magnetic amplifiers in metering direct current on electrolytic cell lines

where η a n d g a r e b o t h pos i t ive real i n t ege r s . The exact express ion for a (ω) given in

equation 10 is a po lynomia l , a n d m a y b e con ­sidered as i t s o w n T a y l o r ' s ser ies e x p a n s i o n in which all coefficients b e y o n d bn a r e zero . If, in equa t ion 47 , Κ is chosen equa l t o («-hg)/2 a n d t h e success ive c o l u m n s of equation 47 a r e e q u a t e d t o t h e successive coefficients of e q u a t i o n 10, t h e fol lowing re ­sults are ob t a ined

BaiT,'-\-Ba2Ti^+ . .,+BaK^b, ]

BaKTK2 = {2\)b2

BaETK^'^inDbn'} (48)

Β α ; , Γ κ " + - = θ |

Βα^Ά'^' + Βα2Τ2''^'+ . . . + ΒακΤκ^'-^'^Ο

It is possible t o choose a n y sequence of increasing m a g n i t u d e s for Ti, Ti . . . Τ κ, provided only t h a t Τκ ^ Td/2. T h e n , e q u a ­tions 48 m a y b e solved for t h e Ba/c v a l u e s . Further, it should b e n o t e d t h a t w i t h t h i s procedure t h e g rea t e r t h e v a l u e of K, t h e better ^ ( ω ) will a p p r o x i m a t e α(ω) .

A procedure exac t ly a n a l o g o u s t o t h a t shown by e q u a t i o n s 47 a n d 48 , for p r o d u c i n g an .4 (ω) which closely a p p r o x i m a t e s α(ω), can be employed t o d e t e r m i n e Βι,ι, 5 ^ . . . Bi,k such t h a t i S ( ω ) will closely a p p r o x i m a t e jbM,

It should b e obse rved t h a t t h e mu l t i p l i e r s Bak obtained f rom e q u a t i o n s 4 8 m u s t b e used at taps located Γ* a b o v e a n d Tk be low t h e center t a p . S imi la r r e q u i r e m e n t s a r e p l aced on the Bbk mu l t ip l i e r s w h i c h w o u l d be o b ­tained from t h e sine t e r m s . If t h e s a m e taps (not necessar i ly equa l l y s p a c e d ) a r e used in de t e rm in ing t h e Bak v a lues a n d t h e Bbk values, t h e n t h e mu l t i p l i e r s m a y b e combined a t each t a p ( o t h e r t h a n t h e c e n t e r tap) exactly a s w a s d o n e for t h e F S C d e ­vice.

erences

1. T R A N S V E R S A L F I L T E R S , H . E . Kal lman. Pro­ceedings, Inst i tute of Radio Engineers, N e w York, N . Y. , vol. 28 , July 1940, pp . 3 0 2 - 1 0 .

2. A P H A S E - P L A N E A P P R O A C H TO T H E C O M P E N ­SATION OP S A T U R A T I N G S E R V O M E C H A N I S M S . Arthur M . Hopkin. AIEE Transactions, vol. 70, pt. I, 1951, p p . 6 3 1 - 3 9 .

3. S I G N A L C O M P O N E N T C O N T R O L , D . J. Gimpel , J. F. Calvert. AIEE Transactions, vol. 71 pt . I I , N o v . 1952, pp . 3 3 9 - 4 3 .

Di iscussion A b r a h a m M . F u c h s ( W e s t i n g h o u s e E lec t r i c C o r p o r a t i o n , B a l t i m o r e , M d . ) : T h i s p a p e r is i n t e r e s t i n g for i t s r e su l t s a n d t h e impl i ­c a t i o n s w h i c h c a n b e d r a w n f rom t h e r e su l t s . F igs . 4 a n d 6 show t h a t t h e c o m p e n s a t e d s y s t e m s h a v e a l inear p h a s e l ag v e r s u s fre­q u e n c y cha rac t e r i s t i c ove r a wide b a n d ­w i d t h . If t h e i n p u t s ignal is l imi ted t o a f requency compos i t i on w i t h i n t h e region of l inear p h a s e l ag ve r sus f requency , t h e n t h e o u t p u t of t h e s e r v o m e c h a n i s m will b e a n exac t r e p r o d u c t i o n of t h e i n p u t d i sp laced b a c k w a r d s in t i m e b y t h e rec iproca l of t h e p h a s e s lope . As t h e s lope is a l so con t ro l ­lab le ( a l t h o u g h a t t h e expense of h ighe r ga in in t h e c o m p e n s a t o r ) , b o t h t h e a c c u r a c y of r e p r o d u c t i o n of t h e i n p u t s ignal a n d t h e t i m e d i s p l a c e m e n t of t h e o u t p u t c o m p a r e d t o t h e i n p u t can b e des igned i n t o t h e s y s t e m .

