A STUDY OF THE PERFORMANCE OF THE 1000 KW MOTOR GENERATOR SET SUPPLYING THE CANBERRA HOMOPOLAR GENERATOR FIELD

AcJ* "r-R S P H Y 3 .S

V

EP-RR 10

T. W. BRADY

First Published: June, 1966

Re-issued: April, 1967

Department of Engineering Physics

Research School of Physical Sciences

THE AUSTRALIAN NATIONAL UNIVERSITY

^ ’ ra, A.C.T., Australia.

HANCOCK

f T J 1 6 3 ̂. A8 7 E P - R R 1 0

TJ163.A 87 EP-RR10.

1924132

2

A . N . U . L I B R A R Y

This book was published by ANU Press between 1965–1991.

This republication is part of the digitisation project being carried out by Scholarly Information Services/Library and ANU Press.

This project aims to make past scholarly works published by The Australian National University available to

a global audience under its open-access policy.

^ üsU»Htn Nat/ona; ^ - ^

f Rs phys.si / ß R A R V

2 9 F E B 1988

A STUDY OF THE PERFORMANCE OF THE

1000KW MOTOR GENERATOR SET SUPPLYING

THE CANBERRA HOMOPOLAR GENERATOR FIELD

by

T. W. BRADY

First Published: June, 1966 Re-issued: April, 1967

Publication EP-RR 10

Department of Engineering Physics Research School of Physical Sciences

THE AUSTRALIAN NATIONAL UNIVERSITY

Canberra, A.C.T. Australia

SUMMARY

Safety studies a re described on the 1000 kW M otor G enerator se t used for exciting the field magnet of the Homopolar G enerator. The studies w ere made by te s ts on the set, and analogue sim ulation of its b e ­haviour. In som e cases, analogue studies w ere f ir s t made onthe re su lts checked by te s ts ; these gave confi­dence that the analogue sim ulation was in fact re liab le .

No attem pts w ere made to deal with "com m on­p lace fau lts" , such as a p a rtia l o r com plete short c ircu it on the set, because the p ro tective apparatus in the sw itch- g ear should handle them.

CONTENTSpage

1 . G eneral 1

2. D escrip tion 1

3. Study 1

3. 1 Backing Oil P re s su re 13.2 Magnet Field for Normal W orking Conditions 13.3 M ains F ailure to M G Set 13 .4 Loss of Feedback Followed by Mains Failu re 53. 5 R eversa l of Excitation with Mains Failure 53.6 Magnet Field fo r E lec trica l Dump Load 10

4. Future W ork 12

5. Conclusions 12

Appendix 1 14

I

1 . GENERAL

This paper d esc rib es studies on the 1000 kW Motor G enerator se t used for exciting the field m agnet of the Homopolar G enerator. The studies w ere made by te s ts on the set, and analogue sim ulation of its behaviour. In some cases analogue studies w ere f ir s t made and the re su lts checked by te s ts ; these gave confidence that the ana­logue sim ulation was in fact c a rr ie d out co rrec tly .

To avoid confusion in the subsequent text the te rm "G enera to r" will be used to d escribe the Homopolar machine p roper. The machine under study w ill be described as the "M G set" o r ju s t sim ply as the "se t" .

2. DESCRIPTION

A schem atic d iagram of the M G set is shown in Figure 1. The c ircu it used fo r analogue studies is given in Figure 2; this is in fact a sim plified version of Figure 1. Appendix 1 gives equations used in the analogue sim ulation, and norm al operating con­ditions of the set with p e rm issib le maximum values.

3. STUDY

3.1 Backing Oil P re s su re

R eferring to Figure 3, th is shows in terlocks in the pilot ex citer field existing at the tim e of w riting. This shows (among o ther things) that there can be no excitation of the pilot exciter, and hence the G enerator field, when th e re is no backing oil p re ssu re . For reasons which w ill be d iscussed la te r, it is desirab le that th is in te r ­lock should prevent the field being applied before the G enerator is s tarted , and that once the G enerator is running, fa ilu re of the backing oil should not prevent field being applied. This is necessa ry because of course m agnet field is necessary for shutdown of the G enerator in some c ircum stances. T herefore the oil p re s su re in terlocks a re shorted by contacts of re lays operated by the speeds of both top and bottom ro to rs (RL 54 and RL 55).

3. 2 Magnet Field for Normal W orking Conditions

R eference to Figure 3 shows also that an in terlock in the pilot exci­te r field is derived from the "Run" c irc u it b reak er of the M G set (see also Figure 1). This means that if the M G set is thrown off the supply the excitation w ill d isappear. Some discussion of th is in terlock w ill appear la te r in Section 3 .6 .

