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3
Selection and application
3-7 Welder circuit applications
(2) Selecting MCCBs
(a) Basic rule
Assuming that the welder is used in the operating condition
illustrated in Fig. 3-22, the MCCB to be used must meet the
following requirements:
(i) The rated current (I
N) of the MCCB is higher than the
thermal equivalent current I
B (IN > IB).
Allowing for possible supply voltage fluctuation, a margin of
some 10% would be recommended.
(ii) The MCCB is not tripped by the primary input current.
The MCCB’s hot-start characteristic curves are positioned
above the points (t
L, IL) and (TL, Ia) so that the currents IL
and Ia (A) would not cause the MCCB to malfunction (Fig.
3-23).
(iii) The MCCB is free from malfunction due to inrush current
when the circuit is closed.
Fig. 3-23 Hot and cold MCCB operating characteristics
TL
IB
IN
IL
Ia
Current
tL
Cold
Hot
Time
(b) Selecting MCCB based on welder ratings
If the operating conditions for the welder are not definite,
the MCCB to be used should be selected by allowing for the
maximum operating limits of the welder considering its ratings
or specifications.
The rated capacity of a resistance welder is indicated in
terms of a 50% on-load factor. Namely, the rated capacity
is defined as an input load that would meet the temperature
rise requirement when the welder is used with a 50% on-load
factor.
If the welder is to be used with a current different from that
available with a 50% on-load factor, it must be used with an
on-load factor that would cause an equivalent temperature rise
observed with a 50% on-load factor or lower.
The relationship between the primary input capacity and the
allowable on-load factor can be stated in an equation as
u 50%
Rated capacity
Primary input capacity
Allowable on-load factor=
2
This equation may be used to examine all possible
combinations of the primary input capacity and the allowable
on-load factor.
(i) Reviewing the thermal equivalent current
With an on-load factor of 100%, the thermal equivalent current
can be stated in equation form as
u
Rated capacity
Rated voltage
Thermal equivalent current=
50
100
(A)
Hence, the rated current of the MCCB must be at least equal
to this value.
(ii) Reviewing the method to prevent malfunctioning
associated with the primary input current
The first step in reviewing the primary input current-time
characteristics of the resistance welder and the hot-start
characteristic of the MCCB is setting the operating time (t
L)
associated with the allowable on-load factor (D) of the welder.
Assuming that the intermittent loading cycle is 1 minute and
hence t
L = 60·D/100 (seconds), the relationship between the
operating time (t
L) and the primary input current (
IL
) must be
represented.
Fig. 3-24 shows the relationship between the primary input
current and allowable operating time for a single-phase 200V
resistance welder rated at 25kVA.
Since the equation
u
Rated capacity
Rated voltage
Primary input current =
50
On-load factor
is derived from the relationship presented above, the
maximum operating limits of the welder can be calculated as
follows:
125A for 50% on-load factor, (t
L = 30 seconds)
280A for 10% on-load factor, (t
L = 6 seconds)
884A for 1% on-load factor, (t
L = 0.6 seconds)
Fig. 3-24 Relationship between maximum primary input current
and operating time
60
30
6
0.6
88.4 125 280 340 884
Operating time t
L
(sec.)
Primary input current IL (A)
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