T h i s b r i g h t s ide of e v a l u a t i n g t h e m e r i t s of th i s t y p e of open- loop c o m p e n s a t i o n is bec louded b y t h e p i c t u r e of h o w well sa id s y s t e m will w o r k w h e n t h e p r o t e c t i v e coa t ­ing of " l i nea r opera t ion '* is r e m o v e d . A s s u m e , for example , t h a t t h e r e is a load t o r q u e p r e s e n t (o r frict ion on t h e o u t p u t ) . T h e ab i l i t y of t h e s y s t e m t o o v e r c o m e t h e load t o r q u e (o r fr ic t ion) is n o t a u g m e n t e d b y t h e c o m p e n s a t o r . T h e ques t ion ar ises w h e t h e r t h e s a m e effort s p e n t in bu i ld ing t h e c o m p e n s a t o r m i g h t n o t b e b e t t e r s p e n t in bu i ld ing t h e b a n d w i d t h ( a n d genera l ly

t h e increased loop ga in ) i n t o t h e con t ro l sys ­t e m where i t n o t only i m p r o v e s t h e follow­ing of t h e i n p u t s ignal b u t a l so m a k e s t h e con t ro l s y s t e m less sens i t ive t o load t o r q u e s o u t p u t friction, e t c .

D . J . F o r d a n d J . F . C a l v e r t : W e wish t o t h a n k A . M . F u c h s for h is in t e r e s t i ng dis­cuss ion of o u r pape r . A s M r . F u c h s p o i n t s o u t , t h e p a p e r deals exclusively w i t h a n open- loop t y p e of sys t em, a n d a s such defi­n i t e ly c a n n o t be expec ted t o exhib i t all of t h e p e r f o r m a n c e charac te r i s t i c s of closed-loop s y s t e m s . A s s u m e t h a t t h e s y s t e m be ing cont ro l led is in itself a feedback sys­t e m h a v i n g undes i r ab l e a m p l i t u d e a n d p h a s e cha rac te r i s t i c s , y e t possessing t h e insens i t iv-i t y t o " n o n l i n e a r i t i e s " of t h e form of o u t p u t friction, load t o r q u e , e tc . , charac te r i s t i c of such feedback s y s t e m s . U n d e r these cir­c u m s t a n c e s , t h e app l i ca t ion of t h e t y p e of c o m p e n s a t o r s descr ibed in t h e p a p e r will i m p r o v e t h e over-al l s y s t e m response a p p r e ­c iab ly . T h u s , t h e open- loop c o m p e n s a t o r t a k e s t h e p lace of t h e e l a b o r a t e correc t ion a n d c o m p e n s a t i n g n e t w o r k s in t h e cont ro l s y s t e m wh ich wou ld be needed t o ove rcome t h e undes i r ab l e a m p l i t u d e cha rac t e r i s t i c c aused b y t h e i n t r o d u c t i o n of h igh gain in t h e feedback loop.

In o t h e r words , a given s y s t e m m a y b e des igned w i t h a s imple h igh-gain amplif ier a s a feedback n e t w o r k , t h u s o b t a i n i n g t h e des i rab le insens i t iv i ty t o o u t p u t friction, load t o r q u e , e tc . , w i t h l i t t l e reference t o a m p l i t u d e a n d p h a s e response . W i t h t h i s accompl i shed , t h e c o m p e n s a t o r ma}^ be de ­signed t o o p e r a t e w i t h t h i s fedback con­t ro l sy s t em, g iv ing i t a v i r t u a l l y c o n s t a n t gain a n d l inear p h a s e cha rac t e r i s t i c over a s wide a f requency b a n d a s s a t u r a t i o n l imi­t a t i o n s will p e r m i t . T h u s , t h e combined s y s t e m will possess t h e des i rab le cha rac t e r ­istics of t h e closed-loop e r ro r r educ t ion m e c h a n i s m t o g e t h e r w i t h t h e des i rab le a m ­p l i t u d e a n d p h a s e con t ro l charac te r i s t i c s of t h e open- loop t y p e of sys t em descr ibed in t h e pape r .

Magnetic Amplifiers in Metering Direct,

Current on Electrolytic Cell Lines

E. A. DOWNING NONMEMBER AIEE

SE V E R A L m o n t h s a g o e v e r y p u b l i c a ­t ion d e v o t e d t o p o w e r a n d t h o s e

covering i n d u s t r y i n g e n e r a l c a r r i e d articles te l l ing t h e s t o r y of m a g n e t i c a m ­plifiers. T h e y t o l d of u s i n g m a g n e t i c amplifiers w i t h s e r v o m e c h a n i s m s for a u t o ­matic p i lo t s , t o c o n t r o l s p e e d s of l a r g e motors , t o r e g u l a t e v o l t a g e s of l a r g e g e n ­erators, a n d a n y n u m b e r of o t h e r a p p l i c a ­tions, in s o m e c a s e s r e p l a c i n g e q u i p m e n t tha t h a d b e e n a c c e p t e d a s s t a n d a r d for years.

T h e a p p l i c a t i o n d e s c r i J D e d h e r e is n o t a s d r a m a t i c a s t h i s b u t o n e t h a t is v e r y i m ­p o r t a n t i n t h e e l e c t r o c h e m i c a l field. T h a t is t h e m e a s u r e m e n t of d -c a m p e r e s .

