Power Supplies for Electrodeposition of Coatings - Advances in

4 Power Supplies for Electrodeposition of Coatings

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C L A Y B O U R N E M I T C H E L L , JR. Elcoat Systems, Inc., 38780 Grand River Rd., Farmington, Mich. 48024

The basic principles of power supply (rectifier) design for electrodeposition of coatings are examined. All system com ponents from the primary ac input to the dc output, along with design criteria, are briefly discussed to aid scientists and production personnel in selecting power systems. Ex perimental measurements were made of current during the electrocoating cycle.

demand

These confirmed that the

most severe transient conditions on the rectifier occur during the high initial current risetimes of 200—1000 μsec, which rapidly decline 25% or more in 1-3 seconds.

" E l e c t r i c a l p o w e r f o r i n d u s t r i a l a n d d o m e s t i c consumers is s u p p l i e d i n the U n i t e d States a n d t h r o u g h o u t t h e w o r l d p r i n c i p a l l y as a l t e r n a t i n g c u r r e n t , a n d as s u c h , i t p e r i o d i c a l l y flows i n opposite directions. F o r i n d u s t r i a l processes r e q u i r i n g essentially u n i d i r e c t i o n a l c u r r e n t flow, i t is necessary to use a p p r o p r i a t e e q u i p m e n t to r e c t i f y t h e current.

Conse

q u e n t l y , a n a l t e r n a t i n g current ( a c ) to d i r e c t c u r r e n t ( d c ) c o n v e r s i o n is effected, a n d t h e e q u i p m e n t is t y p i c a l l y c a l l e d a rectifier, a n a m e w h i c h o r i g i n a t e d d u r i n g t h e early e r a of e l e c t r o p l a t i n g .

Basic Components of a Rectifier T h i s p a p e r discusses t h e basic c o m p o n e n t s of a t y p i c a l p o w e r s u p p l y ( r e c t i f i e r ) , p a r t i c u l a r l y as r e l a t e d to a n electrocoating process.

Considera

t i o n of its d e s i g n a n d f u n c t i o n s i n c l u d e the t y p e of o u t p u t controller, m a n u a l o r a u t o m a t i c c o n t r o l features, m e t e r i n g , p r o t e c t i o n , alarms, a n d p o w e r s u p p l y r i p p l e . T h e b a s i c components of a rectifier are s h o w n i n F i g u r e 1, a n d each of these is d i s c u s s e d i n d e t a i l b e l o w . A c Input. T h e i n c o m i n g i n d u s t r i a l p o w e r u t i l i z e d is t y p i c a l l y threephase ac f o r outputs greater t h a n 5 k w , a n d t h e i n p u t a c voltages m a y 62 In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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r a n g e f r o m 208 t o 550 volts, d e p e n d i n g o n g e o g r a p h i c a l locale a n d t h e age of the plant's e l e c t r i c a l system. T h u s , w h e r e i t is e c o n o m i c a l l y feasible to operate a l o w p o w e r l a b o r a t o r y s u p p l y f r o m a single phase, 120-volt ac c i r c u i t , i t is q u i t e t h e c o n t r a r y f o r a large m a n u f a c t u r i n g i n s t a l l a t i o n . I m m e d i a t e l y f o l l o w i n g t h e i n p u t ac t e r m i n a l s , e l e c t r i c a l c o d e r e q u i r e s a three-phase

d i s c o n n e c t s w i t c h o r c i r c u i t breaker, either i n t e r n a l o r

e x t e r n a l t o t h e rectifier. T h i s m a k e s i t p o s s i b l e t o isolate safely t h e i n coming power for maintenance.

F o l l o w i n g t h e d i s c o n n e c t s w i t c h o r cir

c u i t breaker, there c a n also b e a contactor o r starter

(which may be


r e m o t e l y o p e r a t e d ) to c o n t r o l the i n p u t p o w e r d u r i n g v a r i o u s o p e r a t i o n a l phases.

A c o n t a c t o r i s a m a g n e t i c a l l y a c t u a t e d s w i t c h t o energize a n d

d e - e n e r g i z e the p r i m a r y ac current. A starter is a contactor w i t h t h e r m a l o v e r l o a d sensors that a u t o m a t i c a l l y disengage the starter i n t h e event of excessive a c c u r r e n t f o r a p e r i o d o f t i m e . W h e r e a s a t u r a b l e reactor o r s i l i c o n c o n t r o l l e d rectifier ( S C R ) p r i m a r y c o n t r o l l e r is u s e d , i t is f r e q u e n t l y u s e d as the p o w e r d e a c t i v a t o r t o save t h e a d d i t i o n a l expense of a contactor or starter. Input

Starter

Controller Transformer Rectifier

T^FI

AC

niter

iï^h

DC τ

Feedback Figure 1.

Basic components of a rectifier

Controller. A l t h o u g h t h e c o n t r o l l e r m a y b e l o c a t e d i n the p r i m a r y or secondary currents of t h e p o w e r transformer ( s u b s e q u e n t l y discussed ), most present d a y e l e c t r o c o a t i n g rectifier designs u t i l i z e t h e m i n t h e p r i mary.

T h e p r i m a r y currents are t y p i c a l l y less t h a n t h e secondary, a n d

for c e r t a i n types of s o l i d state c o n t r o l , p r i m a r y c o n t r o l results i n less ripple. T h e simplest a n d least flexible c o n t r o l results f r o m n o t h a v i n g one. T h e o u t p u t voltage is d e t e r m i n e d b y l i n e v o l t a g e c o n d i t i o n s

(±10%),

t r a n s f o r m e r turns r a t i o , a n d l o a d ( — 1 0 / 1 5 % a t f u l l c u r r e n t ) .

