MODEL PCA090480WH
CHAPTER 3
TROUBLESHOOTING
Section 6: Troubleshooting Direct Digital Controller (DDC)
March 2013
3-6
Proprietary/Confidential
Page
95
© 2013 Twist, Inc.
Push the “Start” button and confirm that the start control relay inside the control
panel close and latch – if not, check the wiring sequence for the pushbuttons.
During this sequence, the VFD should start the blower. The control mode relay
will latch and no other motor starters will close until the ALC gets a signal from
the VFD that it is running. Push the “Stop” button after these checks are
performed.
The ALC program requires that all of the temperature probes be present and
operating correctly before the program will run. If the red LED labeled FAULT
turns on, this indicates a fault condition in the ALC program. If this LED is “On”
and the unit will not start, go to the section “Checking Controller Inputs”, below.
2.
Checking Controller Inputs
Each temperature probe is connected to two terminals on the analog input
terminal block. Check the wiring to be sure it is correct.
The input point for the Current Transducer (CT) is a 4 to 20 milliamp input
When the “Start” mode relay is closed, the ALC should begin to start the unit
by signaling the VFD to start the blower. If the start mode relay is closed and
the unit does not start the circuit may not be correct.
The other input points are digital inputs. If the switch is closed, there should be
continuity at the terminal block it is connected to. If the input point is active,
the input LED should light.
3. Checking
Controller
Output
Output points can be checked using the computer. It is possible that the
outputs have wiring problems, blown fuses, or possibly bad relay contacts or
coils. If the ALC LED display is indicating that an output point is on but the
physical device is not responding, check the circuit wiring, circuit breakers and
thermal overloads.
4. Thermal
Overloads
The next most likely cause of a problem is that the thermal overloads are
tripped. Reset the overloads and check the system again. If the overloads trip
on a regular basis, check the amperage of the motor. Check the settings of
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