T h e p a r t i c u l a r i n s t a l l a t i o n h a s b e e n i n e x i s t e n c e for o n l y a m a t t e r of m o n t h s b u t i t is o n e of t h e first i n s t a l l a t i o n s w h e r e m a g n e t i c a m p l i f i e r s h a v e b e e n u s e d i n se r i e s t o m e a s u r e t o t a l d -c a m p e r e s f r o m m o r e t h a n o n e rec t i f i e r . ( F i g . 1 ) . I t is n o t t h e i n t e n t i o n t o d i s c u s s t h e t h e o r y of t h e d e s i g n of m a g n e t i c a m p l i f i e r s b u t t o

r e v i e w t h e r e a s o n s for m a g n e t i c ampl i f i e r s b e i n g s e l e c t e d for t h i s p a r t i c u l a r i n s t a l l a ­t i o n , t o d e s c r i b e t h e m e t h o d of i n s t a l l a ­t i o n , a n d t o r e p o r t o n t h e i r p e r f o r m a n c e

F i r s t , s i n c e t h e r e is m u c h s h n i l a r i t y b e ­t w e e n m a g n e t i c ampl i f i e r s a n d s a t u r a b l e r e a c t o r s , i t m a y b e wel l t o def ine t h e s e a s l i s t ed in t h e p r o g r e s s r e p o r t of t h e A I E E M a g n e t i c A m p l i f i e r s S u b c o m m i t t e e :

A s a t u r a b l e r eac to r is a n a d j u s t a b l e i nduc ­to r in which t h e cu r r en t -ve r sus -vo l t age rela­t i o n s h i p is a d j u s t e d b y c o n t r o l m a g n e t o ­m o t i v e forces app l i ed t o t h e core .

A m a g n e t i c ampl i f ier is a device us ing s a tu r ­ab le r e a c t o r s e i t he r a lone or in c o m b i n a t i o n

Paper 54-178, recommended by the A I E E Chemi­cal, Electrochemical and Electrothermal Applica­tions Commit tee and approved by the A I E E Commit tee on Technical Operations for presenta­tion at the A I E E Winter General Meeting, N e w York, N . Y. . January 18-22, 1954. Manuscript submitted October 22. 1953; made available for printing December 4, 1953.

E. A. D O W N I N G is with the Columbia-vSouthern Chemical Corporation, N e w Martinsville, W . Va.

M. \Y 1 9 5 4 Downing—Magnetic Amplifiers in Metering Direct Current 9 3

BUS Fis. 1 (left). Schematic oi current totalizing

Fis. 2 (right). Simple-type magnetic amplifier

with other circuit e lements to secure ampli­fication or control.

S h o w n i n F i g . 2 i s o n e of t h e s m a l l e r m a g n e t i c a mp l i f i e r u n i t s w i t h a n o u t p u t r a t i n g of 4 0 m i l l i w a t t s . T h i s s m a l l u n i t h a s a s a t u r a b l e r e a c t o r w i t h a d - c c o n t r o l w i n d i n g o n t h e c o r e . T h e e n d s a r e c o n ­n e c t e d t o t h r e a d e d s t u d s t o a c c o m m o d a t e c o n n e c t i o n s t o s h u n t s o r o t h e r s o u r c e s of c o n t r o l v o l t a g e . T h e s e c o n d w i n d i n g o n t h e c o r e h a s a l a r g e r n u m b e r of t u r n s , a n d t h e a-c o u t p u t f r o m t h i s w i n d i n g is r e c t i ­fied b y a s m a l l f u l l - w a v e m e t a l U c r ec t i f i e r , a l s o m o u n t e d in t h e c a s e . T h e t w o w i n d ­i n g s a r e e l e c t r i c a l l y i n s u l a t e d f r o m e a c h o t h e r .

Reasons for Using Magnetic Amplifiers

T h e u s e of m a g n e t i c a m p l i f i e r s for m e t e r i n g c u r r e n t i n c h l o r i n e - c a u s t i c s o d a cell l ine w a s e l e c t e d b e c a u s e t h e y w o u l d e l e c t r i c a l l y i s o l a t e t h e r e c o r d i n g i n s t r u ­m e n t s f r o m t h e cell h u e b u s b a r s , a n d b e ­c a u s e t h e y w o u l d e l i m i n a t e t h e c o s t of i n ­s t a l l i n g a l a r g e s h u n t in t h e m a i n b u s b y t o t a H z i n g t h e o u t p u t of i n d i v i d u a l r e c t i ­fiers. A n y n u m b e r of m a g n e t i c - a m p l i f i e r u n i t s m a y b e c o n n e c t e d i n se r ies t o t o t a l ­ize t h e l a r g e s t s y s t e m s .

T h e first-mentioned f u n c t i o n w a s i m ­p o r t a n t b e c a u s e b u s p o t e n t i a l w a s r e ­m o v e d f r o m t h e i n s t r u m e n t s , t h u s e l i m ­i n a t i n g a m a j o r c a u s e of i n s t r u m e n t f a i lu res , a n d r e m o v i n g t h e h a z a r d t o i n ­s t r u m e n t m e c h a n i c s of c o m i n g in c o n t a c t w i t h t h e h i g h v o l t a g e .