For a

non-sensitive a p p l i c a t i o n , w i t h n o r e g a r d to c o a t i n g thickness o r q u a l i t y , coverage c a n b e o b t a i n e d . H o w e v e r , c o m p e n s a t i o n cannot b e m a d e f o r v a r i a b l e s , i n c l u d i n g t h e most sensitive—the c o a t i n g s o l u t i o n . A t a p s w i t c h c o n t r o l , m a n u a l o r m o t o r o p e r a t e d , offers l i m i t e d r a n g e c o n t r o l ( 3 0 - 1 0 0 % ) i n a discrete n u m b e r o f steps that varies t h e o u t p u t v o l t a g e b u t achieves n o r e g u l a t i o n i n response to l i n e or l o a d changes. V a r i a b l e t r a n s f o r m e r c o n t r o l , m a n u a l o r m o t o r o p e r a t e d , gives a range ( 0 - 1 0 0 % ) o f almost stepless o u t p u t c o n t r o l b u t o t h e r w i s e has t h e

In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

64

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OF

COATINGS

r e g u l a t i o n deficiencies of a t a p s w i t c h . N e i t h e r a t a p s w i t c h n o r v a r i a b l e t r a n s f o r m e r is e c o n o m i c a l l y feasible for h i g h p o w e r p l a n t -installations. T h e i n d u c t i o n r e g u l a t o r is a m o t o r d r i v e n ( a l t h o u g h m a n u a l l y oper able f o r e m e r g e n c y o p e r a t i o n )

m e c h a n i s m w h i c h " b u c k s o u t " or n u l l s

p a r t of the i n c o m i n g ac voltage to p r o v i d e c o n t i n u o u s l y v a r i a b l e c o n t r o l of the o u t p u t voltage. T h i s t y p e of c o n t r o l is i n f r e q u e n t l y e n c o u n t e r e d i n o p e r a t i n g systems o w i n g to the size, cost, a n d l i m i t e d c o n t r o l a c c u r a c y of a c l o s e d l o o p w i t h a m o t o r d r i v e as one element of the system. A saturable core reactor c o n t r o l is f r e q u e n t l y u s e d f o r e l e c t r o c o a t i n g Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

rectifier c o n t r o l , a n d it, p l u s S C R ' s , a c c o u n t f o r p r o b a b l y 9 0 - 9 5 %

of

c u r r e n t p r o d u c t i o n . T h e o p e r a t i o n a l p r i n c i p l e of the reactor is b a s e d o n the i n c o m i n g ac c u r r e n t s b e i n g l o o p e d a r o u n d a core of m a g n e t i c a l l y s a t u r a b l e m a t e r i a l . A bias w i n d i n g is also l o o p e d t h r o u g h the core, a n d d c c u r r e n t is a p p l i e d to this w i n d i n g . I n the absence of a d c bias c u r r e n t , the c o r e m a t e r i a l is u n s a t u r a t e d d u r i n g the ac c y c l e , a n d the

voltage

o u t p u t is m i n i m u m whereas f u l l bias c u r r e n t results i n core saturation ( m i n i m u m i n d u c t i v e r e a c t a n c e ) a n d the m a x i m u m voltage o u t p u t . o u t p u t r a n g e of c o n t r o l is 5 - 9 5 %

The

of f u l l o u t p u t voltage.

O f p a r t i c u l a r interest w i t h this t y p e of c o n t r o l , as w e l l as the S C R , is its effect u p o n the i n c o m i n g s i n u s o i d a l voltage, as s u p p l i e d b y

the

electric u t i l i t y . W h e r e a s the t a p s w i t c h , v a r i a b l e transformer, a n d i n d u c t i o n r e g u l a t o r decrease the a m p l i t u d e of the i n c o m i n g w a v e a n d c o n d u c t t h r o u g h o u t the entire c y c l e , a p h a s i n g c o n t r o l of the saturable

reactor

a n d S C R t y p e d o not alter the a m p l i t u d e b u t rather c o n d u c t f o r o n l y a f r a c t i o n of the c y c l e . T h i s is s h o w n i n F i g u r e 2 w h e r e , for e x a m p l e , c o n d u c t i o n w o u l d o c c u r f r o m 135 to 180 degrees o n the first h a l f of the c y c l e a n d 315 to 360 degrees o n the second h a l f c y c l e . T h e e q u a t i o n for the o u t p u t voltage i s :

where Ε

= peak voltage oit = phase angle of i n c o m i n g voltage (0-x rad) θ ι = angle at i n i t i a t i o n of c o n d u c t i o n p

A s a result of this m e t h o d of v a r y i n g the o u t p u t voltage, the i n c o m i n g voltage a n d c u r r e n t are c h o p p e d i n t o segments.

T h i s results i n w a v e f o r m

d i s t o r t i o n , a n d the o u t p u t r i p p l e is greater t h a n that p r o d u c e d w i t h a tap s w i t c h or v a r i a b l e t r a n s f o r m e r controller. R i p p l e is d i s c u s s e d b e l o w . A n S C R is a s o l i d state p h a s i n g c o n t r o l w h i c h operates o n the same p r i n c i p l e as a saturable reactor.

Its c h i e f differences are a c o n s i d e r a b l y



4.

M I T C H E L L

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Power Supplies

Figure 2.

Phase controller conduction angle

smaller size, greater efficiency, faster response, a n d 0 - 1 0 0 %

operating

range. P r o p o n e n t s of saturable reactor controllers cite t h e i r a b i l i t y to w i t h stand h e a v y c u r r e n t surges, w h i c h m a y o c c u r because of large t a n k loads or s h o r t i n g of t h e tank b y d a n g l i n g o r s c r a p i n g parts.

However, good

e n g i n e e r i n g d e s i g n of a n S C R c o n t r o l l e r protects i t against these surges, as w e l l as the tank a n d l o a d . Manual/Automatic

Control.

M a n u a l a n d automatic

controls a r e

a v a i l a b l e f o r l a b o r a t o r y or p r o d u c t i o n rectifiers. A m a n u a l c o n t r o l estab lishes a n o m i n a l o u t p u t voltage, w h i c h m a y b e affected b y l i n e v o l t a g e or l o a d changes.

I n a n extreme case, the o u t p u t voltage c o u l d d r o p as

l o w as 2 5 % , f o r example. A n a u t o m a t i c c o n t r o l samples the o u t p u t voltage a n d / o r c u r r e n t a n d compares i t w i t h a reference

voltage.

Differences b e t w e e n t h e o u t p u t

a n d reference constitute a n error s i g n a l w h i c h is u s e d , after a p p r o p r i a t e s i g n a l c o n d i t i o n i n g , to alter the o u t p u t a p p r o p r i a t e l y . T y p i c a l r e g u l a t i o n a c h i e v e d i n p r a c t i c e is ± 2 % f o r a saturable reactor o r i n d u c t i o n r e g u lator a n d ±i%

o r better f o r a n S C R .

