WTC MedWeld 3005, Operator'S Manual

Page: 37 / 206

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In this way, any drift of component values on the SCIB can be
detected and corrected for. Self-calibrate can also detect a
damaged or disconnected secondary current-sensing coil.
The SCIB contains the necessary connectors to be installed in-line
on the cable which normally goes from the weld timer to the
firing board. An additional connector accepts the output of an
air-wound (Rowgowski) secondary current sensing coil.
Two bi-color (red and green) LEDs indicate operating modes and
any error conditions.
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Power for the SCIB comes from the +12 VDC which the weld
timer supplies to the firing board. Maximum current draw is 35
mA from the +12 V supply and 25 mA from the –12 V supply.
The SCIB is designed for use with air-core current sensing coils
which meet the IEC standard of 150 mV output per kA with a
sinusoidal 50 Hz current waveform (0.477
µ
Hy mutual
inductance). On 60 Hz, these coils output 180 mV per kA. To
allow for variance in current-sensing coils, the welding control
have a setup parameter to allow setting the mutual inductance
between 0.05 and 0.99
µ
Hy. The SCIB secondary current coil
input terminals present a 1,000-ohm load to the coil.
The SCIB has three current ranges which are set with suitcase-
type jumpers on the circuit board. These ranges are 0–25 kA, 0–50
kA and 0–100 kA. Higher currents, up to any desired value, can
be obtained by using a special jumper setting together with an
external resistor network on the current-sensing coil.
Due to the averaging nature of the V/F converter used to
measure current, the V/F converter counts, accumulated over an
interval (usually ½ cycle) represent the rectified average value of
the current, not the RMS value. For a sinusoidal AC current
waveform (100% current; SCR ON all the time), the RMS value of
the current is 1.111 times the rectified average value. This ratio of
RMS-to-average current gradually increases to a value of about 2
at very low (5%) current settings.
Note that, when measuring DC secondary currents, none of these
RMS-to-average considerations apply. Accumulated V/F counts
represent DC current directly; no multiplier is necessary. Also,
the proportionality constant between counts and secondary
current per half-cycle is different at 50 and 60 Hz, due to the
different periods of those power line frequencies.
The following table shows the relationship between V/F counts
per half-cycle and secondary current for the three defined current
ranges at 60 Hz and 50 Hz: