THEORY OF OPERATION
MOTOMAN 3-11 Spot Welding Manual for Medar
Figure 3-12 Automatic Voltage Compensation (AVC)
The middle line indicates where the SCR is gated when the instantaneous line
voltage is equal to the reference voltage. If the line voltage drops, less current will
flow, so the SCR should gate at the leftmost line in order to allow more of the
waveform to conduct. If line voltage increases, more current will flow, so the SCR
should gate at the rightmost line in order to allow less of the waveform to conduct.
Thus, AVC regulates voltage to maintain a somewhat consistent level of current on
the secondary circuit.
3.7.3 Constant Current Mode
If the weld current is programmed in kilo-amps (kA), you are using Constant
Current Mode. In Constant Current Mode, current feedback is required.In order to
work properly, the turns ratio must be programmed properly. In order for the weld
control to dynamically adjust the firing rate to meet the requested current, it must
sample the current dynamically and multiply by the turns ratio. Constant current
can be considered the “smart mode” of operation.
Constant current control allows the programming of weld current in terms of actual
secondary current (for example, 12.5 kA). Just as AVC dynamically adjusts the
SCR firing angle to ensure that a constant percentage of available current is
delivered, Constant Current regulation monitors the current delivered and
dynamically adjusts the SCR firing angle to ensure that the desired amount of
current is delivered, regardless of the available current.
NOTE: Available current must be greater than the current being delivered.
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