59
comes:
rated voltage
ra
e
capac
y
I
e
= x duty factor
=
200
85 + 10
3
x 0.5 = 300A
Hence, the MCCB rated current becomes:
I
MCCB
= I
e
x 1.15 = 300 x 1.15 = 345A
(i.e., the next higher standard value).
The relationship between the duty factor, which
does not exceed the working limitations, and the maxi-
mum permissible input I
β
at the above duty factor is:
β
I
e
I =
β
300
=
β
If the total period is taken as 60 seconds and the
duty factor is converted into the actual period during
which current flows, the above relationship can be
expressed graphically as in Fig. 7.6. Thus, although
the thermal equivalent current is 300A, the maximum
permissible input current for a duty factor of 50% (30
seconds current flow) is 425A. For a duty factor of
6.25% (3.75 sec current flow) it is 1200A. Even if the
secondary circuit of the welder were short circuited,
however, the resultant primary current would only in-
crease by about 30% over the standard maximum
welding current. If this is 400kVA, the maximum pri-
mary current I
βmax
is:
primary voltage
standard maximum input
I
max
= x 1.3
200
400 x 10
3
x 1.3 = 2600A=
β
Hence the maximum input current I
β
should be re-
stricted to 2600A.
The 75% hot-start characteristic of the 350A Type
NF400-SP breaker is shown by the broken line in Fig.
7.6, and the temperature-rise characteristics up to the
upper limit of the welder, by the solid line. To ensure
protection of the welder from burnout, the delay-trip
characteristic is selected at higher than the solid line;
however, to establish MCCB protection criteria, it is
necessary to look at each welder individually.
Operating time (s)
30
3.75
0.6
425
Primary input current (A)
Type NF400-SP·350A 75%
hot start
10"
2"
300026001200
Fig. 7.6 Welder Temperature Rise and MCCB Trip Curve
7.4.2 MCCB Instantaneous Trip and Trans-
former Excitation Surge
When a welding-transformer primary circuit is closed,
depending upon the phase angle at the instant of clo-
sure, a transient surge current will flow, due to the
super-imposed DC component and the saturation of
the transformer core.
In order to prevent spurious tripping of protective
devices resulting from such surges, and also to main-
tain constant welding conditions, almost all welders
currently available are provided with a synchronized
switch-on function, with or without wave-peak con-
trol.
With synchronized switch-on, the measured ratio
between the RMS value of the primary current under
normal conditions and the maximum peak transient
current ranges from √
2 ~ 2.
For nonsynchronized soft-starting-type welders the
measured ratio is a maximum of 4.
Maximum instantaneous transient surge excitation
currents for various starting methods are as follows:
Synchronized switch-on welders with wave peak con-
trol:
I
max
= 2 x I
max
Synchronized switch-on welders without wave peak
control:
I
max
= 2 x I
βmax
Nonsynchronized switch-on welders with soft start:
I
max
= 4 x I
βmax
Nonsynchronized switch-on welders without soft start:
I
max
= 20 x I
βmax
If synchronized switch-on is employed, the tran-
sient surge excitation currents are relatively consis-
tent, so that the relationship I
max
= 2 I
βmax
is suffi-
cient.
For a synchronized switch-on type welder of maxi-
mum primary input (I
βmax
) = 2600A
I
max
= 2 x I
βmax
= 2 x 2600 = 5200A
Since MCCB instantaneous trip currents are speci-
fied in terms of RMS value, I
inst
is as follows:
2
I
max
I
inst
= = 3680A
2
5200
=
The MCCB should be selected so that I
inst
is smaller
than the lower tolerance limit, of the instantaneous
trip current.
7.4.3 Arc Welders
An arc welder is an intermittent load specified. The
MCCB rating can by selected by converting the load
current into thermal-equivalent continuous current. If
this is taken as the rated current, however, the cur-
rent duration per cycle will become relatively long, with
the attendant danger of thermal tripping of the MCCB.
In the total period of 10 minutes, if the duty factor is
50%, a 141% overload exists for 5 minutes; if the duty
factor is 40%, a 158% overload exists for 4 minutes;
and if the duty factor is 20%, a 224% overload exists
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