tacho signal E(in) through Ch.1 of ADC and compare E(in) with

E(set)

Yes Is E(in) = E(sel)

?

Yes Is

r------..C'.:-~~~~~~~~.::-+---< 5(s) > 8(ul) Call subprogram Cycle to ?

calculate the effective trigger angle atop) and to emit a set of six trigger pulses at 60 degree

intervat with N-cycles

A

Figure 6 -- FlclI' chart i"or industrial starting and slip regu lation.

as a feedback e lement which gene rates the in stantaneous

analog vo ltage signal , proportional to the speed o f the motor. The AID conve rte r based on IC-8009 was used as DAS, which conve rts the anal og vo ltage s igna l into

dig ita l fo rm for supply to the llIi croprocesso r.

System Software The software has bee n developed in suc h a way th at ,

the microprocessor initi a li s ing the stack poin te r and as-

s igning port PA as input port and ports PB and PC as

output ports, preserves starting de lay number o(s) [An­

nex ure - 1], upper limit o(ul ), lower li m it 0( 11 ) of de lay

number. delay number correspondin g to 120 deg ree

o( 120), de lay number corresponding to 60 degree 0(60)

and the se t va lu e correspond in g to the se t speed in

memo ry loca ti on:;: M to (M+6), and ca ll s subroutine 'Sync ' for synch roni sati on o r ac s ig nal w ith so ftware and to detcct the starting inst ant of R-phase vo ltage. Then,

MUKHOPADHYAY & DAS : MICROPROCESSO R BASED INDUSTRIAL STA RTER IO(}7

Table I - T riggering sequence and conduction period (in degree) of the th yri stors

Triggeri ng Phase

angle( "",degrees)

R B

0 <"" < 60 Y R B Y

Firing

sequence

T H I

T H6

T I-n T H2 T H5 T H.+

T H4

Firing

instant,

degrees

0< + 60

"" + 120 "" + I SO

"" + 240 "" + :100

"" - 60

Conduct ion peri od

wi th respec t to R-phase, degrees

to 180

0< + 60 to 240

"" + 120 to 30G 0< + 180 to 360

"" + 240 to 60

oc + 300 to 120

"" - 60 to 120 y

R T HI "" - 60 + 60 "" - 60 + 60 to 180

60 < "" < 120 B T H6 "" - 60 + 120 "" - 60 + 120 to 240

Y

R B

T H3 oc - 60 + 180 "" - 60 + 180 to 300 T H2 "" - 60 + 240 "" - 60 + 240 to 360 T H5 "" - 60 + 300 "" - 60 + 300 10 60

T H5 "" - 120 "" - 120 10 60 B Y T H4 "" - 120 + 60 "" - 120 + 60 10 120

120 < "" < 360 R T H I 0< - 120 + 120 "" - 120 + 120 10 IXO

B Y

R

TH 6 "" - 120 + I SO "" - 120 + 1110 to 240 T H3 "" - 120 + 240 "" - 120 + 240 to :WO T H2 0< - 120 + 300 "" - 120 + 300 10 360

it starts the operation of dig ital starte r using the subrou­

tine 'Cycle'. The di g ita l sta rte r, be ing complete ly based

on so ft ware, replaces the conventional starte rs gener­a ll y used in indi vidu al dri ves . The fl ow cha rt s (Fi gures

6 and 7) fo r soft starte r have been so deve loped th at, the

microprocessor compares the sta rting de lay number 8(s)

w ith 8( 120). As 8 (s) > 8 ( 120), the microprocessor com­

putes operationa l de lay number 8 (op) [= 8(s) - 8( 120)],

ca lls subroutine 'Redge' to detect the zero c ross in g in­

stant of ri sing edge of the R-phase vo ltage, makes a de­

lay corresponding to the de lay number 8(op) and re leases

a set o f six tri gger pul ses at a time inte rva l of 60 degree

to tri gger the th yri stors in the sequence THS, TH4, TH I ,

TH 6, TH3 and TH2, respvcti ve ly, during both positi ve and negati ve hal f cycles, and continues the cyc li c opera­tion fo r N-cycles to s tab ili se the speed of the mac hine.

When 8(s) < 8( 120), the mi croprocessor compares

8(s) w ith 8 (60) . As 8 (s) > 8 (60), the m icroprocessor

computes 8 (op) [= 8 (s) - 8 (60)] to re lease the trigger

pul ses afte r a ti me de lay correspond ing to 8(op) from

the in stant of detecting the ri s in g edge at the ze ro cross­

ing instant of R-phase vo ltage. In thi s case, the trigger

pul ses are re leased to tri gger the thyri sto rs in the sequence T H4, T H I , TH 6, TH 3, TH 2 and TH S, respec ti ve ly. each at a timc in terval of 60 degree . The procedure is fo l­

lowed fo r N-cyc lic operat ion.

