Watlow QPAC Modular SCR Power Control Service - Manual (Page 26)

26 WATLOW QPAC Service Manual
Troubleshooting, Chapter 5
Q33
Problem: Full or Partial Uncontrollable Heat
The SCR Switch is Shorted
Remove the control card and power up the QPAC. The output should be OFF. If
the unit is still full ON, remove power and disconnect the load wires and measure
across L1 and T1 with an ohmmeter. If resistance is low (5Ω or less), the SCR is
shorted and must be replaced. See Page 5 for ordering a replacement SCR.
A Heater is Partially Shorted
Disconnect the control card; the output should be OFF. If not, disconnect the
power and heater, and measure the heater resistance. It should match the heater
KW rating.
Card has the Wrong Bias Adjustment
The bias adjustment is set too low and needs to be readjusted. See the Bias and
Gain Adjustment procedure with the appropriate control card on Page16.
The Control Card is Bad
First, try to re-seat the control card. Make sure there is a good connection between
the control card and the mother board. Power down the control and remove the
control card. Power up without the control card installed. If there is no heat, the
control card is bad. Replace the control card. See Page 5 for ordering.
An Excessive Input Signal
Check the input signal for proper calibration (i.e. 4-20mA at 4mA). The output
should be full OFF. At 20mA the output should be full ON.
The QPAC Mother Board is Bad
Refer the the mother board troubleshooting procedure.
Problem: Partial Heat
One of Three 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 - 29.
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.
The Zero Cross or Phase Angle Jumper is in the Wrong Location
Move the jumper. See the Zero Cross Calibration procedure on Page 15.