Entron EN1000 series

ENTRON Controls, LLC. • 700120S • Page 25
3.8 CONTACTOR SPECIFICATIONS
To help in selecting the proper SCR contactor size for application, use the following “rule of
thumb” for sizing SCR contactors for various size transformers.
x 3 = Maximum Current Demand
Example 1: Using 75 KVA transformer at 230 VAC:
Maximum Current Demand = x 3 = 978 Amperes
Example 2: Using 250 KVA transformer operating at 460 VAC:
Maximum Current Demand = x 3 = 1630 Amperes
The multiplier factor of 3 in this formula assumes a reasonable secondary configuration of an
8" x 12" throat to a secondary of 13" to 18", with a poor power factor of about 40%, having a
necessary adjustment on the welding control of greater than 50 percent current.
A multiplier factor of 2.5 may be used when a machine’s power factor is 45% or better. A
multiplier factor of 5 or 8 may be required for machines with large secondaries with power
factors of 30% or poorer.
When applying the above “rule of thumb”, two other parameters must be considered. Conduction
Time – the time the welding transformer is energized and the Duty Cycle – the ratio of
Conduction Time to the complete cycle time (including part handling). These are factors that
can substantially alter the selection of a contactor with regard to demand current.
The shorter the Conduction Time and Percent Duty Cycle, the greater the current switching
capability of a contactor. Conversely, longer Conduction time and higher Duty Cycle reduce
the current switch capability of the contactor.
Figure 3-15 shows suggested relationships for Current Demand, Duty Cycle and Conduction
Times. All curves on chart are shown in 30 cycle (60 Hz) conduction time. Assuming maximum
30 cycle conduction time and using Figure 3-15, following recommendations would be made
for above examples:
Example 1: For 75 KVA transformer operating at 230 VAC, recommended contactor
size would be 1200A SCR contactor for Percent Duty Cycle of approximately 14% or less.
Example 2: For 250 KVA transformer operating at 460 VAC, recommended contactor
size would be 1200A SCR contactor for Percent Duty Cycle of approximately 20% or less.
DUTY CYCLE
Duty Cycle is the percent of the time the weld current is on. A convenient formula for calculating
Duty Cycle is:
% Duty Cycle =
Transformer KVA x 1000
AC Line Voltage
75 x 1000
230
250 x 1000
460
Weld Time (in Cycles) x Number of welds per minute
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