DEVELOPMENT OF THE TRANSISTOR ARC WELDING INYERTER WITH PULSED AND DIRECT CURRENT

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Nov 2, 2013 (3 years and 5 months ago)

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DE
V
EL
OPMENT

OF

THE


TRANSISTOR

ARC

WELDING

INYER
TER


WITH
PULSED

AND

DIRECT


CURRENT

Wu


X i a n p i n g,

F a n g


C h e n f u, Hu


Y a j u n
Z h e n j i a n
g

S h i p b u i l d i n g

I n s t i t u t e

ABSTRACT

A transistor arc welding inverter
w i t h

pulsed and
d i r e c t

curr
ent is introduced in this paper. A
c i r c u i t

for arc
i g n i t i o n

with low current is equipped in this inverter and a

methematical model of
the
c i r c u i t

is studied. The success of realizing
i g n i t i o n

arc with low current increases the
r e l i a b i l i t y

of the inverter.
The principle of obtaining pulsed current is discussed. The measured
waveform shows that the waveform of output rectangular current
w i t h

spike in the forward of pulse
edage is very accurate and it is beneficial to the
s t a b i l i t y

of welding arc. The way of i
ncreasing
the
r e l i a b i l i t y

of the inverter is also studied preliminary in this paper.

1.
INTRODUCTION

Now ad ay s, tra ns is to r a rc w el di ng p ow er
sources are usually used in welding thin
p l a t e
and extra
-
thin plate. These power sources are
expensive. In addition
, the weight and volume
of these power sources are greater because of
main fre quency tranf ormer being empl oied. In
the type of analogous transistor arc welding
power source, transistors are used as variable
resistance and the power consumption upon
transis
tors are great, or the efficiency of
these power sourc es is very low. After arc
welding inverter was invented, the problems
mentioned a bove ha ve been solved fairly, and
the satisfactory effects have been obtained in
weldi ng thi n plat e and ex tra
-
thi n plat e
by
using MOSFET arc welding inverter[1] and high
frequency pulsed arc welding inverter[2]. But
it is d iffic ult to bu y pow er MO SFET in Ch ina

market, and the power MOSFET is very expensive,
whereas, it is easier and cheaper to buy power
transistor in China
market. Therefore, the
economic benefit in developing transistor arc
welding inv erter
w i t h

puls ed a nd d irect
current is obvious.

2.
THE CHARACTERISTICS OF THE INVERTER

The principle diagram of the inverler is
shown in
f i g.
1. Half bridge
c i r c u i t

is
emploied

in this arc welding inverter. Power
transistors are controled by the method of PWM
and the converse frequency is 15000 Hz. This
inverter is co mposed of main
c i r c u i t,
electronic control
c ir cu i t,

the circuit for arc
i g n i t i o n

w i t h

low current and the control

c i r c u i t

for external characteristics. The
inverter possesses these properties such as
follows.

main circuit


/N/22OV

The
electronic
control
circuit

The circuit
for arc igni
-
tion with low
current

The control
circuit for
external cha
-
ra
cteristic

f i g.l

The principle diagram of the inverter

31


C D Th e o p e r.
-
ci r u i t v o lt a g e i s l ov e:.
Because the tim e constant of the wel ding loop
is v er y s ma ll a n d th e sp e ed o f r es po ns e i s
q u i t e

qu ic k, s o th e ar c we ld in g i nv er te r
posse sse s e xcel len t dy nam ic
char act eri stic.
Therefore, the property of
i g n i t i o n

arc is
v er y go o d a n d t h e a r c co u l d b e i g n it e d a t
lower ope n
-
circ uit vo ltage. The op en
-
cir cuit
voltage of this inverter is 35v.

(2)

Th e o ut p ut cu rr e nt wa ve f or m s of th e

inverter are rectagnular and direct, the

pulse

frequency may be regulated froa 1 to 200 Hz.

T he p ul s e h e i gh t, t h e p u l se w i dt h an d th e

k ee p i n g a r c c ur r e n t,
a l l

o f t h e m ma y b e

reg ul ate d re spe ct iv ely. It is b en efi ci al to

adjust welding specification.

(3)

The e xte rn al ch ar ac ter is ti c c ur ve is

shown in f
ig.2. The slope of the curve is very

steep.


fi g.