3. 3 Mains Failu re to M G Set

For this argum ent it is assum ed that the in terlocks of P ara . 3. 2 do not exist, and that the pilot ex c ite r rem ains on its supply when the se t is thrown off the mains.

Et

w,oo<y i »• v

n n <! n

I I KV MAINS

I_______—RLSR/l

l̂ l------------ |~l

A ut»., 7r»n«f*n*»c S M « r 1140 KVA ICftOH* 5«jn'Wr*n*w%

E x c ite r : 4 6 V. 145 A. 7 5 0 R P N\.

Figure J. 1000 kW Motor G enerator Schematic diagram M otor and G enerator Control.

S y n c h ro n o u s M o to r :1 5 0 0 H R, I - O p f * PW.5 0 c p v 1 1 ,0 0 0 V . 6 2 A 7 5 0 A R M .

NOTES- O C B 0.1 Circuit 6r«aW»cC.P • Cornroutbt iny Pole*.CPSTG' C o m p t n M ^ i ^ W in d in g O i f f ! D i f f e ren t i a l .C o m ' C u m u l a V w « .

T O R : Tim« Relqy.

MM- m .

C a b l e N u m b e r s .

2 4Cable C.-ffl

2 5(TKCable C.

2 4 0 V o l t A. C. S u p p ig S r# TRACING O M 17 2 2 . NOTE6 .

PILO

T m

ain

exci

ter

ma

in

gen

era

tor

m

ag

ne

tEX

CIT

ER.

3_____ ( O _____CL p -J

p̂ ü&Mö&ö_/n

Figu

re

2.

Sim

plifi

ed M

otor

Gen

erat

or D

iagr

am f

or A

nalo

gue

Stud

ies.

4

'iSOiX lOOWREMOTECONTROL. POTENTIOMETER.

BACKING 0\L PRESSURE ’'‘TOP.'

E>ACK\NG OU-PRESSURE "BOTTOM.

RE 5 4 T Speed felavj R L 5 5 J operating a*V

G enerator sp eed o*P 100 R .P M .

O.C.B. RUN

PILOT EXCITER.

LOCALCONTROL POTENTIOMETER.

3 5 0 jl lOOW.

Figure 3. In terlocks in pilo t ex citer f ie ld

STUDY 5

The re su lts of a te s t on the M G set a re shown in Figure 5. These confirm very closely analogue studies made in the past. It is seen that fo r an in itial field c u rre n t of 2000 am ps the speed fa lls quickly and the machine in fact re v e rse s its ro ta tion . This re v e rsa l is due to the tim e constant of the m ain field c ircu it of the set (V , i e in Figure 2); when the machine reaches f i r s t s tandstill some field s till ex ists.

These re su lts dem onstrate that in these c ircum stances a loss of m ains fa ilu re is safe. The decay of magnet cu rren t should be noted. This does not decay in a m anner very m arkedly different from the decay when the machine is ru n ­ning norm ally and all excitation of-the M G se t is rem oved (Figure 4).

3 .4 Loss of Feedback Followed by Mains Failure

This condition is rea lly an extension of 3. 3. It is supposed that winding ?rb" on the m ain ex c ite r (F igure 2) becom es open c ircu ited when the machine is norm ally excited. The f i r s t re su lt of this would be fo r the cu rren t to r is e consid ­erab ly in the m agnet winding as the effect of winding "b" is to m aintain negative feed­back. The cu rren t in the m agnet field would r is e to about 3000 am ps. It is assum ed that when th is happens the se t is thrown off the supply due to the overloading o r some o ther cause. The perform ance of the se t under these conditions is shown in F igure 6. This perform ance is derived from an analogue study, but it is expected to be accura te , since com parison of analogue sim ulation and actual te s t was so good in 3. 3.

It is obvious that even under these conditions perform ance of theM G set is safe.

3. 5 R eversa l of Excitation with Mains Failure

It has been custom ary in the p ast to fo rce the field of the m agnet to the re v e rse d irec tion by fully rev e rsin g the pilot ex c ite r field. There a re som e re a ­sons fo r wanting to do th is . F irs t, the G enerato r may have to be slowed quickly by regenera tive braking, and second a re la tive ly quick way to rem ove m agnet energy is to re v e rse excitation in th is m anner, and then cancel it as the m agnet cu rren t goes through zero.