If 8 (s) < 8 (60), then the microprocessor de tects the

ri s ing edge of the zero c ross ing of the R-phase, ca ll s the

de lay subroutine corresponding to 8 (s), and generates

and outputs the trigger pul ses at an inte rva l o f 60 degree

to trigger the thyri stors in the sequence TH I , TH6, T \-I 3. TH2, THS and TH4 for N-cycles to s tabili se speed of the moto r. The firin g sequence for d iffe rent range of trig­ger angle has been given in Table I .

Then, it rece ives instantaneous signal from T G, prop­

erl y di giti sed th rough AID converte r, and compares it

w ith the set va lue of the speed stored prev iously in some

memory locati on. If the instantaneous speed is less than

the set va lue, microprocessor reduces the de lay nu mber

8 (s) by one and call s the subroutine aga in to en hance

thc in stantaneous speed . In thi s way, the machine att ;lins the set speed through d ig ita l sta rter.

With the change o f suppl y vo lt age or load , the speed o f the machine may va ry. If the instantaneous speed ex­

ceeds the set va lue, the microprocessor ca ll s the subrou­

tine 'Cyc le' to ca lculate the effec ti ve tri gger ang le 8(op)

and outputs a set o f six trigger pul ses at an inte rva l of 60 deg ree and repeats the cyclic operc: ti on for N-cycle.

Aga in , a ft er receiving the instantaneolls tacho signal through A DC , microprocessor compares it w ith the set speed and perfo rms the s im il ar cyc li c operati on o r tri g-

1008 J SCI IND RES VOL. 59 DECEMB ER 2000

Compare o(s) with 0(120)

Yes

Compare o{s) with 0(120)

Yes

No

Call subprogram to detect rising edge of R-phase, call

delay subprogram corresponding to o(op) and

emit a set of six trigger pulses with 60 degree interval to

trigger the thyristors in the sequence TH1, TH6, TH3, TH2, TH5 and TH4 respectively of N­

cyclic operation

Compute o(op) = o(s) - 0(60)

Call subprogram to detect rising edge of R-phase, call

delay subprogram corresponding to o(op) and

emit a set of six trigger pulses with 60 degree interval to

trigger the thyristors in the sequence TH4, TH1 , TH6, TH3, TH2 and TH5 respectively of N­

cyclic operation

Compute 8(op) = o(s) - 8(120)

Call SUbprogram to detect rising edge of R-phase, call

delay subprogram corresponding to atop) and

emit a set of six trigger pulses with 60 degree interval to

trigger the thyristors in the sequence TH5, TH4, TH1 , TH6, TH3 and TH2 respectively of N­

cyclic operation

Figure 7 - Fl ow chari i"or suhprogrillll eyc' le.

gering if the instantaneous speed becomes equal to the set speed, But, if the in stantaneous speed is different rrom the se t speed, the microprocessor increases or dec reases O(s) according to whether E( in) is greater th an or less lhan E(set), respecti vely, co mpares the new O(s) with the upper and lower limiting value O( ul ) and 0(11 ), as the

case Illay be, co llects either O(ul) or 0( 11) according to iile ~iluat i o n. and preserves it 1'01' the CU lliilllldlioll orlhc

tr iggering phenomena and hence the rotati on of the mo­tor.

Th rough the procedure described above the micro­processor, maintaining the Illotor speed at set value UIl­

der the supply or load f lu ctuati on. It also co ntrols the triggering angle at the uppe r alld lowe r li miting \'aluc under the rigoloLls fill ctuat ion or su pp l y \' 0 1 tage or lowl.

MUKHOPADHYAY & DAS : MICROPROCESSOR BASED INDUSTRIAL STARTER

... : .... :: ... : .... :.. . . .... : r .. .. . . . ... . .. . ... . .... . ..

... . I: · ···3r~·

Starting time =5 S

Direct starting

~.

.. j ..

x -axis: 1 V / Div. Y-axis: 5 S / Div .

.t=.: ... . . . .. . . ; ..... .

. .

StiUting time = 23 S

Digital starting

. Figure ~ - Speed response osc ill ogram du,ing starting ..

s .

B

A A:

·s

Trigger angle = 138° Delay number = 8A H

f

s ·

-"-I---'-' ~-......,

B

Trigger angle = 90° Delay number = SA H

s

A

1

. A . .

:s B

Trigger angle = 96° De!~y nlimber = 60 H

Trigger angle = 74° Delay number ="4A H

Figllre l) - Stawr Cli lTenl wavd"onn at different tri gger angle.

.. A .

"

1009

1010 J SCI IND RES VOL. 59 DECEMB ER 2000

s R

: '-1 . : ~ . . . ~ .

Speed recovery time = 1.4 S

No load to 50 % load

S R

sd~~~ry~:. · .. tune 1.4 S . . .

, . ~

No load to 100 % load

Speed recovery time = 2.0 S

No load to 125 % load

. .. . .. . . . . : . . .... . . : . .. . .. .