Transformer. T h e t r a n s f o r m e r serves t w o p r i m a r y p u r p o s e s : ( 1 ) S t e p - u p o r s t e p - d o w n of t h e p r i m a r y l i n e v o l t a g e to t h e neces sary s e c o n d a r y voltage, so that i n c o n j u n c t i o n w i t h t h e selected c i r c u i t c o n f i g u r a t i o n , t h e r e q u i r e d d c v o l t a g e is o b t a i n e d . ( 2 ) I s o l a t i o n of the l o a d c u r r e n t f r o m t h e l i n e c u r r e n t , m a k i n g i t possible to g r o u n d either p o l a r i t y of the d c o u t p u t , or neither.


66

ELECTRODEPOSITION

Rectifier or Rectifier/Control.

OF

COATINGS

I n the c o n t r o l a n d / o r r e c t i f i c a t i o n

( s h o w n i n F i g u r e 1) w h i c h m a y o c c u r i n the s e c o n d a r y c i r c u i t of t h e transformer, the c o n t r o l l e r is u s u a l l y i n the p r i m a r y . I n the cases w h e r e the c o n t r o l l e r is i n the secondary, it c o u l d be a saturable core reactor b u t is m o r e t y p i c a l l y S C R ' s or S C R ' s a n d d i o d e rectifiers. W i t h a p r i m a r y c o n troller, diodes are u s e d i n the secondary because o n l y r e c t i f i c a t i o n is required. S C R s i n the s e c o n d a r y c i r c u i t of the t r a n s f o r m e r p e r f o r m the same f u n c t i o n as i n the p r i m a r y — p h a s e c o n t r o l of the a l t e r n a t i n g voltage. T h i s Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

t y p e o f c o n t r o l is p a r t i c u l a r l y advantageous

i f the i n c o m i n g p r i m a r y

v o l t a g e is greater t h a n 600 volts a c — f o r e x a m p l e , 2400 or 4160

volts.

A p r i m a r y S C R c o n t r o l l e r w o u l d r e q u i r e a s t e p - d o w n i s o l a t i o n trans f o r m e r to r e d u c e the i n c o m i n g voltage to a v a l u e ( 2 2 0 - 5 5 0 volts

ac)

c o m p a t i b l e w i t h the S C R p e a k reverse voltage r a t i n g . A s e c o n d a r y S C R is subjected to the potentials t y p i c a l of electrocoating voltages a n d there fore does not r e q u i r e a d d i t i o n a l t r a n s f o r m a t i o n . D i o d e s are c o m b i n e d w i t h S C R ' s for cost r e d u c t i o n ( f o r c o m p a r a b l e c u r r e n t a n d voltage r a t i n g , d i o d e s are cheaper t h a n S C R ' s ) a n d s i m p l e r S C R triggering requirements.

H o w e v e r , w h e r e the c u r r e n t o u t p u t re

quires the p a r a l l e l i n g of S C R ' s , c a r e f u l e n g i n e e r i n g is necessary to assure b a l a n c i n g of the S C R c u r r e n t s — a m o r e d i f f i c u l t task t h a n b a l a n c i n g o n l y d i o d e currents.

I n a d d i t i o n , the S C R - d i o d e c o m b i n a t i o n p r o d u c e s

a

greater r i p p l e o u t p u t t h a n a p r i m a r y S C R or a l l S C R secondary c o n troller.

T h i s necessitates g r e a t l y i n c r e a s e d

filtering

requirements

where

the specifications i n d i c a t e a l o w r i p p l e ( 1 0 - 2 0 % ) at short c o n d u c t i o n cycles ( 2 5 - 3 0 % of f u l l voltage o u t p u t ) .

Operation of a Rectifier Circuit Protection. C i r c u i t p r o t e c t i o n appears i n v a r i o u s f o r m s a n d serves the m u l t i p l e purposes of p e r s o n n e l a n d e q u i p m e n t

protection,

m i n i m i z a t i o n of fire h a z a r d , a n d p r o t e c t i o n of the p r o d u c t b e i n g electrocoated.

F u s e s m a y be u s e d to protect S C R ' s , d i o d e s , transformers, a n d

other c o m p o n e n t s , o f t e n i s o l a t i n g the f a u l t y p a r t or s i m u l t a n e o u s l y shut t i n g the rectifier off. C i r c u i t breakers are u s e d for the same p u r p o s e a n d r e q u i r e o n l y r e - e n g a g e m e n t rather t h a n r e p l a c e m e n t to resume o p e r a t i o n . I n t h e event of i n a d e q u a t e c o o l i n g of S C R ' s , d i o d e s , transformers, or other c o m p o n e n t s , t e m p e r a t u r e sensors are u s e d w h i c h protect b y s h u t t i n g off a p a r t of or the entire system. E l e c t r o c o a t i n g rectifiers operate i n i n d u s t r i a l plants i n w h i c h there m a y also b e w e l d e r s , large motors, or other e l e c t r i c a l m a c h i n e r y w h i c h c a n cause l i n e v o l t a g e transients.

T h e s e transients m a y b e as large as

several h u n d r e d volts a n d m a y cause d a m a g e to S C R ' s , diodes, a n d other


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M I T C H E L L

s o l i d state devices, unless p r o p e r l y p r o t e c t e d .

Protection usually con

sists of s n u b b e r n e t w o r k s — r e s i s t o r / c a p a c i t o r c o m b i n a t i o n s w h i c h b y p a s s the transient a r o u n d t h e p r o t e c t e d c o m p o n e n t ; s e l e n i u m surge suppressors w h i c h absorb t h e transient a n d dissipate i t as heat, zener d i o d e s , or gas d i s c h a r g e devices. I n a n electrocoating o p e r a t i o n , t a n k shorts c a n o c c u r because the p a r t ( a n o d e ) is o v e r s i z e d o r sufficiently a g i t a t e d b y t h e s o l u t i o n m o v e m e n t to t o u c h t h e tank w a l l ( c a t h o d e ) .