2 External characteristic curve
of the inverter

(4) The process of soft turning on voltage
and current has been realized. When turning on
the inverter, the output voltage increases
g r a du a l l y w i t h t h
e c on t r o l p u l s e wi d t h
gradually increases from narrow to wide. While
the control pulsewidth reaches the
d e f i n i t e
value, the pulsewidth doesn't increase again
and th e o pen
-
ci rc uit vo lta ge is se t at
the
same time. The process of soft turning
on
voltage has be
en realized. In the process
ignition arc * by using tungsten extrode to
contact with workpiece in TIG welding, the

short
-
circuit current set by the
c i r c u i t

for
arc ignition with low current is 3A. In the
process of ignition, the current increases
gradually

from 3A to greater with the curve of
e x t e r n a l c h a r a c t e r i s t i c s l o w l y a n d
co n ti nu o us l y ch a ng es. W h il e t he c u rr en t
reaches the definite value, the current is
m ai n t a i ne d c o ns t a n t w i t h th e h e lp o f

32
c on t r o l

c i r c u i t

for external charge:

t h e

inverter. The proce
ss of
s o f t

turning

the current has beer, realized.

3.
THE CIRCUIT FOR ARC IGNITION WITH LOW
CURRENT

The me thod of pu lse widt h mod ula tion is
adopted to controd this inverter. The more
narrow the pulsewidth is, the lower the output
voltage
w i l l

be and vice ve
rsa.The arc welding
inverter always works in the mal
-
conditions,
the load often changes from open
-
circuit to
short
-
circuit and the output voltage often
greatly changes. Especially, when the
c i r c u i t
is shorted for arc ignition
w i t h

great current,
the surge
current passing through transistors
may rea ch a fe w
t i m e

more than t hat u nder
the condition of normal arc loaded. It is
disadvantageous for transistors to be used
safely. In order to solve this problem, the
c i r c u i t

for arc ignition with low current is
equ i
p ped to re ali ze so ft t ur ni ng o n t he
current and to increase the
r e l i a b i l i t y

of the
inverter.

The
p r i n c i p l e

of s oft t urning on the
current is shown in
f ig.
3. When short
-
circuit
for arc ignition, the output current which is
set by the circuit for arc ignitio
n
w i t h

low
current is 3A. (the curve 1 in fig.3). After



1

I n 1



1 I I

0 3

i
f

fig. 3 The principle of soft turning
on the current

a r c i s ig n i t e d, a l o ng t h e c u r v e o f
volt
-
amperage characteristic of the arc,
welding
current gradually increases
with the
curve of
external characteristic changes
continuously
from point 1 to point n. If the
welding current
if corresponding to point n is
equal to
definite value, the curve of external
characteristic doesn't change and the current
doesn't increase eith
er. The increasing rate
of th e cu r re n t f r om lo w er to g re a te r


the
of

ir slow, that ir. the process of soft turning on
the current has been realized, and it prevents
power transistors from damaging by over surge
current.

The principle diagram of the circuit for ar
c
initiation with low current is shown

in
fig.4.












fig. 4


The principle diagram of the
c i r cu i t
for

arc initiation with low current

where, U
t



exte rnal s ignal;

K
1



s ys te m e le me nts b et wee n po int


A in
f i g.l

and the out
put of the inverter,
which change voltage signal into current;

K
2



the current negative feedback
element;

K
2
( l
-
e
-
t/
τ
)


t h e c u r r e n t
positive feedback element.

Fig.4 may be expressed as following equation,

K
1

U
S

i
(t) =
-----

-------

(1)

1+К
1

K
2

e
-
t/
τ

At the transient when the output loop is
short for arc ignition,
t=0
+
, the
short
-
circuit current is equal to

K
1

U
S

i
(
0
+
) =

-------


(2)

1

+ K
1

K
2

Ob vi ou sl y, th e s ho rt
-
c ir c ui t cu r re nt f or
arc ignition is the minimum. When K
1
, K
2
, U
S

and
τ

being a rrange d prop erly, t he pro per cu rrent
for arc ignition, and proper increasing rate of
the current may be obtaine d. In this system,
the value of i(0
+
)

s et t l ed

is 3A.

If t
-
∞, then i(∞)
-
K
l
U
S
.