I n t h e c a s e of c e r t a i n t y p e s of i n s t r u ­m e n t s i t is n e c e s s a r y t h a t t h e y b e i n s u ­l a t e d f r o m t h e h i g h e r v o l t a g e s . T o c i t e a n

e x a m p l e : A few y e a r s a g o t h e r e w a s a n a u t o m a t i c c o n t r o l a p p l i c a t i o n u s i n g c o n ­t a c t - m a k i n g m i l l i v o l t ( m v ) m e t e r s . T h e first c o n t r o l l e r i n s t a l l e d e m p l o y e d a v a n e a n d o s c i l l a t o r coi l t o d e t e c t a l i g n m e n t of t h e s e t p o i n t e r a n d i n d i c a t o r . A s t h e i n d i c a t o r a p p r o a c h e d t h e s e t p o i n t e r i t w a s r e p u l s e d v i g o r o u s l y , m a k i n g i t i m ­p o s s i b l e t o r e a c h t h e c o n t r o l p o i n t . O t h e r i n s t r u m e n t s fa i led w h e n t h e i n s u l a t i o n b r o k e d o w n t o g r o u n d . All of t h e s e in ­s t r u m e n t s w e r e q u a l i t y i n s t r u m e n t s a n d w o u l d h a v e o p e r a t e d s a t i s f a c t o r i l y if p r o p ­e r l y u s e d . P o s s i b l y t h e s e o p e r a t i n g p r o b l e m s w o u l d h a v e b e e n a v o i d e d b y t h e i s o l a t i o n p r o v i d e d b y m a g n e t i c ampl i f i e r s .

T h e a d v a n t a g e s of e l i m i n a t i n g t h e l a r g e t o t a l i z i n g s h u n t t o m e a s u r e t h e c u r r e n t i n t h e cel l s t r i n g w e r e l a r g e l y e c o n o m i c a l . T h e c o s t of t h e m a g n e t i c amp l i f i e r s a t t h e t i m e w a s l ess t h a n t h e c o s t of a 5 0 - m v t o t a l i z i n g s h u n t of a d e q u a t e c a p a c i t y . I t t u r n e d o u t l a t e r t h a t a g r e a t e r s a v i n g w a s r e a l i z e d b y b e i n g a b l e t o o m i t s e v e r a l f ee t of b u s t o t h e cel l l i n e . I t h a d b e e n t h e p r a c t i c e t o l o c a t e all s h u n t s in t h e r ec t i f i e r r o o m , for t h e p u r p o s e of p l a c i n g t h e m in a l ess c o r r o s i v e a t m o s p h e r e t h a n t h e ce l l r o o m . E l i m i n a t i o n of t h e b u s s e c t i o n t o t h e t o t a l i z i n g s h u n t in t h e r ec t i f i e r r o o m s a v e d n e a r l y 1 t o n of c o p ­p e r . T h i s r e p r e s e n t s a c o n s i d e r a b l e s a v ­i n g s w h e n t h e f a b r i c a t i o n a n d e r e c t i o n c o s t of t h e c o p p e r is i n c l u d e d . A l s o c o p ­p e r w a s o n a l l o c a t i o n a t t h a t t i m e .

A n o t h e r s a v i n g , w h i c h d i d n o t r e c e i v e p r i m e c o n s i d e r a t i o n w a s e f f ec ted b y t h e m a g n e t i c a m p l i f i e r s . T h i s w a s e v i d e n t i n c o m p a r i n g t h e losses i n t h e m a g n e t i c a m p l i f i e r t o t h e lo s ses i n a t o t a l i z i n g s h u n t . T h e t o t a l losses of f o u r m a g n e t i c ampl i f i e r u n i t s w o u l d b e less t h a n I V 2 w a t t s , w h e r e t h e losses i n a 2 0 , 0 0 0 - a m -p e r e 5 0 - m v s h u n t w o u l d b e 1,000 w a t t s , w h i c h a m o u n t s t o m o r e t h a n 8 ,000 k i l o ­w a t t - h o u r s p e r y e a r .

Installation

T o i n s t a l l a c o m p l e t e se t of m a g n e t i c -ampl i f i e r e q u i j n n e n t , o t h e r accesso r i e s a re r e q u i r e d . T h e s e a r e a v o l t a g e regulator . , a n i s o l a t i n g t r a n s f o r m e r , a v o l t a g e s t ep -d o w n t r a n s f o r m e r , a n d a p a n e l w i t h po ­t e n t i o m e t e r s for s e t t i n g t h e r a n g e a n d zero a d j u s t m e n t s . O n e v o l t a g e r e g u l a t o r a n d s t e p - d o w n t r a n s f o r m e r c a n s e r v e severa l m a g n e t i c - a m p l i f i e r u n i t s , t h e r e f o r e , on ly o n e is r e q u i r e d for e a c h i n s t a l l a t i o n .

