Curtis PMC 1204X/1205X/1209/1221 Manual
47A-5
APPENDIX A
the short is removed or the controller turned off. This 10-µs-on, 10-ms-off timing produces very low power
dissipation in the drivers while protecting them from the shorted load.
Fault detect monitors M- to confirm that it periodically goes high. If M- remains low for more than 20 ms,
a fault is assumed (internal or external to the controller); the forward/reverse contactor driver is turned off and
the logic board pulses are disabled. The controller will remain off until the proper input switch sequencing
is re-applied. If the fault is still present, the controller will remain off; otherwise, it will turn back on. Fault
detect is inhibited (fault inhibit) for throttle inputs greater than 80% to avoid sensing a false fault when the
bypass contactor turns on at 90% throttle input (bypass looks like full speed at M-). The inhibit release timer
keeps fault detect inhibited for about 50 ms after the throttle request decreases below 80% duty cycle to allow
time for the bypass contactor to drop out. This delay avoids detecting a slowly releasing bypass contactor as
a fault.
The emergency reverse input causes the controller to go into plug braking mode with increased current limit.
Increasing the current limit is necessary because the standard plug braking current limit is usually not high
enough to slow the vehicle fast enough for emergency reversing. The emergency reverse switch is externally
wired to a relay that simultaneously selects “reverse” and “full speed”. The vehicle is suddenly reversed (if in
forward) and the controller goes into enhanced plug braking mode to slow the vehicle to a stop before it
accelerates away from the operator in reverse.
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