Obviously, it corresponds to a opened
-
loop
sy s te m,

a n d t he
current amplificatin
coefficient is great. From t=0
*

to t
-

, the

current increases with the time increases. The
max im um c ur re nt i s
K
l
U
S
. B ec au se t he a rc
we l
di n g i n ve r te r is ye t e q ui p p ed
w i t h

t h e

control
ci rc u it

for external
ch arac ter.
when
the
ci r cu i t

is designed,
l e t

the
maximum

welding
curretn
i
f
max
<
K
l
U
S
. While the output
current
controled by the Circuit for
ig nit ion

w ith

low current reaches the
de fi ni te
current if set
by the control
c i r cu i t

for
ext ernal
character istic, the cu rrent is
maintained
constant
wi t h

the help of control
circuit for
external characteristic. Mean
-
while, the
circui t

for arc
ignit ion

w ith

low
c ur ren t is
cut o ff au to na ti cl ly. B y t hi s
time,
i
f
<
i
f
max
<

K
l
U
S
.

Th e v ol t ag e a nd cu r re nt wa v ef or m i n t h e
process of
ign it io n

arc is shown in fig.5. The













Fig. 5 The voltage and current waveform in the
process of
i gni ti on

arc


process was operated manually and was recorded
by light
-
beam
oscil lograp h. It can be se en
tha t th ere i s a lo we r sur ge c ur re nt id a t
the transient when the output loop is shorted.

The surge current obtained by the capacitance
in out put loop dis char ging is lowe r th an
welding current if in stable state, and it is
benef
ical to
i g n i t e

arc, but can't impact
power transistors. Then the current increases
along a s traig ht
-
l ine from 3A to w eldi ng
curren t if, w hich is muc h stab le.

T he
short
-
circuit time for arc ignition is much
short, a nd the ti me meas ured i s less th an
0.04S.

A
fte r short
-
c ircuit, the volta ge
recovery rate is quick, or it indicates that
the invert er posse sses a go od dynam ic
characteristic [3]
,
So that the arc can be
ignited
e as i l y

at low open
-
circuit voltage
with low short
-
circuit current. The success of
ignition

arc with low current prevents power
transisto rs from impac ting by over sur ge
current, caused by short
-
circuit, and it
increases the
r eliab ilit y




33

B


of the inverter.

4.
THE CONTROL CIRCUIT FOR EXTERNAL
CHARACTERISTIC

The principle diagram of t
he
c i r c u i t

is
shown in fig.6.







-














fig.6 The principle diagram of control
circuit for external characteristic

where,
U
g



a adjustable external signal
fig.
6
«
ay be expressed as following equation







K
1


K
1


i
f(t>

=

--------

U
g(t)

(3)

1 + K, K
2

The output current i
f(
t)

nay be regulated
by regul
ating
U
g
( t )
,

which may be a voltage
signal in direct or rectangular waveform
controled by switching manipulation.

When
U
g
(t
)

is a direct voltage signal, the output of the
inverter is a direct current shown in
fig.
5.
U
9
(
t
) is a pulsed voltage signal shown
in fig. 7.


volt
-
acpcragc characteristic cf the
between
two curves of external chara
cteristic
. Where,
U may be regulated
w i th

U
p
;
i
k

may be
regulated
w i t h

U
k
;

the pulsewidth and pulse
frequency of output current may be regulated
with t
1

and t
2
. In th
is inverter, these
parameters mentioned above may be regulated
respectively. The waveform of arc voltage and
pulsed current recorded by light
-
beam
oscillograph is shown in
fig.
9. It can be
seen

that arc voltage is maitained constant, that
is, the arc
works along a lever line of
volt
-
amperage characteristic of the arc; and
the waveform of welding current is rectangular
with spike in the forward of pulse edge. The

pulsed current is very accurate and it
is

benefical to the
stability

of welding arc.
So

tha
t the arc welding inverter is


satisfactory arc welding power source
for

welding thin
p l a t e

with low current in
TIG

welding.




