Allen-Bradley PowerFlex 70 Reference Manual page 95

If the drive is operated in a low ambient condition the drive may exceed rated
levels of current before the monitored temperature becomes critical. To guard
against this situation the drive thermal overload also includes an inverse time
algorithm. When this scheme detects operation beyond rated levels, current limit
may be reduced or a fault may be generated.
Operation
The drive thermal overload has two separate protection schemes, an overall RMS
protection based on current over time, and an IGBT junction thermal manager
based on measured power module temperature and operating conditions. The
drive may fold back current limit when either of these methods detects a
problem.
Overall RMS Protection
The overall RMS protection makes sure the current ratings of the drive are not
exceeded. The lower curve in
operation. In normal duty, the drive is rated to produce 110% of rated current for
60 seconds, 150% of rated current for three seconds, and 165% of rated current
for 100 milliseconds. The maximum value for current limit is 150% so the limit
of 165% for 100 milliseconds should never be crossed. If the load on the drive
exceeds the level of current as shown on the upper curve, current limit may fold
back to 100% of the drive rating until the 10/90 or 5/95 duty cycle has been
achieved. For example, 60 seconds at 110% will be followed by 9 minutes at
100%, and 3 seconds at 150% will be followed by 57 seconds at 100%. With the
threshold for where to take action slightly above the rated level the drive will only
fold back when drive ratings are exceeded.
If fold back of current limit is not enabled in [Drive OL Mode], the drive will
generate a fault when operation exceeds the rated levels. This fault can not be
disabled. If current limit fold back is enabled then a fault is generated when
current limit is reduced.
Figure 18 Normal Duty Boundary of Operation
1.80
1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
1.00
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
Figure 18 shows the boundary of normal-duty
10.00 100.00
Time (Seconds)
Drive Overload
1,000.00
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