Speed recovery time = 1.4 S

SO % load to no load

Speed recovery time = 1.4 S

1 00 % load to no load

8 R

. .. 1: .. L .~ . ~

Speed recovery time = 2.2 S

125 % load to no load x - axis: 500 mV / Div. Y-axis: 1 S / Div.

Fi gure 10 - Speed response oscillograms during load variat ion.

As the applied vo ltage is not free from harmon­ics, the controller could not be applied for heavy duty operation of a high powered motor.

It may be menti oned that the upper limit of tri gger angle is necessary to overcome the inertia , and the lower limit is necessary to avoid misfiring occurring because of local short c ircuit due to tran sients during two consecuti ve tri ggerin g

in highl y inductive c ircuit.

Limitation The deve loped schemc is unique and advantageo ll s in

man y res pec ts with d limitation of the zonc of tri ggerin g operati on due to high inductance of stator co i Is.

Experiment On the basis of the proposed scheme, a prototype

model was developed and implemen ted in the labo­ratory for start ing and to regulate the slip of a three phase squi rre l cage induct ion motor [A nnex ure 21·

MUKH OPADHYAY & DAS : MI CROPROCESSO R BASED 1 DUSTR1AL STA RTER 1011

The speed response curves fo r direc t starting and di g i­tal starting us ing so ftware-based starte r, re fe rred to as industri al starte r, a re presented in Figure 8. The sta rting

time as we ll as steps o f gradual speed inc rement ca n be

a ltered by alte ring only the number o f cyclic operati ons

(N) with every operating de lay number <S( s) in dig ital

startin g:. The oscill ogram of the stator current for different

de lay number (tri gger ang le) is presented in F igure 9, from whic h the upper limit of tri gger ang le has been de­te rmined. In these osc illograms, po ints ' A' and ' 8 ' rep­

resent the triggering instants at pos iti ve and negati ve half

cycles, respecti ve ly. In c losed-l oop contro l, the initial operatin g De lay

number is stored in a pa rti cul ar me mory loca ti on fo r the

motor running at a speed with 33 per cent s lip o f the motor, which is cons idered as the set speed . The osc ill o­

grams showing the speed response characte ri sti cs corre­

sponding to sudden change o f load was recorded and presented in Fi gure 10. In these speed response curves, the points'S ' and ' R ' refer to speed fluctuati on and speed

recovery instants, respecti ve ly.

Conclusion The present in vestigati on hig hlights the replacement

of conventi onal e lectromagne ti c starte r or e lectroni c c ir­

cuitry by microprocessor-based dig ital contro lle r with a

software package for starting and slip regulati on, the reby e liminating complicated hardware by s imple ones, pro­

viding sequential and accurate contro l, and enhancing re liability o f the sys tem.

Development of inte lligent software for calcul ating e ffec ti ve trigger angle and de li vering a set o f s ix trigger

pulses sequenti a lly without loss of synchroni sm may be

c laimed as one of the important deve lopment in the area o f three phase vo lt age co ntrol.

Development o f software-based starte r uti I is i ng the same contro lle r refe rred to as industri a l s tarter, e lim i­

nates hard ware-based e lectro magne ti c o r e lec tro n ic

starter, reduc ing additi onal cost of insta llati on. The start­

in g time may be adjusted as pe r requ irement to suit the machine acce le ratin g time.

References I Ramamoorty M, Thvri.l'lOrs {Il/{llheir opplic({/io ll .l' (A ffili ated

East-West Press Pri vate Limited, New Delhi ) 1977. 92. 2 Kuhota H, Matsuse K & Amono M , A ew In verter Fed Induc­

tion Motor Drive with Microprocessor-based Stale Obse rver for Est imat ing Torque, Proc IECON '84, 1111 CO/(/, Illd O ('CII<III.

COll lrollllSl rllII/ (Cat. No. 84CH 1991-9) (Tokyo, Japan ) (22-:26 October 1984), 2 pp 857-862.

3 Lewandowski M , Microprocessor-contro lled PWM modulator, Proc lASTED S\'IIlP, Idelll, Model Sillllllatioll (Paris. France) (22-24 June 1987), pp 268 - 27 1.

4 Addoweesh K E. Induction Motor Speed Cont ro l Using a Mi­croprocessor-based PWM Inverter, IEEE Trans Illd Eleclroll . 36 ( 1989) , 5 16.

Annexure 1 It is poss ible to compute de lay time correspond ing to

the tri ggerin g ang le in te rms of a group o f in structi on. a

pre-set number of times. The nu mber of repetiti on of a

bas ic time de lay has been refe rred to in thi s pape r as

"de lay number" and is denoted by 8.

Annexure 2 The inducti on motor under tes t had the fo ll ow in g

spec ificati ons on its name pl ate: 3-ph ase, Star connec ted , Squirre l Cage, 4 15V, SA (line)

1425 rev / min for 50 Hz supply.