Indeed the s im p lest way to d issipate m agnet energy is the way just described . When the pilo t ex c ite r is rev e rsed quickly, the voltage Vg a c ro ss the g en era to r re v e rse s while the cu rren t Iq m aintains the sam e d irection . (F igure 2).If a t o r about th is tim e the se t is thrown off the supply, then the magnet cu rren t will acce le ra te the ro to r. The re su lts derived by analogue m eans a re shown in F igure 7.

This result is in one respect a simplified one since windage effect was not included in the computation - this would tend to lessen the maximum speed shown in the figure. Nevertheless, this fault is obviously very dangerous and cannot be tolerated. Even a partial reversal of excitation causing a small increase in speed is not desirable.

STUDY 6

=tü2ETime - seconds

Figure 4. Recorded curves of magnet current on application and loss of excitation.

•sdrav -

Ulfa

ca

§o0 »CG

0 )

•H d ' H -

7

Fig

ure

5. L

oss

of m

ains

to

mot

or g

ener

ator

set

; re

cord

ed

deca

y cu

rves

.

•sdure - pxaid

H 'd 'H - paodS

wT3§o0 )CG

• pH

Eh

8

Figu

re

6. Lo

ss o

f mai

ns to

mot

or g

ener

ator

set

und

er f

orw

ard

exci

tatio

n; A

nalo

gue

study

.

’SduiB - p j o i j ;8 uSb]A[

*H ä H - psadS

COTD§O0)CQ

• i - l

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9

Figu

re 7

. Lo

ss o

f mai

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mot

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set

und

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ever

se e

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A

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.

STUDY 10

Exam ination of the re su lts shows a slight drop in speed at f irs t.This is because the voltage Vg does not re v e rse im m ediately due to the tim e con­stan t of the m ain field c irc u it (V i. as before).

It is now obvious that the main, if not the only, purpose of the in te r­locks described in Section 3. 2 is to prevent th is fault occurring with the consequent destruction of the M G se t. Faults described in Sections 3. 3 and 3. 4 have been shown to be safe.

F u rth e r re fe rence to these re su lts w ill be made in Section 3.6 .

3. 6 Magnet Field fo r E lec trica l Dump Load

We co n sid er here m eans for m aintaining field c u rren t so that the energy of the g en era to r can be e lec trica lly "dumped” in the case of em ergency. The desirab ility of such a method of dumping the energy is not d iscussed in th is paper.We shall assum e that it is in fact requ ired , and that the m eans ex ist to absorb thegen era to r energy even w ith p a rtia l magnet field.

T here a re four conditions to consider, these a re :

1. The M G set running norm ally with the m agnet field excited.2. The M G se t running norm ally with the m agnet field not ex­

cited.3. The M G set thrown off the supply but with the magnet field

excited a t th is tim e.4. The M G se t thrown off the supply with the magnet unexcited

a t th is tim e.

We shall consider these in tu rn in detail

1. This p resen ts no problem , all conditions a re norm al and the energy of the g en e ra to r could be dumped im m ediately.

2. In th is case m agnet field may be forced as quickly as p o ss i­ble and the energy of the g en era to r dumped when the field has reached a to lerab le value.

3. This re su lts in the perform ance given by Figure 5. The magnet field decays le isu re ly and th e re is am ple tim e available to take a pulse before the field has decayed to a sm all quantity.

4. H ere the magnet field m ust be forced as the M G set is ru n ­ning down. Obviously the ro tational energy of the M G se t is tra n s fe rre d to the m ag­net field. F igure 8 shows actual te s ts c a rrie d out on the set. It can be seen that the cu rren t r is e s to about a q u a rte r of full field strength . It is however fa irly fla t topped and p e rs is ts fo r a considerab le tim e. C onsiderable forcing is requ ired to obtain this

W *a H - paads

Fig

ure

8.

Los

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otor

gen

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et w

ith f

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d ex

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tion;

STUDY 12

field cu rren t a s can b e seen from the curve of ia in Figure 8, and in fact almost maxirrum excitation is applied to the pilot exciter. (Compare norm al value of ia in Appendix I . )

It hollows that if method 4 is to be used in p rac tice , then the in te r ­locks shown in F igure 3 cannot be used, since these would prevent excitation being applied to the M G s e t and hence the studies of 3. 3, 3. 4, 3. 5 can be held to apply. In terlocks a re req u ired which prevent the unsafe condition of pa ra . 3. 5 only; but these in terlocks a re not easy to apply as they m ust operate under som e tran s ien t con­ditions. An e a s ie r so lu tion might be to make it im possible to re v e rse the excitation on the pilot exciter quickly by a m echanical a rrangem ent on the control knob. It would be possible to make such a device while s till retain ing the ability to in c rease the field quickly in e ither d irec tio n from zero, a quality which is e ssen tia l if the dem ands of pa ra . 3. 6. 2 and 3. 6 .4 a re to be satisfied .

