Unitek UNIBOND 2, User Manual

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2.07. L I N E / L O A D REGULATION - I n t h e VOLTAGE FEEDBACK Mode, t h e Line Regulation o f
the Unibond i s 1% t o t a l f o r a t13% change i n i n p u t l i n e v o l t a g e . T h e Load Regulation
is 1% f o r an 800 amp change i n output c u r r e n t . T h e response t i m e i s approximately
0.5 ms.
2.08. A C C U R A C Y - The absolute accuracy o f t h e PULSE AMPLITUDE Thumbwheel s e t t i n g s ,
measured a t t h e sensing p o i n t , i s 13% o f s e t t i n g , 1 5 mv, i n t h e VOLTAGE FEEDBACK
Mode. T h e output voltage f o r any given s e t t i n g w i l l r e p e a t w i t h i n 0.1%. T h e width
of t h e output pulse w i l l b e l i n e a r w i t h i n 0 . 0 5 ms o f t h e value s e t on t h e PULSE WIDTH
Thumbwheels.
2.09. V O L T A G E FEEDBACK MODE - I n t h i s mode, t h e Unibond w i l l maintain constant
voltage, a t t h e sensing p o i n t , a s preset on t h e PULSE AMPLITUDE Thumbwheels. WHEN
THE UNIBOND I I I S USED I N THE VOLTAGE FEEDBACK MODE ITS OUTPUT WILL BE IDENTICAL TO
THAT OF THE UNITEK UNIBOND I , Stock Numbers 01-124-01 through 0 5 , AND OUTPUT
TRANSFORMER, Stock Numbers 9-001-01 o r 10-105-01 through 0 4 , OPERATED I N THE MANUAL
MODE.
2.10. V O LTA G E + CURRENT FEEDBACK MODE - I n t h i s mode, t h e Unibond w i l l continuous-
ly increase t h e Pulse Amplitude t o a maximum o f t w i c e , o r reduce i t t o a minimum o f
one-half, o f t h e value p r e s e t on t h e Pulse Amplitude Thumbwheels i n an e f f o r t t o
maintain t h e output voltage-current product a s indicated i n Figure 2 4 . T h i s Mode i s
designed t o be e f f e c t i v e f o r Weld Resistances between 1 and 1 5 milliohms. T h e r e -
sistance connected between t h e Output Transformer Terminals and t h e Welding Load
should n o t exceed 1 . 5 milliohms. T h e resistance o f t h e Models 4 0 , 4 6 , 8 6 , 8 7 , 1 2 7 ,
and 147 Weld Heads a r e 1 . 5 milliohms o r l e s s . T h e performance o f t h e system w i l l b e
reduced a s t h e resistance between t h e Terminals o f t h e Output Transformer and t h e
Sensing Point i s increased o r i f t h e Weld Resistance i s l e s s than 5 milliohms.
2.11. V O L T A G E + CURRENT FEEDBACK MODE - HOW I T WORKS - The welding process uses
both h e a t and pressure t o form a weld. Electrode pressure i s used t o f o r c e t h e metals
together. H e a t , generated by t h e resistance o f t h e workpieces t o t h e f l o w o f e l e c -
t r i c i t y, e i t h e r melts t h e m a t e r i a l a t t h e i n t e r f a c e o r a t l e a s t reduces i t s strength
to a l e v e l where t h e surface becomes p l a s t i c . When t h e f l o w o f c u r r e n t stops, t h e
electrode f o r c e i s maintained f o r t h e f r a c t i o n o f a second required f o r t h e weld t o
cool and s o l i d i f y.
Figure 3 i l l u s t r a t e s why t h e workpieces m e l t a t t h e i r i n t e r -
face. T h e surfaces o f metal a r e q u i t e rough i f t h e y a r e e x -
amined on a molecular s c a l e . W h e n t h e metals a r e forced
together with a r e l a t i v e l y small f o r c e some o f t h e peaks make
contact. O n those peaks where t h e contact pressure i s s u f f i -
ciently high t h e oxide l a y e r breaks, forming a l i m i t e d number
of metal- t o -metal bridges. T h e weld current i s d i s t r i b u t e d
over a l a r g e area a s i t passes through t h e b u l k metal. A s i t
approaches t h e i n t e r f a c e , t h e current i s forced t o f l o w
through these m e t a l l i c bridges. T h i s "necking down" increases
the current d e n s i t y, generating enough h e a t t o cause melting.
As t h e f i r s t o f these bridges m e l t and collapse, new peaks
come i n t o contact, forming new bridges and a d d i t i o n a l current
paths. T h e resistance o f t h e molten metal i s higher t h a n t h a t
of t h e new bridges, s o t h a t t h e current f l o w t r a n s f e r s from
bridge- t o -bridge. T h i s process continues u n t i l t h e e n t i r e
interface i s molten. W h e n t h e current stops, t h e electrodes
rapidly cool t h e molten metal which s o l i d i f i e s ,
forming a weld.
CURRENT
Figure 3