T h e G-volt i s o l a t i n g t r a n s f o r m e r s were s u p p l i e d m o u n t e d o n t h e p a n e l w i t h cal i­b r a t i n g p o t e n t i o m e t e r s . T h e o t h e r i t e m s w e r e r e c e i v e d a n d m o u n t e d s e p a r a t e l y . T h e m a g n e t i c - a m p l i f i e r u n i t s a r e fur­n i s h e d w i t h a s e t of c a l i b r a t e d s h u n t l e ads w h i c h w e r e i n c l u d e d in t h e i n i t i a l c a l i b r a ­t i o n . T h e s e l e a d s a r e s h o r t , m a k i n g i t n e c e s s a r y t o m o u n t t h e m a g n e t i c - a m ­plif ier u n i t n e a r t h e s h u n t of e a c h r ec t i ­fier. I n t h i s i n s t a l l a t i o n t h e s h u n t s a r e m o u n t e d b e h i n d t h e c a t h o d e b r e a k e r s , so t h e m a g n e t i c - a m p l i f i e r u n i t s were m o u n t e d o n t h e f r a m e in t h e r e a r of t h e c i r c u i t b r e a k e r , F i g . 3 .

All of t h e o t h e r c o m j ^ o n e n t s m a y b e m o u n t e d w h e r e v e r i t is m o s t conv^enient . T h e i r l o c a t i o n h a s n o effect o n t h e pe r ­f o r m a n c e of t h e e q u i p m e n t . Al l of t h e c a l i b r a t i n g p o t e n t i o m e t e r s w e r e m o u n t e d in o n e l a r g e m e t a l b o x w i t h g a s k e t e d c o v e r , b e s i d e t h e c a t h o d e b r e a k e r s . T h e l o c a t i o n m a d e t h e m c o n v e n i e n t for t e s t ­i n g , a n d t h e m e t a l e n c l o s u r e s offers s o m e p r o t e c t i o n f r o m c o r r o s i v e a t m o s p h e r e .

A p p a r e n t l y t h e m a n u f a c t u r e r d i d n o t c o n s i d e r t h e a t m o s p h e r e a r o u n d c h e m i c a l p l a n t s w h e n d e s i g n i n g t h e p o t e n t i o m e t e r p a n e l , b e c a u s e t h e r e w a s n o e n c l o s u r e of a n y k i n d f u r n i s h e d w i t h t h e m , F i g . 4 .

B e c a u s e t h e m a g n e t i c ampHfier h a s t o s o m e d e g r e e t h e c h a r a c t e r i s t i c s of t h e a-c i n s t r u m e n t c u r r e n t t r a n s f o r m e r t o c a r r y a c o n s t a n t c u r r e n t r e g a r d l e s s of t h e r e s i s t ­a n c e in t h e e x t e r n a l c i r c u i t , t h e p o t e n t i o m ­e t e r s a r e p r e c i s i o n - w o u n d w i t h m a n -g a n i n w i r e h a v i n g a low t e m p e r a t u r e co­efficient of r e s i s t a n c e t o e l i m i n a t e e r r o r s f r o m t e m p e r a t u r e c h a n g e s .

T h e s t e p - d o w n t r a n s f o r m e r , v o l t a g e

9 4 Downing—Magnetic Amplifiers in Metering Direct Current M . \ Y 1 9 5 4

Fig. 3. Magnetic amplifier and shunt

regulator , a n d r e c o r d i n g i n s t r u m e n t w e r e installed in t h e r e m o t e p a n e l in t h e s t a t i o n control r o o m . T h e w i r i n g b e t w e e n t h e c o m p o n e n t s of t h i s s y s t e m r e q u i r e s v e r y little spec ia l a t t e n t i o n . T h e s h u n t l e a d s mus t b e t i g h t a n d t h e r e s i s t a n c e of l e a d connect ions k e p t a s l o w a s p o s s i b l e . T h e resistance of t h e c o m p l e t e d -c c o n t r o l c i r ­cuit is 0 . 0 5 0 + 5 = 0 .010 o h m . A r e s i s t ­ance c h a n g e of 0 .0001 o h m w o u l d i n ­troduce a 1-per -cent e r r o r . D e p e n d i n g upon t h e i n s t a l l a t i o n , s h i e l d e d l e a d s might b e r e q u i r e d b e t w e e n t h e p o t e n t i o m ­eters a n d r e c o r d e r s t o e l i m i n a t e i n d u c e d voltages in t h e l e a d s .

Performance

T h e p e r f o r m a n c e of s u c h a n i n s t a l l a t i o n is m e a s u r e d l)y t h e m a i n t e n a n c e r e q u i r e d and t h e d e g r e e of a c c u r a c y in r e c o r d i n g current . T o d a t e t h e m a i n t e n a n c e h a s been nil. T h e ec iu i ] )ment is l ess t h a n 1 year old a n d it m a y b e s o m e w h a t j ^ r e m a -ture t o f o r m c o n c l u s i o n s , b u t t h e r e i s a