Similarly, i n a conveyor opera

tion, a part might drop a n d become wedged. Consider, for example, the Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

effect of a c a r b o d y w h i c h , w h i l e r e m a i n i n g c o n n e c t e d t o the anode, becomes j a m m e d against t h e tank. T h e several possibilities w h i c h c o u l d o c c u r i n c l u d e b u r n i n g a h o l e i n the tank, m e l t i n g p a r t of the b o d y , or d r a w i n g s u c h excessive c u r r e n t that t h e rectifier is d e s t r o y e d . T h i s t y p e of p r o t e c t i o n is d i s c u s s e d b e l o w . F i n a l l y , some degree of p r o t e c t i o n is offered b y a u d i b l e a n d / o r v i s u a l w a r n i n g systems i n w h i c h it is d e s i r e d to i n d i c a t e a m a l f u n c t i o n without

ceasing o p e r a t i o n .

failsafe measures

H o w e v e r , these s h o u l d b e s u p p o r t e d b y

i n the event the w a r n i n g signals are u n n o t i c e d o r

unheeded. O u t p u t Control. T h e o u t p u t c o n t r o l c a n r a n g e i n c o m p l e x i t y f r o m the s i m p l e m a n u a l setting of a t a p s w i t c h or v a r i a b l e t r a n s f o r m e r c o n t r o l to a n a u t o m a t i c v o l t a g e / c u r r e n t c o n t r o l that w i l l regulate to less t h a n ±1%

i n voltage a n d c u r r e n t w i t h 0 - 1 0 0 % v o l t a g e a n d c u r r e n t c o n t r o l

range. T h i s is a c c o m p l i s h e d b y t h e s a m p l i n g , f e e d b a c k , error s i g n a l g e n eration t e c h n i q u e p r e v i o u s l y n o t e d . T h e o u t p u t v o l t a g e a n d c u r r e n t are t y p i c a l l y r e a d o n ± 2 % meters, unless m o r e expensive a n d accurate meters are specified b y t h e customer. D e s p i t e the s e e m i n g p a r a d o x of a better t h a n ± 1 % c o n t r o l a n d ± 2 % meters, this indicates r e p e a t a b i l i t y (as d e t e r m i n e d b y instruments o f greater a c c u r a c y ) of t h e o u t p u t setting over a n e x t e n d e d p e r i o d of t i m e or w h e n s w i t c h e d r e p e a t e d l y off a n d o n whereas t h e o u t p u t c a n o n l y b e set w i t h i n ± 2 % . A m p e r e - h o u r meters are often specified i n e l e c t r o c o a t i n g installations to measure the t o t a l i n t e g r a t e d a m o u n t of e n e r g y e x p e n d e d f o r a c o a t i n g application.

T h e system c a n b e d e s i g n e d so that n o t o n l y are t h e t o t a l

ampere-hours r e c o r d e d , b u t a s i g n a l c a n b e p r o v i d e d to r e p l e n i s h solids or other constituents as they are d e p l e t e d . A c o n t r o l that is c o m m o n l y specified i n c o n v e y o r i z e d e l e c t r o c o a t i n g installations is the h i g h - l o w v o l t a g e c o n t r o l . I f the c o n v e y o r is s t o p p e d f o r m a n y m i n u t e s a n d parts are left i n t h e t a n k at f u l l c o a t i n g v o l t a g e , excessive film thickness results. N o t o n l y is this expensive b u t o f t e n t h e c o a t i n g o b t a i n e d is u n d e s i r a b l e , a n d the p a r t m u s t b e s t r i p p e d a n d r e coated. T u r n i n g the rectifier off i m m e d i a t e l y a l l o w s d i s s o l u t i o n of t h e film


68

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COATINGS

a l r e a d y d e p o s i t e d , a n d c o u l d result i n start-up w i t h a tank f u l l of n e a r l y u n c o a t e d parts, thus c r e a t i n g a n a b n o r m a l c u r r e n t d e m a n d o n the r e c t i fier.

T o a v o i d this p r o b l e m , the rectifier is p r o g r a m m e d so that w h e n

the c o n v e y o r is s t o p p e d , the v o l t a g e is s w i t c h e d to a l o w e r v a l u e — a p p r o x i m a t e l y 50 volts. It m a y r e m a i n at this l o w e r v a l u e , or a t i m e r m a y t u r n the u n i t off i f the c o n v e y o r r e m a i n s stationary f o r longer t h a n a g i v e n t i m e , t y p i c a l l y five m i n u t e s . Rectifiers w i t h a u t o m a t i c controls are t y p i c a l l y d e s i g n e d to sense the o u t p u t c u r r e n t a n d regulate the o u t p u t s u c h that it cannot exceed the Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

m a x i m u m of w h i c h the rectifier is c a p a b l e .

T h i s is a c h i e v e d i n p h a s i n g

t y p e controls b y d e c r e a s i n g the o u t p u t voltage to a v a l u e s u c h that o n l y m a x i m u m c u r r e n t is p r o d u c e d . T h u s , i n the event of a fixed short i n the tank or at the rectifier o u t p u t , the meters m i g h t i n d i c a t e that m a x i m u m c u r r e n t was b e i n g p r o d u c e d at zero voltage.

T h i s is not p o s s i b l e since

O h m ' s l a w is s t i l l a p p l i c a b l e , b u t a l o w resistance short requires so l i t t l e voltage for f u l l o u t p u t c u r r e n t that the v o l t a g e appears to be n i l , e s p e c i a l l y o n a 300- or 500-volt scale. T h e d c o v e r l o a d is a c u r r e n t - l i m i t i n g d e v i c e w h i c h also senses a c u r rent m a x i m u m a n d either greatly reduces the o u t p u t or shuts it off en tirely.

