Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 162)

162 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 3 Diagnostics and Protection
Low Speed Operation
When operation is below 5 Hz, the IGBT duty cycle is such that heat builds up
more rapidly in the power device. The thermal manager increases the calculated
IGBT temperature at low output frequencies and causes corrective action to take
place sooner. Consult technical support when prolonged operation at low output
frequencies is required so proper drive derating can be applied. Also consider that
when a drive is in current limit the output frequency is reduced to try to reduce
the load. This works fine for a variable torque load, but for a constant torque load
reducing the output frequency does not lower the current (load). Lowering
current limit on a constant torque load pushes the drive down to a region where
the thermal issue becomes worse. In this situation the thermal manager increases
the calculated losses in the power module to track the worst case. So if the
thermal manager normally provides 150% for 3 seconds at high speeds, it can
only provide 150% for one second before generating a fault at low speeds. Some
applications, such as hoisting and lifting, can benefit from the disabling of
current limit fold back.
Faults Faults are events or conditions occurring within and/or outside of the drive.
These events or conditions by default are considered to be of such significant
magnitude that drive operation is discontinued. Faults are annunciated by the
STS (Status) indicator on the drive, a HIM, communications network and/or
contact outputs.
Drive Response to Faults
When a fault occurs, the fault condition is latched, requiring the user or
application to perform a fault reset to clear the latched condition. The condition
that caused the fault determines the user response. If the condition that caused
the fault still exists after a fault reset, the drive faults again and the fault condition
is latched.
• In response to a fault, the drive takes a predetermined action based on fault
type. Drive response to some fault types are user configurable. With non-
configurable faults the drive output is turned off and a “coast to stop”
sequence occurs. The Troubleshooting section of PowerFlex 750-Series
Programming Manual, publication
750-PM001 , provides details on both
types of faults.
• The fault code is entered into the first buffer of the fault queue (see Fault
Queue below for rules).
• Additional data on the status of the drive at the time that the fault
occurred is recorded. This information is always related to the most recent
fault queue entry captured by P951[Last Fault Code]. When another fault
occurs, this data is overwritten.