Such an application needs fu rth e r thought.

C onsider Figure 9, which shows the rundown of the M G se t with no excitation. The pu rpose of th is figure is fo r com parison with Figure 8, and to show that em ergency ap art, the magnet field may be forced som e tim e a fte r s ta r t of rundown because speed decays slowly.

4. FUTURE WORK

Some fu rth e r w ork may be done on the c h a ra c te r is tic s of the se t under the conditions of 3 .5 , but w ith p a rtia l instead of full re v e rsa l to determ ine what amount of field rev e rsa l, if any, is to le rab le . This w ork would obviously have to be done on a com puter at f ir s t and then carefu lly checked on the M G set.

It should be rea lised tha t even with the in terlocks described in Section 3. 2 (figure 3), a certa in am ount of danger ex ists with violent re v e rsa ls of excitation. This is because of the tim e constant of the m ain field c ircu it. In o ther w ords, given a re v ­e rsa l, there may be an in c rease of speed before the gen era to r voltage (Vg) is cancelled. This danger is elim inated at the tim e of w riting by ensuring that the forw ard cu rren t does not exceed say 500 am ps when field re v e rsa l is applied. This figure of c u rre n t is ra th e r a rb itra ry , but is considered safe; however, it ought to be verified by fu tu re study.

It is proposed in the future to insta ll autom atic control on the M G se t e ith e r to control field cu rren t o r gen era to r voltage (Vg) o r both. This paper is concerned with safety and not with stab ility as such, n evertheless the autom atic control system m ust be compatible with any safety fea tu res o r system s installed , and any c ircu it changes made in the interests of stability o r quick response m ust be consisten t w ith o ther desirab le qualities of the M G set controls.

5 . CONCLUSION

This paper deals with safety of the M G se t with p a rtic u la r re fe rence to its perform ance with the Homopolar G enerator Magnet. No attem pts have been made to deal with what might be called commonplace faults, such as a p a rtia l o r com plete sh o rt c ircu it on the set, because the p ro tec tive apparatus in the sw itchgear should take ca re of th is o r s im ila r conditions.

14

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. M

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gen

erat

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wn;

no

exc

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reco

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val

ue.

35

APPENDIX I

Equations (refer to Figure 2)

d <|>N<|> = r i + 92, 300 ——e e d 6

d i

volts

Note 6 = (n i + n. L + n i )e a a d o c c

now

(500 i + 1400 ^ + 500 i )amp turns

with due regard to sign of currents.

dK ̂ p̂-V = N <f> = 0.18 1 + 779 — — . — g-g g g d I dt

o volts

note <$ = (n i + n I )g e s g

(280 i + n I ) e s g' amp turns

(see 2. 2 of this appendix for value of n )s

dNT = T + T + T + . 00326 M . m g Fw I dt

lbs. ft

where T = 78.0 + .000498 N2 Fw lbs. ft

(friction and windage torque)

T = .0075 i N I e lbs. ft

(1 )

(2)

(3)

(4)

(5)

( 6 )

(7)

(8)

(Iron lo ss torque)

Appendix I 16

A lso T = 7.03 g

1g g lbs. ft

(9)

Hence T . N = 7 .03 V I g g g H. P.

- vg =<m + rf> * b + 2 5 1 44(10)

(11)

(note 173 ohms is the value of )

d <J>and V = r i

a a a

or 19 = 10 ia

Notes: 1. <P (<5 )e e

2. <P (6 )g g

3. K (I ) g

+ 90volts

d <J>+ go — g-

dt volts

(12)

(13)

is the magnetic characteristic of the main exciter (not shown here).

is the magnetic characteristic of the main generator (not shown h ere).

is the H. P. G. field magnet ch aracteristic (not shown here).