It is n o r m a l l y set s l i g h t l y h i g h e r ( 5 - 1 0 % ) t h a n the

automatic

c u r r e n t l i m i t , a n d as a s a f e g u a r d to it. H o w e v e r , it is possible to set i t l o w e r t h a n the c u r r e n t l i m i t p o i n t to g u a r d against o v e r l o a d i n g the r e c t i fier c a p a b i l i t y . F o r e x a m p l e , if too m a n y parts are i n a n o v e r l o a d e d coat i n g tank, this forces the rectifier to l i m i t c u r r e n t a u t o m a t i c a l l y . reduces the c o a t i n g voltage, a n d i n a d e q u a t e

film

This

coverage c a n result,

w h i c h m a y not be d i s c o v e r e d u n t i l i n the field. W i t h the d c o v e r l o a d set l o w e r ( 5 - 1 0 % ) t h a n current, the u n i t is shut off a n d the c o n v e y o r over l o a d i n g is i m m e d i a t e l y detected. Various claims have been made for pulsed p o w e r i n electrocoating a p p l i c a t i o n s , a n d it c a n b e p r o g r a m m e d i n t o rectifiers that are d e s i g n e d f o r this a p p l i c a t i o n . H o w e v e r , its efficacy has not b e e n sufficiently estab l i s h e d to w a r r a n t the a d d i t i o n a l costs i n v o l v e d ( 2 5 - 1 0 0 % ). If the e l e c t r o c o a t i n g a p p l i c a t i o n is b a t c h r a t h e r t h a n c o n v e y o r , the i n i t i a l c u r r e n t d e m a n d m u s t be c o n s i d e r e d w h e n p o w e r is first a p p l i e d . E i t h e r the rectifier m u s t h a v e a c u r r e n t c a p a b i l i t y to d e l i v e r the instant

of c u r r e n t d e m a n d , or a p p r o p r i a t e c u r r e n t l i m i t i n g m u s t

first be

i n c l u d e d to p r o t e c t the rectifier. Input A c Requirements. T h e p o w e r factor ( ^ 1 . 0 ) of a rectifier is the ratio of the p o w e r i n p u t ( k i l o w a t t ) to the k i l o v o l t - a m p e r e (leva) i n p u t , w h i c h is p r o p o r t i o n a l to t h e p r o d u c t of root-mean-square l i n e v o l t a g e a n d current.

(rms)

T h e p o w e r factor is 0.90-0.92 at f u l l o u t p u t

b u t d r o p s s i g n i f i c a n t l y at r e d u c e d v o l t a g e a n d c u r r e n t o w i n g to c h a n g i n g


4.

M I T C H E L L

69

Power Supplies

l o a d c o n d i t i o n s a n d t h e w a v e f o r m d i s t o r t i o n created b y a p h a s i n g t y p e controller. T h e efficiency ( ^ 1 . 0 ) of a rectifier is t h e ratio of t h e p o w e r o u t p u t ( k w ) to t h e p o w e r i n p u t ( k w ) . A n electrocoating rectifier has a n effi c i e n c y of 0.92-0.95 at f u l l o u t p u t voltage a n d current, w h i c h decreases s o m e w h a t w i t h l o w e r e d o u t p u t voltages. T h e decrease results f r o m f a i r l y constant

p o w e r losses i n components

whereas

t h e l o a d p o w e r c o n s u m p t i o n becomes less a n d t h e r a t i o is

s u c h as t h e w i r i n g a n d diodes


lowered. O u t p u t D c Requirements. T h e o u t p u t voltage r e q u i r e d is p r i n c i p a l l y a f u n c t i o n of t h e electrocoating m a t e r i a l , r e q u i r e d t h r o w - p o w e r ( a b i l i t y to coat d e e p recesses, e t c . ) , c o a t i n g t i m e , a n d t a n k parameters.

T h e cur

rent r e q u i r e d is m a i n l y d e t e r m i n e d b y the a m p e r e - h o u r / p o u n d ( o r c o u l o m b / g r a m ) specification of t h e m a t e r i a l , t h e n u m b e r of p o u n d s p e r u n i t area f o r a g i v e n c o a t i n g thickness, a n d the area of m e t a l i n the tank to be coated.

C o n s i d e r a t i o n m u s t also b e g i v e n to the t y p i c a l c u r r e n t vs.

t i m e characteristic of the c o a t i n g process. Power Supply Ripple. A l t h o u g h a rectifier o u t p u t is r e f e r r e d t o as d i r e c t current, i t is n o t i n a n y sense as d i r e c t a n d s m o o t h a c u r r e n t as that o b t a i n e d f r o m a battery.

It c a n b e s m o o t h e d w i t h r i p p l e

t y p i c a l l y a c o m b i n a t i o n of i n d u c t a n c e a n d capacitance,

filtering,

to almost a n y

extent, b u t the cost increases p r o p o r t i o n a t e l y . T h e variations i n t h e d c o u t p u t result f r o m the three phases o f alter n a t i n g currents f l o w i n g i n t h e p r i m a r y a n d w h i c h m a y b e d i s t o r t e d b y the controller. T h e resultant rectified o u t p u t represents t h e o v e r l a p p i n g of the tops a n d i n v e r t e d bottoms of t h e t r a n s f o r m e d p r i m a r y voltages, a n d t h e e n s u i n g crests a n d valleys. T h e s e constitute t h e r i p p l e variations i n t h e d c o u t p u t , w h i c h are m e a s u r e d w i t h a " t r u e " r m s m e a s u r i n g i n strument.

T h e " t r u e " results f r o m the i n a b i l i t y o f o r d i n a r y ac p a n e l a n d

p o r t a b l e meters to r e a d the correct r m s v a l u e of t h e a l t e r n a t i n g c o m p o nent. T h e f o r m ( r i p p l e ) factor is t h e r m s a l t e r n a t i n g c o m p o n e n t d i v i d e d b y the average d c v a l u e ( this c a n b e ascertained w i t h a n y average r e a d i n g meter ). T a p s w i t c h a n d v a r i a b l e transformer

c o n t r o l l e d rectifiers h a v e

r i p p l e of 4 . 5 - 5 . 0 % t h r o u g h o u t the entire range of c o n t r o l .

a

Saturable

reactors a n d S C R ' s h a v e a f u l l o u t p u t v a l u e of 4 . 5 - 5 . 0 % , w h i c h increases to 5 0 - 1 0 0 % at l o w o u t p u t voltages ( 1 0 - 2 5 % ). A t y p i c a l l y specified v a l u e of r i p p l e c o r r e c t i o n is 1 0 % r i p p l e at 1 0 % voltage a n d current. O p i n i o n i n t h e electrocoating i n d u s t r y varies as to t h e necessity or degree of r i p p l e filtering r e q u i r e d .

T h e r e is some e v i d e n c e that r i p p l e

contributes to increased h e a t i n g effects i n the d e p o s i t e d film, a n d there is s u s p i c i o n that c o a t i n g r u p t u r e m a y b e r e l a t e d to r i p p l e crests. P a r t of


70

ELECTRODEPOSITION

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the d i f f i c u l t y arises f r o m differences i n r i p p l e o u t p u t of l a b o r a t o r y a n d p r o d u c t i o n rectifiers, a n d this contributes to v a r i a t i o n i n the c o r r e s p o n d i n g results.

