Rockwell Automation SD3000 Drive Configuration, Programming - Manual (Page 74)

Note that, unlike Control Block tasks on AutoMax Processors, UDC tasks cannot run on a
hardware or software event basis. The EVENT parameter cannot be specified in the SCAN-LOOP
block in UDC tasks. This means that there is no timeout for execution of the UDC tasks. If the UDC
task is scanned to the SCAN-LOOP block and CCLK is not on, the task will simply wait without
timing out.
Note that no other control blocks are permitted before the SCAN-LOOP block. BASIC statements,
however, are permitted before the SCAN-LOOP block.
5. Other Control Block and BASIC statements or functions
This portion of the task consists of the logic specifically required for the application. This portion
of the UDC task (after the SCAN-LOOP block) is the only part of the task that executes after the
initial scan of the task, after a STOP ALL command and subsequent Run command, or after
power is cycled to the rack.
6. Motor thermal overload protection
Electronic thermal overload protection for Distributed Power drives is normally provided by the
THERMAL OVERLOAD block. The following briefly describes how the THERMAL OVERLOAD
block works, how to program the block, and what adjustments are possible. Each UDC task must
contain a THERMAL OVERLOAD block, unless motor thermal overload protection is provided by a
hardware device. See J-3676, the Control Block Language instruction manual, for the structure of
the block.
CAUTION: Electronic motor overload protection must be provided for each motor in a Distributed Power
drive application to protect the motor against excessive heat caused by high currents. This protection can be
provided by either the THERMAL OVERLOAD software block or an external hardware device. Applications in
which a single power module is controlling multiple motors cannot use the THERMAL OVERLOAD software
block and must use an external hardware device or devices to provide this protection. Failure to observe this
precaution could result in damage to, or destruction of, the equipment.
The THERMAL OVERLOAD control block is used to create a model of the temperature in a single
device, such as a motor or power module, controlled by a DPS drive and to turn on an alarm
when an overload condition exists. The block calculates a rise in temperature based on current
feedback. When operating above 100%, if the rise in temperature exceeds the programmed limit,
the OVERLOAD output will turn on. After the overload condition is detected, the rise in
temperature must return to the 100% condition before the drive will be allowed to turn on again.
The operation of the block is programmed through four block input parameters: LIM-BAR,
THRESHOLD, TRIP-TIME, and I-FDBK. The value used for LIM-BAR must be the same value
entered as the motor overload ratio during drive parameter configuration. The value used for
THRESHOLD selects the percent of full load current at which overload is detected. The value used
for TRIP-TIME selects the time, in seconds, within which the block must detect an overload after a
step from 100% current to LIM-BAR. The main input to the THERMAL OVERLOAD block is
I-FDBK. I-FDBK represents current feedback from the PMI in counts (register 21 1/1 21 1), scaled
so that LIM-BAR is 4095 counts.
The main output from the block is OVERLOAD. This boolean will be turned on when a thermal
overload is detected. The OVERLOAD output must be programmed in a Ladder Logic task to turn
off the drive when the fault is detected. The block also has an output called CALC-RISE. Current
feedback is squared, scaled, passed through a Lag filter, and then written to CALC_ RISE,
Consider an example in which LIM-BAR is defined to be 150% of full load current, THRESHOLD is
114%, and TRIP-TIME is 60 seconds. When I-FDBK is at 100%, CALC-RISE will reach a steady
state value of 1000, (100 /
% ** 2 / IO). With THRESHOLD at 114%, the trip point for CALC-RISE will
be 1300, (114 % ** 2 / 10). If I FDBK is at steady state 100% and then is stepped to 1 50%,
CALC-RISE will integrate up to 1300 in 60 seconds and OVERLOAD will turn on. The OVERLOAD
output will stay on until the rise decays to less than 1000. If I-FDBK remains less than 114%,
CALC-RISE will remain less than 1300 and OVERLOAD will not turn on.
4-4