Fig. 4. Calibrating potentiometers

firm bel ief t h a t i t wi l l r e m a i n v e r y low. T h e s u b j e c t of a c c u r a c y k n i t s i n c lose ly

w i t h t h e s u b j e c t of t e s t i n g a n d c a h b r a -t i o n , s o t h e y wil l b e d i s c u s s e d t o g e t h e r . T h e m a g n e t i c a m p l i f i e r d o e s n o t r e p l a c e a n y e x i s t i n g e q u i p m e n t b u t i s a n o t h e r d e ­v i c e a d d e d t o t h e s y s t e m . I t m a y i m ­p r o v e a c c u r a c y of t h e s y s t e m b y i m p r o v ­i n g o p e r a t i o n of t h e o t h e r i n s t r u m e n t s b e ­c a u s e m a i n t e n a n c e r e q u i r e m e n t s a r e d e ­c r e a s e d . A m a g n e t i c - a m p l i f i e r i n s t a l l a ­t i o n i s n o t a s e a s i l y c a l i b r a t e d a s s o m e o t h e r i n s t a l l a t i o n s b e c a u s e of t h e a d d i ­t i o n a l e q u i p m e n t r e q u i r e d . T h e t e s t s h o u l d b e m a d e o n t h e j o b s i t e b e c a u s e t h e m a g n e t i c ampHf i e r a n d a s s o c i a t e d e q u i p m e n t a r e n o t e a s i l y r e m o v e d .

T h e c o n t r o l c i r c u i t r e q u i r e s a p p r o x i ­m a t e l y 5 a m p e r e s a t 50 m v f r o m t h e s h u n t a t r a t e d m a g n e t i c - a m p l i f i e r i n p u t . S i n c e t h e c u r r e n t m u s t b e v a r i a b l e , i t is n e c e s ­s a r y t o r e m o v e t h e l e a d s f r o m t h e s h u n t . A b a t t e r y a n d v a r i a b l e r e s i s t o r a r e s u b ­s t i t u t e d fo r t h e s h u n t a s a d - c p o w e r s u p ­p l y for t h e c o n t r o l c i r c u i t . I f t h e t e s t is m a d e w i t h t h e m a g n e t i c - a m p l i f i e r e q u i p ­m e n t i n i t s n o r m a l p o s i t i o n , t h e u s u a l a -c p o w e r s o u r c e is u s e d w i t h o u t c h a n g e s i n c o n n e c t i o n s .

S i n c e t h e s h u n t s a r e o u t of t h e c i r c u i t s d u r i n g t e s t i n g , a n y e r r o r s i n t r o d u c e d b y t h e s h u n t s a r e n o t i n c l u d e d i n t h e t e s t . H o w e v e r , i n a c t u a l o p e r a t i o n t h e s h u n t is t h e s o u r c e of d -c c o n t r o l p o w e r a n d t h e r e ­fo re s h u n t e r r o r s a r e i n c l u d e d i n t h e m e a s u r e m e n t . A n y e r r o r s i n t r o d u c e d b y t h e m a g n e t i c - a m p l i f i e r s y s t e m a r e a d d e d t o o r s u b t r a c t e d f r o m t h e s h u n t e r r o r .

T h e p u b l i s h e d a c c u r a c y of c o m m e r c i a l m a g n e t i c - a m p l i f i e r u n i t s is 0 .25 p e r c e n t , a n d i t is u n d e r s t o o d t h a t t h e t e s t s o n s ing le u n i t s a t t h e m a n u f a c t u r e r s ' l a b o r a ­t o r y i n d i c a t e a h i g h e r d e g r e e of a c c u r a c y .

T h e t r a n s f e r o r i n p u t - v e r s u s - o u t p u t c u r v e of t h e m a g n e t i c amp l i f i e r is n o t a s t r a i g h t l i n e b u t r e s e m b l e s a n e l o n g a t e d S, F i g . 5 . If a s t r a i g h t l i ne A - B ( F i g . 5) w e r e s u p e r i m p o s e d o n t h e t r a n s f e r c u r v e , t h e r e is s u p p o s e d l y n o d e v i a t i o n f r o m t h e s t r a i g h t Hue g r e a t e r t h a n 1 / 4 of 1 p e r c e n t . T e s t s m a d e o n u n i t s i n t h e p l a n t s h o w t h e m a g n e t i c - a m p l i f i e r u n i t s we l l w i t h i n t h e s e l i m i t s . W h e n t h e s t r a i g h t l i n e is e x t e n d e d , i t is f o u n d t h a t t h e i n d i c a t e d o u t p u t is 0 .58 m v w i t h n o c u r r e n t i n t h e c o n t r o l c i r c u i t . R e p o r t e d l y t h i s 0 . 5 8 - m v o u t p u t w i t h z e r o c o n t r o l c u r ­r e n t is t r u e fo r a l l m a g n e t i c - a m p l i f i e r u n i t s .

T h e r e is o n l y o n e c a l i b r a t i n g a d j u s t ­m e n t w h i c h is t h e r a n g e a d j u s t m e n t o n t h e m a g n e t i c - a m p l i f i e r e q u i p m e n t . T h i s fixes t h e t r a n s f e r c u r v e a t o n e p o i n t for r e l a t i o n of i n p u t m v . T h e s l o p e of t h e t r a n s f e r c u r v e v a r i e s a s t h e r a n g e is v a r i e d , b u t i t i s

A-OUTPUT OF M-A Β - MEAN SLOPE OF M-A C - INSTRUMENT DESIGN

MILLIVOLTS AT SHUNT

Fig. 5. Transfer curve and receiving-instru­ment design curve

c o n s i d e r e d t h a t i t i n t e r s e c t s t h e axis a t 0 .58 m v for a l l r a n g e s e t t i n g s .

