Tweco Troubleshooting Power Supply - Manual (Page 11)

11
• Blown fuse in wall fuse/disconnect box or phase missing from power
distribution.
• Loose connection on power cord.
• C.P1 (On/Off SW) defective.
• Shorted inrush SCR in inverter module input rectifier bridge.
• Defective power supply PCB (PCB1, PCB4, PCB5)
• Defective CCM
Missing Phase
The power supply checks for 3 phase voltage at both the power supply input
terminals and at D1 (3 phase bridge on horizontal panel behind front panel)
coming through C.P1 (front panel On/Off SW). Measure the 3 phases at the input
terminals L1-L2; L2-L3; L1-L3. Do not measure from Phase to neutral. On D1
measure the 3 phases on the red, white & black wires coming from the bottom of
C.P1. If present at the input terminals but missing at D1 it is likely that C.P1 is
bad.
Shorted SCR
When power is first applied the input contactor for one inverter closes
momentarily then opens. This keeps repeating a number of times during which
code 2-1 will flash. After a while contactor may stay on and you could cut
however allowing this to continue can damage the contactor and possibly the
rectifier bridge and input capacitors.
When power is first applied the AC input voltage is rectified to DC and allowed
to slowly charge the input capacitors through a pair of resistors. Then when the
capacitors are fully charged an SCR (part of the input bridge) is turned on
connecting the DC directly to the capacitors. With the SCR shorted very high
currents, limited only by the line impedance, flow through the rectifier bridge and
into the filter capacitors. If you have a missing phase the line voltage drops every
time that phase should be supplying voltage. The missing phase detection circuit
detects the drop. The high currents caused by no inrush limiting cause the voltage
to drop momentarily each time the contactor closes. The missing phase detection
circuit interprets this drop as missing phase thus the 2-1 code.
To determine if SCR is shorted, find the inverter module(s) on the left side of
the unit. The input bridge is to the left end of the module (rear of unit).