Practical Considerations F o r those not f a m i l i a r w i t h rectifiers, yet i n v o l v e d i n their selection,


a f e w g u i d e l i n e s that c a n be u s e d are: ( 1 ) T h e rectifier s h o u l d b e d e s i g n e d w i t h r e g a r d for the safety of o p e r a t i n g p e r s o n n e l , the parts b e i n g coated, a n d the rectifier. P r o p e r de s i g n a l l o w s f o r access to a l l parts of the u n i t b u t interlocks shut off a l l p o w e r i f e n t r y is a t t e m p t e d w h i l e e n e r g i z e d . T h e o u t p u t c u r r e n t s h o u l d b e l i m i t e d to the m a x i m u m of w h i c h the u n i t is safely c a p a b l e , a n d p r o v i s i o n s h o u l d b e m a d e to shut d o w n i n the event of a tank short. T h e o v e r a l l d e s i g n s h o u l d c o n f o r m to N E M A a n d J I C e l e c t r i c a l standards. ( 2 ) T h e d e s i g n a n d c o n s t r u c t i o n s h o u l d i n c o r p o r a t e c o m p o n e n t s of i n d u s t r i a l q u a l i t y a n d g o o d e n g i n e e r i n g practices to m a x i m i z e r e l i a b i l i t y . M a i n t e n a n c e m u s t be possible w i t h r e g u l a r p l a n t p e r s o n n e l , a n d a l l parts of the u n i t m u s t b e c a p a b l e of r a p i d r e p l a c e m e n t i n the event of a f a i l u r e . M a i n t e n a n c e s h o u l d p r e f e r a b l y be o n a subsystem basis, s u c h as p r i n t e d c i r c u i t b o a r d s , or a r e p l a c e a b l e chassis, so that service p e r s o n n e l are not r e q u i r e d to test c o m p o n e n t s or to b e i n t i m a t e l y f a m i l i a r w i t h the c i r c u i t r y . ( 3 ) T h e rectifier m u s t i n c l u d e a p p r o p r i a t e features that e m p h a s i z e e c o n o m i c a l o p e r a t i o n , p r o v i d e flexible o p e r a t i o n , a n d h a v e a m i n i m u m of m a i n t e n a n c e r e q u i r e m e n t s a n d m i n i m u m obsolescence.

Figure 3.

Current vs. time (HT-300). Horizontal: 3 sec/div; vertical: 0.8 amp/div.

Electrocoating Current Measurements T h e c u r r e n t as a f u n c t i o n of t i m e i n a n e l e c t r o c o a t i n g c y c l e is not constant, as i n a p l a t i n g b a t h , f o r e x a m p l e , b u t rather exhibits a large i n i t i a l d e m a n d w h i c h decreases as the d e p o s i t e d l a y e r accumulates.

In

this respect the c y c l e is s i m i l a r to that of a n o d i z i n g . T h e c u r r e n t c a n be


4.

MITCHELL

71

Power Supplies

o b s e r v e d w i t h a n oscilloscope that v e r t i c a l l y d i s p l a y s t h e voltage d r o p across a c a l i b r a t e d resistor that is i n series w i t h t h e c u r r e n t flow w h i l e s w e e p i n g h o r i z o n t a l l y . T h e oscilloscope sweep is set t o b e i n i t i a t e d near t h e start o f t h e c o a t i n g c y c l e . A p e n recorder, w h i c h has a m u c h s l o w e r response t h a n a n oscilloscope, w i l l c o r r e c t l y d i s p l a y this c u r v e 5 - 1 0 sec onds after the i n i t i a l surge b u t w i l l n o t d i s p l a y the r a p i d c u r r e n t t r a n s i t i o n


at t h e b e g i n n i n g .

Figure 4.

Current vs. time (LT-300 v). Horiz: 3 sec/div; vert: 0.8 amp/div.

Figure 5.

Current vs. time (15303-300 v). Horiz: 3 sec/ div; vert: 1.6 amp/div.

L a b o r a t o r y experiments w e r e c o n d u c t e d to demonstrate t h e o s c i l l o g r a p h i c d i s p l a y of electrocoating c u r r e n t vs. t i m e , p a r t i c u l a r l y of t h e i n i t i a l surge. M e a s u r e m e n t s w e r e m a d e w i t h P a r k e r C o . B o n d e r i t e E P - 1 t r e a t e d panels i n a s o l u t i o n v o l u m e of a p p r o x i m a t e l y 1.7 liters, a c h r o m e p l a t e d steel c a t h o d e w i t h a n area of 90 s q c m , a n a n o d e area of a p p r o x i m a t e l y 100 s q c m ( o n e s i d e ) , a n d a n a n o d e - c a t h o d e s p a c i n g of 10 c m . T h e solution temperature was 2 4 ° C .

Coatings used were S h e r w i n W i l

liams K S 3 0 A ( h i g h t h r o w ) , K S 3 0 B ( l o w t h r o w ) , a n d P P G S15303 ( h i g h m e d i u m t h r o w ) , a n d i n a l l cases a p o t e n t i a l of 300 volts d c w a s u s e d . T h e results a n d e x p e r i m e n t a l a r r a n g e m e n t are s h o w n i n F i g u r e s 3 - 1 1 .


72

ELECTRODEPOSITION

O F COATINGS

I n F i g u r e 3 t h e h i g h t h r o w K S 3 0 A e x h i b i t e d a p e a k c u r r e n t of 3.4 a m p , w h i c h d r o p p e d to h a l f t h e i n i t i a l c u r r e n t i n a b o u t 0.5 sec. I n F i g u r e 4 the l o w t h r o w K S 3 0 B h a d a n i n i t i a l p e a k c u r r e n t of 2.7 a m p a n d d r o p p e d to h a l f this v a l u e i n a b o u t 3 sec. F i g u r e 5 shows that t h e S15303 h a d a n i n i t i a l c u r r e n t p e a k of 7.5 a m p w h i c h d r o p p e d to h a l f this a m o u n t i n a p p r o x i m a t e l y 1 sec. T h e results i n F i g u r e 6 w e r e o b t a i n e d u n d e r t h e same c o n d i t i o n s as those i n F i g u r e 5 except that t h e h o r i z o n t a l w a s c h a n g e d to 0.5 s e c / d i v i s i o n r a t h e r t h a n 3 s e c / d i v i s i o n . I n this case the i n i t i a l p e a k c u r r e n t is Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

8 a m p , w i t h a s e c o n d p e a k w h i c h occurs a b o u t 1.2 seconds after t h e first. T h e s e c o n d p e a k w a s later established as b e i n g t h e c u r r e n t surge t o the rear of the a n o d e w i t h respect to t h e cathode. F i g u r e 7 is a d u p l i c a t i o n of F i g u r e 3 except that t h e h o r i z o n t a l w a s c h a n g e d to 2 m s e c / d i v i s i o n to demonstrate better the i n i t i a l surge. T h e

Figure 6.