T h e r e w e r e m i n o r d i s c r e p a n c i e s be ­t w e e n field c a l i b r a t i o n s a n d f a c t o r y t e s t s . T h i s w a s e x p l a i n e d b y s t a t i n g t h e t e s t i ng fac i l i t ies a t t h e f a c t o r y w e r e m o r e accu ­r a t e t h a n t h o s e i n o u r p l a n t . N o d o u b t t h e f a c t o r y t e s t i n g fac i l i t i es a r e b e t t e r , b u t a n i n s t r u m e n t t o b e p r a c t i c a l m u s t b e cali­b r a t e d w i t h n o r m a l t e s t i n g e q u i p m e n t . T h e field s e t t i n g s w e r e a c c e p t e d for final c a l i b r a t i o n .

T h e t r u e a c c u r a c y of t h e m a g n e t i c ampl i f i e r is m e a s u r e d b y i t s ab i l i t y t o t r a n s f e r a s i g n a l f r o m t h e s o u r c e t o a r e ­c e i v i n g i n s t r u m e n t . T h i s n o t only re­q u i r e d n e a r l i n e a r c h a r a c t e r i s t i c s of t h e m a g n e t i c - a m p H f i e r u n i t , b u t t h e o u t p u t c h a r a c t e r i s t i c s of t h e m a g n e t i c amplif ier m u s t m a t c h t h e r e c e i v i n g i n s t r u m e n t s . S i n c e all m a g n e t i c ampl i f i e r s h a v e s imilar c h a r a c t e r i s t i c s , t h e d e s i g n specif icat ions of t h e r e c e i v i n g i n s t r u m e n t a r e d u p l i c a t e d for a l l i n s t a l l a t i o n s .

T e s t s c o n d u c t e d on t h e e q u i p m e n t in­s t a l l e d s h o w s t h a t t h e m a x i m u m over-al l e r r o r is 0 .3 p e r c e n t . O v e r a l a rge por ­t i o n of t h e r a n g e t h e e r r o r is m u c h less t h a n t h i s . I t i s a s s u m e d t h a t if t h e u n i t s w e r e c a l i b r a t e d for a n a r r o w e r work ing r a n g e , c o m p a r a b l e t o a c t u a l w o r k i n g con­d i t i o n s , t h e e r r o r c o u l d b e r e d u c e d below 0.2 p e r c e n t .

U p t o t h i s p o i n t al l d i s c u s s i o n h a s been o n t h e b a s i s of u s i n g nu l l b a l a n c e i n s t ru ­m e n t s t o m e a s u r e t h e magne t i c - ampHf ie r o u t p u t . A s a n e x p e r i m e n t , a r egu l a r s w i t c h b o a r d i n s t r u m e n t w a s u s e d a n d t h e r e a d i n g s s h o w e d t h a t t h e m a g n e t i c - a m ­pHfier o u t p u t is l i n e a r w i t h r e s p e c t t o t h e i n p u t . If o n e w a n t e d t o e l i m i n a t e h i g h v o l t a g e c o m p l e t e l y f r o m t h e pane l s , i t

MAY 1 9 5 4 Downing—Magnetic Amplifiers in Metering Direct Current 9 5

c o u l d b e d o n e w i t h m a g n e t i c a m p l i f i e r s a n d a s p e c i a l l y c a l i b r a t e d m - v m e t e r t o in ­d i c a t e a m p e r e s .

Conclusion

B y i s o l a t i n g i n s t r u m e n t s f r o m b u s p o t e n t i a l s , t h e m a i n t e n a n c e h a s b e e n r e ­

d u c e d a n d t h e a v e r a g e a c c u r a c y i m p r o v e d o n t h e r e c o r d i n g i n s t r u m e n t s . T h i s h a s b e e n ver i f ied h e r e a n d , a f t e r o n l y a f ew m o n t h s ' o p e r a t i o n of o n e se t , t h e r e w e r e r e q u e s t s t o i n s t a l l t h e m o n o t h e r u n i t s . T h e a c c u r a c y of t h e m a g n e t i c amp l i f i e r is e q u a l t o o r b e t t e r t h a n m o s t c o m m e r c i a l i n s t r u m e n t s o n t h e m a r k e t t o d a y . F o r

l o a d - t o t a l i z i n g t h e i n i t i a l c o s t of t h e m a g ­n e t i c a m p l i f i e r s c a n u s u a l l y b e offset b y e l i m i n a t i n g t h e l a r g e t o t a l i z i n g s h u n t s .

Reference

1. M A G N E T I C - A M P L I F I E R A P P L I C A T I O N S I N D - C C O N V E R S I O N S T A T I O N S , W . A . Derr, E . J . Cham. AIEE Transactions, vol. 72, pt . I l l , April 1953. pp. 2 2 0 - 2 9 .