2. Normal Operating Conditions

(note - norm al operating speed of the set is 750 r .p. m . )

2.1 Main exciter

i = 1 .9 ampsSi

ijj = “9- 394 amps (negative sign indicates bucking current)

i = 0c

V = 51 vo ltse

*e 12. 5 amps

Appendix I 17

2 .2 G enerato r

Vg

— 325 volts

Ig

= 1800 amps

Tg

= 5,480 lbs. ft

TFw= 358 lbs. ft

TI = 72 lbs. ft

Tm = 5,735 lbs. ft

M - 15, 000 lbs. ft'

n — 0s

note a lso r^ = 650 ohms

3. P e rm iss ib le Maximum Values

3 .1 Main ex c ite r

H >

V

5. 0 am ps

1. 0 am ps

4. 0 am ps

250 voltse

i = 25 am ps

3.2 G enerato r

Vg

Ig

480 volts

2,100 amps

R I

P u b lica tio n s by D epartm en t of E ngineering P h y sics

No._________ A uthor__________________ T itle

E P -R R 1 H ibbard , L. U. C em enting R o to rs fo r theC a n b e rra H om opolar G en era to r

E P -R R 2 C arden , P . O. L im ita tions of R ate of R iseof P u lse C u rre n t Im posed by Skin E ffect in R o to rs

E P -R R .3 M arsh a ll, R. A. The D esign of B ru sh es fo rthe C a n b e rra H om opolar G en e ra to r

E P -R R 4 M arsh a ll, R . A.

E P -R R 5 Inall, E . K .

E P -R R 6 Inall, E . K.

E P -R R 7 Inall, E . K.

E P -R R 8 B rady , T . W.

E P -R R 9 Inall, E . K.

The E lec tro ly tic V ariab le R es is ta n ce T e s t L oad/Sw itch fo r the C a n b e rra H om opolar G en e ra to r

The M ark II Coupling and R o to r C en tering R e g is te rs fo r the C a n b e rra Hom opo- l a r G en e ra to r

A Review of the S pecifica­tions and D esign of the M ark II Oil L ubricated T h ru s t and C en tering B earin g s of the C an b e rra H om opolar G en era to r

P ro v in g T e s ts on the C a n b e rra H om opolar G en­e r a to r w ith the Two R oto rs Connected in S e rie s

N otes on Speed B alance C on tro ls on the C an b e rra H om opolar G en e ra to r

T e s ts on the C an b e rra H om opolar G en e ra to r A rran g ed to Supply the 5 M egaw att M agnet

F ir s tP ub lished R e -issu e d

May, 1959 A pril, 1967

Sept., 1962 A pril, 1967

Jan ., 1964 A pril, 1967

May, 1964 A pril, 1967

Oct. , 1964 A pril, 1967

Nov. , 1964 A pril, 1967

F e b . , 1966 A pril, 1967

M ar. ,1966 A pril, 1967

May, 1966 A pril, 1967

P u b lic a tio n s by D epartm en t of E ngineering P h y s ic s (C on t.) R2

No. A uthor T itleF i r s t

P ub lished

E P -R R 10 B rady, T .W . A Study of the P e rfo rm a n c e of the 1000 kW M otor G en­e r a to r Set Supplying the C an b e rra H om opolar G en­e r a to r F ield

June, 1966

E P -R R 11 M acleod, I.D .G . In s tru m en ta tio n and C ontrolof the C an b e rra H om opolar G en era to r by O n-L ine Com ­p u te r

E P -R R 12 C arden , P .O . M echanical S tre s s e s in an Infin ite ly Long H om ogeneous B it te r Solenoid w ith F in ite E x te rn a l F ield

O c t . , 1966

J a n . , 1967

E P -R R 13 M acleod, I.D .G. A Survey of Iso la tion A m pli- F e b . , 1967f ie r C irc u its

E P -R R 14 Inall, E . K. The M ark III Coupling fo r Feb. , 1967the R o to rs of the C an b e rra H om opolar G en e ra to r

E P -R R 15 B ydder, E . L . On the In teg ra tio n of M ar. ,1967Liley, B.S. "B o ltzm an n -L ik e”

C ollision In te g ra ls

E P -R R 16 Vance, C . F . S im ple T h y ris to r C irc u its M ar. ,1967to P u ls e -F ir e Ign itrons fo r C ap ac ito r D ischarge

E P -R R 17 B ydder, E . L . On the E valuation of E las tic Sept. ,1967and In e la s tic C ollision F r e ­quencies fo r H ydrogen ic-L ike P la sm a s

E P -R R 18 S tebbens, A.W ard, H.

The D esign of B ru sh es fo r the H om opolar G en e ra to r a t The A u stra lian N ational U n iversity

M ar. ,1964

R e-issu ed

A pril, 1967

A pril, 1967

S e p t. , 1967

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