16 Rockwell Automation Publication PFLEX-IN006E-EN-P - July 2013
Preface
General Precautions,
Continued
ATTENTION: The “adjust freq” portion of the bus regulator function is
extremely useful for preventing nuisance overvoltage faults resulting from
aggressive decelerations, overhauling loads, and eccentric loads. It forces the
output frequency to be greater than commanded frequency while the drive's
bus voltage is increasing towards levels that would otherwise cause a fault.
However, it can also cause either of the following two conditions to occur.
1.) Fast positive changes in input voltage can cause uncommanded positive
speed changes.
• For PowerFlex 700H drives, an “OverSpeed Limit” fault will occur if the speed
reaches [Maximum Speed] + [Overspeed Limit]. If this condition is
unacceptable, action should be taken to 1) limit supply voltages within the
specification of the drive and, 2) limit fast positive input voltage changes to
less than 10%. Without taking such actions, if this operation is unacceptable,
the “adjust freq” portion of the bus regulator function must be disabled (see
parameters 161 and 162).
• For PowerFlex 700S drives, an “Abs Overspd Det” fault will occur if the speed
reaches [Rev Speed Limit] - [Abs OverSpd Lim] or [Fwd Speed Limit] + [Abs
OverSpd Lim]. If this condition is unacceptable, action should be taken to 1)
limit supply voltages within the specification of the drive and, 2) limit fast
positive input voltage changes. Without taking such actions, if this operation
is unacceptable, disable the bus regulator by setting parameter 414 [Brake/
Bus Cnfg], bit 3 “Bus Reg En” to zero (0).
2.) Actual deceleration times can be longer than commanded deceleration times.
• For PowerFlex 700H drives, a “Decel Inhibit” fault is generated if the drive
stops decelerating altogether. If this condition is unacceptable, the “adjust
freq” portion of the bus regulator must be disabled (see parameters 161 and
162). The “Decel Inhibit” fault can be disabled by setting parameter 238 [Fault
Config 1] bit 6 “Decel Inhib” to zero (0).
• For PowerFlex 700S drives, a “Vref Decel Fail” fault is generated if the drive
stops decelerating altogether. If this operation is unacceptable, disable the
bus regulator by setting parameter 414 [Brake/Bus Cnfg], bit 3 “Bus Reg En” to
zero (0). This fault cannot be disabled in the PowerFlex 700S.
Note: For both drives, installing a properly sized dynamic brake resistor or external
dynamic brake will provide equal or better performance in most cases.
Important: These faults are not instantaneous. Test results show they can take
2…12 seconds to occur.
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