Di iscussion

E . J . C h a m ( W e s t i n g h o u s e E lec t r i c Cor ­p o r a t i o n , E a s t P i t t s b u r g h , P a . ) a n d R . J . R a d u s ( W e s t i n g h o u s e E lec t r i c C o r p o r a t i o n , P i t t s b u r g h , P a . ) : T h i s p a p e r is a v e r y ab l e c o n t r i b u t i o n t o a n e w field of m a g n e t i c -amplif ier a p p l i c a t i o n . T h e in s t a l l a t i on d e ­scr ibed is one of t h e first t o use sma l l m a g ­ne t i c ampl i f iers c o n n e c t e d t o i n d i v i d u a l rectifier s h u n t s t o to t a l i ze t h e ind iv idua l rectifier c u r r e n t s t o o b t a i n a n ind ica t ion of t h e t o t a l c u r r e n t flowing in a n e lec t ro ly t ic cell l ine.

M r . D o w n i n g h a s p o i n t e d o u t t h e a d v a n ­t ages of t h i s m e t h o d of to t a l i z ing a s com­p a r e d t o a s y s t e m us ing a m a s t e r s h u n t . I t is of i n t e r e s t t h a t t h i s m e t h o d a lso h a s a d v a n t a g e s ove r a s y s t e m u s i n g a single la rge c u r r e n t t r a n s d u c t o r a r o u n d a to t a l i z ­ing b u s . W h e n all of t h e d-c to t a l i z ing b u s passes t h r o u g h a c u r r e n t t r a n s d u c t o r , i t s size becomes a large factor in a n in s t a l l a t i on . I n add i t i on , field ca l ib ra t ion o r field checks a r e imprac t i c a l because of t h e difficulty in­vo lved in o b t a i n i n g an a c c u r a t e m e a s u r e of t h e t o t a l c u r r e n t wh ich usua l ly invo lves

t h o u s a n d s of a m p e r e s . A s t h e a u t h o r p o i n t e d o u t , t h e ind iv idua l

sma l l m a g n e t i c ampl i f iers c a n b e checked in t h e field q u i t e r ead i ly . T h e e lec t r ica l q u a n ­t i t i e s i nvo lved can b e m e t e r e d easi ly a n d a c c u r a t e l y w i t h s t a n d a r d i n s t r u m e n t s a n d e q u i p m e n t .

I t will b e well t o p o i n t o u t t h a t t h e lower p o r t i o n of t h e t r ans fe r c u r v e of F ig . 5 is non l inea r in excess of 1/4 p e r cen t . T h i s n o n l i n e a r i t y of o p e r a t i o n is of n o conse ­q u e n c e i n t h e in s t a l l a t i on desc r ibed b y M r . D o w n i n g , s ince t h e rectifier u n i t s a r e oper­a t e d a t a p p r o x i m a t e l y full load. T h i s m e a n s t h a t t h e m a g n e t i c amplif iers a r e oper ­a t e d a t all t i m e s in t h e l inear sec t ion of t h e i r t r ans fe r c u r v e s .

L a t e r d e v e l o p m e n t h a s p r o d u c e d a p u s h -pu l l t y p e of u n i t w h i c h exh ib i t s a t r ans fe r c u r v e t h a t is l i nea r for t h e full r a n g e of s h u n t mi l l ivol t s , t h u s e l i m i n a t i n g non l inea r o p e r a t i o n for t h e lower va lues of s h u n t mil l i ­volts .^ M o s t r e c e n t d e v e l o p m e n t h a s p r o ­duced a new t y p e of u n i t w h i c h c o m b i n e s t h e " t o - z e r o " c h a r a c t e r i s t i c of t h e p u s h - p u l l t y p e of u n i t a n d t h e c o n s t a n t c u r r e n t c h a r ­ac te r i s t i c of t h e t y p e desc r ibed in M r . D o w n i n g ' s p a p e r .

REFERENCE

1. I N D U S T R I A L A P P L I C A T I O N S OF T R A N S D U C T O R S , R. J. Radus . Proceedings, National Electronics

<:onference, Chicago, 111., 1953.

E . A. D o w n i n g : T h e discussion presen ted b y M r . C h a m a n d M r . R a d u s is g rea t ly a p ­p rec i a t ed , p a r t i c u l a r l y since i t br ings ou t t h e app l i c a t i on of large c u r r e n t t r a n s d u c ­t o r s .

I h a v e n o t h a d t h e o p p o r t u n i t y of work­ing w i t h t r a n s d u c t o r s w h e r e tens-of-t h o u s a n d s of a m p e r e s a r e h a n d l e d , b u t it is n o t difficult t o imag ine t h e p rob lem of ob­t a i n i n g t h e l a rge c u r r e n t s for ca l ib ra t ion , or in d e t e r m i n i n g a c c u r a t e l y t h e va lue of t he t e s t c u r r e n t s .

T h e " t o - z e r o " t y p e of m a g n e t i c amplifier is r e q u i r e d for some appHca t ions , b u t i t has l i t t l e a d v a n t a g e w i t h ch lor ine cell lines o t h e r t h a n t h e fact t h a t t h e rece iv ing ins t ru­m e n t s a r e n o t c a l i b r a t e d w i t h a zero bias. T h e s imple t y p e of m a g n e t i c amplif iers a re a c c u r a t e a n d easi ly ca l i b ra t ed . I n m y opin ion t h e s e fea tu res a r e preferred if it is necessa ry t o select be tween these a n d " t o -z e r o " cha rac te r i s t i c s .

9 6 Downing—Magnetic Amplifiers in Metering Direct Current M A Y 1 9 5 4