WTC MedWeld 3005, Operator'S Manual

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If water circulation stops or is interrupted while the power is on,
leakage current through the water in the hose between the SCR
tangs will cause gas bubbles to form. Current will arc across these
voids, weakening or destroying the hose. Putting the water into
circulation again develops pressure in the cooling circuit,
consequently causing the hose to rupture. Therefore, WTC does
NOT recommend the use of water savers with these SCRs.
When magnetic contactors are used, they remove power from the
SCR module and prevent destruction of the hose. If cooling
manifolds are used (rather than hose), leakage current through
the water can cause the same destructive action to occur while
power is being applied. This can destroy the cement holding the
manifold together, resulting in serious water leaks. (WTC’s
warranty specifically does NOT cover such failures.)
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If a cooling hose needs replacing, use a certified non-electrically
conductive hose. Its inside diameter must NOT exceed 3/8”. The
hose between SCR tangs must be at least 18 inches long across the
power voltages, where SCR tangs are directly cooled.
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The SCR module has three main electrical ratings:
• PIV—Peak inverse voltage rating,
• Insulation voltage rating and
• Nominal current rating.
The PIV rating represents the maximum voltage allowed across
the SCR inverse parallel pair before damage occurs. The PIV for
the standard SCR module (p/n #304-0081) is 1,800 V. If the PIV is
exceeded, the control attempts to protect the SCR by activating
the shunt trip.
The insulation voltage rating represents the maximum voltage
allowed between the SCR tang (L or H) and ground before
damage can occur. The insulation voltage rating of a standard
SCR module (p/n #304-0081) is approximately 2,500 V.
continuous and 3,200 V. for transients.
The nominal current rating is the maximum current (at 50% duty
cycle and 0.5 sec. maximum on-time per weld) that the SCR can
withstand before damage occurs. (For a weld on-time exceeding
0.5 sec., the SCR should NOT be used above its continuous-duty
rating. This is 70 percent of the nominal current rating.)
The nominal current rating for a standard SCR (p/n # 304-0081) is
1,100 A. at 50% duty cycle and 0.5 sec. maximum on-time per
weld. If using this SCR for on-time longer than 0.5 sec., the
maximum allowable current is reduced to 70 percent, or 770 A.