Current vs. time (15303-300 v). Horiz: 0.5 sec/div; vert: 1.6 amp/div.

Figure 7. Current vs. time (HT-300 ν). Horiz: 2 msec/div; vert: 0.8 amp/div.


4.

73

Power Supplies

M I T C H E L L

p o w e r s u p p l y r i p p l e (>—4.5% ) is c l e a r l y s h o w n , a n d this accounts f o r t h e w i d t h o f t h e c u r r e n t trace s h o w n i n t h e p r e v i o u s figures. T h e s l i g h t d i s t o r t i o n of the first c y c l e of r i p p l e is c a u s e d b y chatter of t h e r e l a y u s e d to a p p l y d c p o t e n t i a l . H o w e v e r , despite t h e i n c r e a s e d h o r i z o n t a l m a g n i f i c a t i o n , i t is d i f f i c u l t to estimate the surge r i s e t i m e , b u t i t is p r o b a b l y o n t h e o r d e r of 200 ^sec.


10

cm

(4")

A n o d e #1 (Front)

A n o d e #2 (Rear)

Cathode

0.95

Figure 8.

cm

(3/R")

Anode placement for outside front and rear measurements

F i g u r e 8 shows the e x p e r i m e n t a l a r r a n g e m e n t u s e d to o b t a i n t h e d a t a of F i g u r e 9. T h e anodes consisted of t w o p a n e l s w i t h t h e i r i n t e r i o r sur faces m a s k e d . Separate a n o d e leads ( c o m m o n c a t h o d e ) w e r e c o n n e c t e d to t h e p o w e r s u p p l y , a n d t h e a n o d e currents w e r e m o n i t o r e d separately. A n o d e surface 1 ( f r o n t ), nearest t h e c a t h o d e , is r e f e r e n c e d to the abscissa l i n e w h i c h is o n t h e b o t t o m ; a n o d e surface 2 ( r e a r ) , furthest f r o m the c a t h o d e , is r e f e r e n c e d to t h e abscissa l i n e w h i c h is s e c o n d f r o m t h e b o t t o m i n F i g u r e 9. T h e p e a k c u r r e n t o n a n o d e 1 is 4 a m p , a n d o n a n o d e 2 it is 0.8 a m p ; b o t h are a p p r o x i m a t e l y 0.8 a m p w i t h i n 1 . 5 - 2 sec after p o w e r is i n i t i a t e d .

T h e p e a k f o r a n o d e 2 occurs a b o u t 0.5 sec after that f o r

anode 1. F i g u r e 10 is a sketch of t h e e x p e r i m e n t a l a r r a n g e m e n t f o r o b t a i n i n g the d a t a s h o w n i n F i g u r e 11. T h e anodes a g a i n consisted o f t w o panels, b u t t h e exterior surfaces w e r e m a s k e d . Separate leads e n a b l e d separate m o n i t o r i n g as before, w i t h anodes 1 ( f r o n t ) a n d 2 ( r e a r ) r e f e r e n c e d as



74

ELECTRODEPOSITION

O F COATINGS

Figure 9. Current vs. time (HT-300 ν) outside front and rear. Horiz: 0.5 sec/div; vert: 0.8 amp/div. i n F i g u r e 9. A n o d e 1 exhibits a p e a k c u r r e n t of 1.5 a m p a n d decreases to h a l f this a m o u n t i n 0.2 sec. A n o d e 2 peaks to 0.8 a m p 0.15 sec after i n i t i a t i o n of p o w e r a n d decreases to h a l f this v a l u e i n 0.95 sec. F i g u r e 12 is a g r a p h s h o w i n g t h e c u r r e n t c y c l e f o r a p h o s p h a t e d m e t a l l i c area of ca. 1000 s q f t w h i c h is e n t e r i n g a large tank.

electrocoating

T h e voltage remains constant at a b o u t 170 volts t h r o u g h o u t t h e

45-second p e r i o d .

T h e c u r r e n t d e m a n d is i n i t i a l l y 1350 a m p , decreases 10

\

cr-,

(4")

!

! Cathode 0.95

Figure 10.

cm

(3/8")

Anode placement for inside front and rear measurement


4.

75

Power Supplies

MITCHELL

to 1000 a m p i n 0.3 sec, a n d is d o w n to 800 a m p i n 4.3 sec, g r a d u a l l y decreasing

thereafter.

F i g u r e 13 has t h e same v e r t i c a l scale as F i g u r e 12, b u t t h e h o r i z o n t a l is c h a n g e d to 0.2 s e c / d i v i s i o n . T h e faster h o r i z o n t a l s w e e p exhibits less of the t o t a l c y c l e b u t m o r e of t h e i n i t i a l rise. E v e n at this sweep rate, the risetime of t h e i n i t i a l p u l s e is q u i t e short. L a b o r a t o r y measurements of samples i n s m a l l tanks i n d i c a t e a risetime of 2 0 0 - 1 0 0 0 /xsec. A c u r v e of this t y p e c a n b e i n t e g r a t e d to y i e l d t h e t o t a l n u m b e r of c o u l o m b s , a n d t h e w e i g h t of t h e s a m p l e w i l l g i v e the t o t a l grams of d e Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

p o s i t e d m a t e r i a l to d e t e r m i n e the c o u l o m b / g r a m r a t i o .

