Watlow QPAC Modular SCR Power Control Service - Manual (Page 20)

20 WATLOW QPAC Service Manual
Troubleshooting, Chapter 5
Q01
Problem: Partial Heat
This problem can be broken down into two categories:
Uncontrollable Heat
One of Two SCRs has Shorted (Half Waving is Continuous)
This condition exists when one of two SCRs is shorted, allowing half of the AC
sinewave to pass through the heater uncontrolled. See Page 19 to test for shorted
or open SCRs.
The Bias Adjustment is Too High
Recalibrate the bias and gain adjustments by following the procedure on Page 16.
Insufficient Heat
One of Two SCRs Won’t Turn ON (Half Waving)
This is most likely a problem with the mother board. A gate signal may be missing.
Refer to the mother board troubleshooting procedure on Page 28.
The Gain Adjustment is Too Low
Recalibrate the bias and gain adjustments by following the procedure on Page 16.
There is an Insufficient Control Signal
Verify that the signal from the temperature control is correct (i.e. 4-20mA, 1-5VDC,
etc.) Refer to the control card section, or the control user’s manual.
A Partially Open Heater(s)
Check all the connections from the circuit breaker to the heater. Measure the
heater current. Next, measure the voltage across the heater. If voltage is present
and no current is flowing, there is a bad connection or heater. Disconnect the
heater from the power control. Check the resistance of each element. The resis-
tance should correspond with the heater KW rating. If not, replace the heater.