Curtis PMC 1209B/1221B/1221C/1231C Manual
44
A-3
APPENDIX A
limit comparison is inhibited during the off interval by the current limit disable circuit. The current limit is
set as follows:
1.
During manufacture, the current limit is set to the model
’
s nominal rating; it is also
user-adjustable by a trimpot (current limit adjust) to currents lower than the nominal
rating.
2.
During operation at extreme high or low temperatures, current limit is reduced to
protect the controller from damage. From a thermal sensor (temp sense) on the
heatsink, signals are produced to cut back the current limit at temperatures above
75
°
C (above 85
°
C for the 1231C) or below -25
°
C (overtemp, undertemp). The
controller
’
s operating frequency shifts to 1 kHz (
“
B
”
models) or 1.5 kHz (
“
C
”
models) during overtemperature operation, producing an audible tone to alert the
operator.
3.
During plug braking operation, the current limit is reduced to give an appropriate
motor braking torque. The plug braking current is set during manufacture; it is also
user-adjustable by a trimpot (plug current adjust).
4.
The
fixed plug option
provides one level of plug braking current independent of
throttle position (provided it is at least minimally applied). For the
fi
xed plugging
option, the plug current limit reference is derived from the current limit reference.
5.
The
variable plug option
provides variable plug braking current corresponding to the
position of the throttle. This allows much smoother braking under control of the
operator. For the variable plugging option, the plug current limit reference is derived
from the throttle input scaling.
The transition to the plug braking mode is detected (plug detect) by monitoring the voltage across the plug
diode. When this diode becomes forward biased, it indicates that the motor
fi
eld has been reversed and the
controller has gone into plug mode. The current limit is reduced as described, and the frequency of the
oscillator is reduced from 15 kHz to 1 kHz, to allow
fi
ner control of the controller output while plugging.
During plug braking operation, the acceleration circuit is reset to a low level so that when drive operation
resumes, the controller will go through a normal acceleration ramp. When the motor has come to a stop, the
plug diode will again become reverse biased and the controller will revert to normal drive operation.