However, an

a m p e r e - h o u r m e t e r o r a c h a r t r e c o r d e r w i l l g i v e reasonable

accuracy

because t h e i n i t i a l surge exists f o r s u c h a short t i m e . A n a m p e r e - h o u r meter is c o m p l e t e l y acceptable, f o r e x a m p l e , i n c o n j u n c t i o n w i t h a n elec t r o c o a t i n g rectifier f o r i n t e g r a t e d m e a s u r e m e n t of the c u r r e n t to restore d e p l e t e d solids a u t o m a t i c a l l y o n a p e r i o d i c basis.

Figure 11. Current vs. time (HT-300 ν) inside front and rear. Horiz: 0.1 sec/ div; vert: 0.4 amp/div. 1500

1000 Current (Ampere) 500

0 0

10

20 Time

Figure 12.

30

40

50

(Second)

Current vs. time during electrocoat of 1000-sq ft area


76

ELECTRODEPOSITION

O F COATINGS

Since t h e d a t a f r o m t h e l a b o r a t o r y experiments are s i m i l a r t o that o b t a i n e d f r o m a p r o d u c t i o n f a c i l i t y ( e s p e c i a l l y i n i t i a l s u r g e ) , i t is c o n c l u d e d that t h e l a b o r a t o r y d a t a c a n b e u s e f u l i n d e s i g n i n g a p o w e r s u p p l y f o r electrocoating a p p l i c a t i o n s . F u r t h e r , t h e most r i g o r o u s d u t y r e q u i r e ment, i n terms of current d e m a n d , occurs d u r i n g t h e i n i t i a l c o a t i n g p e r i o d . H o w e v e r , this i n t e r v a l is short c o m p a r e d w i t h t h e t o t a l c o a t i n g t i m e , a n d this represents a transient c o n d i t i o n m o r e t h a n a steady-state o n e . T h e i n i t i a l current surge c a n b e p r e d i c t e d because i t represents t h e resistance of t h e c o a t i n g m a t e r i a l b e t w e e n t h e a n o d e a n d cathode, as Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch004

d e t e r m i n e d b y the specific resistance of t h e m a t e r i a l , electrode

areas,

electrode s p a c i n g , a n d a p p l i e d p o t e n t i a l . T h i s is m o r e easily d e t e r m i n e d for a n e x p e r i m e n t a l a r r a n g e m e n t t h a n f o r a c o m p l i c a t e d shape i n large production.

1000

Current (Ampere) 500

0 0

1 Time

Figure 13.

2

(Second)

Initial current surge during electrocoat of 1000sq ft area

F i n a l l y , n o t e that t h e i n i t i a l surge of 1350 a m p s h o w n i n F i g u r e 12 is e q u i v a l e n t to a n i n c o m i n g p r i m a r y ac c u r r e n t of 2 0 - 2 5 amp//xsec ( ne glecting circuit resistance).

Consequently, for the power supply circuit

designer, c o m p o n e n t s s u c h as S C r V s m u s t a n d c a n b e a p p r o p r i a t e l y se l e c t e d a n d p r o t e c t e d to ensure that r e l i a b l e o p e r a t i o n is o b t a i n e d u n d e r these transient c o n d i t i o n s .

Summary T h e i n f o r m a t i o n presented h e r e o n rectifiers w i l l h o p e f u l l y g i v e those better a c q u a i n t e d w i t h other aspects of electrocoating a n i n s i g h t i n t o a n i m p o r t a n t c o m p o n e n t of this system.

This information w i l l enable the

p r o s p e c t i v e p u r c h a s e r of s u c h e q u i p m e n t to ask k n o w l e d g e a b l e questions and

to u n d e r s t a n d some of t h e tradeoffs that m u s t b e c o n s i d e r e d , i n


4.

conjunction

77

Power Supplies

MITCHELL

with

the manufacturer,

to r e a l i z e

t h e most

economical

p e r f o r m a n c e , r e l i a b i l i t y , a n d safety.

Acknowledgment T h e a u t h o r a c k n o w l e d g e s h e l p f u l discussions w i t h M . K o l t u n i a k , V i c e - p r e s i d e n t of E n g i n e e r i n g .


Bibliography Sandford, J. E . , "Electrocoating: The Charge is O n , " Iron Age (Nov. 2, 1967). DeVittorio, J. M . , "The Electro-Mechanical Aspects of System Design for the Electrodeposition of Water Borne Coatings," The Sherwin-Williams Co. Streeter, K. L . , " D C Power Supplies for Electropainting," 1st Conference on Electropainting for the Seventies, Westinghouse Brake & Signal Co., Ltd. Koch, R. L . , "High Gloss White Electropaint," 1st Conference on Electropainting for the Seventies. Yeates, R. L . , "Electropainting," Draper, Teddington, England, 1966. Burnside, G . L., Brewer, G . E . F., Electrophoretic Coating Process, U.S. Patent 3,200,057 (1965). Oster, T . H., Cyclical Current Reversal for an Electrophoretic Deposition, U.S. Patent 3,200,058 (1965). "Canada Leads the World in Electrocoat Efficiency," Modern Finishing Methods (1969). Mitchell, C., " D C Control Systems for Electrocoating Applications," A S T M E paper FC69-127 (1969). Finn, S. R., Hasnip, J. Α., "Electrodeposition: A Current-Time Relationship," J. Oil Colour Chemists Assoc. (1965) 48. Cooke, Β. Α., Strivens, Τ. Α., "The Non-ohmic Nature of Conduction in the Electrodeposition of Paint Films," J. Oil Colour Chemists Assoc. (1968) 51. Ashby, D . , "Rectifiers to Provide Current for Electrodeposition Process," Trans. Inst. Metal Finishing (1964) 41. Burnside, G . L . , Brewer, G . E . F., Strosberg, G . G . , Igras, R. Α., "Prediction of Current Requirements (Ford process)," J. Paint Technol. (1966) 38 (493). LeBras, L . R., "Electrodeposition Theory and Mechanisms," J. Paint Technol. (1966) 38 (493). Berry, J. R., "Electrodeposition of Paint—I," Paint Technol (1963) 27 (12). Berry, J . R., "Electrodeposition of Paint—II," Paint Technol. (1964) 28 (1). Gloyer, S. W . , Hart, P. P., Cutforth, R. E . , "Electrodeposition: Theory and Practice," Official Digest (Feb. 1965). Tawn, A. R. H . , Berry, J. R., "The Electrodeposition of Paint: Some Basic Studies," J. Oil Colour Chemists Assoc. (Sept. 1965) 48. RECEIVED

May

28,

1971.


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