*2



T


fig. 8 The current jumps between two curve of
external characteristic


fig.7 Pulsed external signal

U
p

t=t
1

where,
U
g
(t )

=

(4)

U
k

t=t
2

because of U
g
(t)

jumping periodically
between
U
p

and
U
k
, correspondently, the output
curr
ent jumps periodically between
i
p

and
i
k

too according to the equation

3. It can be
seen from fig.8 that the working point of the
arc jumps periodically along the curve of

O.5s

fig.9

The waveform of pulsed arc

1.5s

34


lr(O

1s

5.
A FEW MEASURES FOR I
NCREASING THE



equation 5 is adiited, that
is,
RELIABILITY OF THE INVERTER


V
CE

= V
BE
+V
F(D2)
+V
F(D3)
-
V
F(D
1
)


To a great extent, the reliability of

transistor arc welding inverter depends on

the

rel
iability of power transistors. Therefore,

power transistors used must meet the needs of

certain technical denands. In addition, it

must be considered that protecting power


transistor from damage according to the


specific conitions when the circuit is

designed, which is more important, following

protective measures have been adopted in


this inverter
.

(1)

As mentioned above, the circuit for arc

ignition with low current has been equipped in

the inverter and over surge current impacting

power transistors caused by ignition arc at

short
-
circuit transient has been greatly

decreased.

(2)

In order to decrease th
e changing rate

of the current when the load is changed

suddenly, it is proper to appropriately

increase output loop inductance.

(3)

The output voltage of the arc welding

inverter varies in a great range. So that the

control pulsewidth also varies in a gre
at

range. When the circuit is unloaded, the

pulsewidth is the widest and the current

passing through transistors is the lowest,

When drived under a great constant current,

transistors will work at a over saturate

state. It is harmful to switching off po
wer

transistors. Further more, it may make two

transistors conduct at the same time and

transistors will be damaged. When the output

loop is shorted, the control pulsewidth is

very narrow and the current passing

through power transistors is quite gr
eater,

thus the power consumptionon switchingis very

great and it is disadvantageous to use power

transistors safely. In order to solve the

problems mentioned above, a quasi
-
saturate

circuit and a circuit for reverse driving base

electrode are equipped in

the inverter besides

two measures as (1) and (2).

A circuit for driving base electrode and

clamping potential is adopted in the inverter

to realize transistors working at

quasi
-
saturate state. The circuit is shown

in fig.10.

When the transistor is con
ducted, the

voltage drop upon passage A
-
B
-
E is equal to

that upon passage A
-
C
-
E, that is,

V
BE
+ V
F(D2)
+ V
F(D3)

= V
CE

+ V
F(D1)

(5)

If Dl is at the state of forward bias, the

3
5

(6)

driving signal

fig. 10 Quasi
-
saturate circuit


Because the forward voltage droping upon diode
usually equals to 0.6
-
0.8v, therefore
V
CE
equals
approximately to 1.9v. Actually, D, is emploied as
a overflow valve, and it makes excessive current
of driving base electrode n
ot to pass through base
electrode. If the current passing through
collecting electrode decreases with the load
decreases,
V
CE

usually decreases too. The lower
V
CE

neans that the extent of saturate is deep and
storage time is long. The quasi
-
saturate circui
t
makes
V
CE

to be the sum of
V
BE

and the voltage
droping upon diode. So that it prevents the
condition mentioned above from taking place.

It
decreases the switching power consumption and
increases the reliability of the power source.


6.
CONCLUSION

A trans
istor arc welding inverter with direct
and pulsed current was developed. The satisfactory
output pulsed current waveform was obtained by
changing external characteristic curve
periodically with the help of control circuit.

In
addition, a circuit for arc ig
nition with low
current is equipped in this arc welding inverter,
and ignition arc with low current at low voltage
is realiged. At the same time, the reliability of
the power source is increased further by equipping
quasi
-
saturate circuit in the iverter.


REFERENCE

[1] Huang Shisheng, Wu Yuehua, Wu Xianping,
«
Transactions of the China Welding Institutions
No. 2,1986

[21 Wu Xianping, Huan Shisheng,
«
Journal of
Zhenjiang Shipbuilding Institute
»
,No. 2
-
3 1988

[3] Huan Shisheng, Arc welding power
sources,
«
Mechan
ical Industry Press
»
, 1980

[4) Richard L.Bonkowski,
«
IEEE Trans. Indu.
AppL.
»
, 1986, Vol. IA
-
22,No. 2 pp. 240
-
243