Rockwell Automation 20D PowerFlex 700S AC Drives with Phase II Control Reference Manual - Manual (Page 142)

142 Rockwell Automation Publication PFLEX-RM003E-EN-E - January 2011
Chapter 1 Detailed Drive Operation
The drive determines that the encoder has faulted based on a combination of
hardware detection and monitoring the rate of change of the motor speed. The
hardware fault detection is based on illegal encoder states and improper encoder
switching patterns. The rate of change of motor speed detection is determined by
a rate of change greater than a user-defined speed change.
Hardware detection of feedback loss for the feedback option cards is based on the
type of device used and specific fault detection implemented on the feedback
option card. The rate of change of motor speed detection is the same
implementation as for encoder feedback.
Speed feedback loss ride through is not allowed in position mode. If the encoder
loss ride through function was allowed in a positioning system it is likely that the
alternate feedback device will supply an arbitrary position feedback value when
an encoder loss is detected. This, in turn, could result in unintended motion in
the drive system.
Permanent Magnet Motors
Permanent Magnet motors require the use of an absolute feedback device (for
example, a resolver, Stegmann, or Heidenhain device). Therefore, the speed
feedback loss ride through function cannot be used with Permanent Magnet
motors (in other words, bit 16 “Auto Tach Sw” of parameter 153 [Control
Options] cannot be set when parameter 485 [Motor Ctrl Mode] is set to 2 “PMag
Motor”).
Speed Feedback Loss Ride Through Operation
Parameter 151 [Logic Command], bit 2 “TackLoss Rst” provides a manual
switch between primary and alternate speed feedback devices with a “0” to “1” bit
transition. A transition from “1” to “0” does not cause a change in operation.
Parameter 152 [Applied LogicCmd], bit 2 “TachLoss Rst”, shows the status of
parameter 151 [Logic Command], bit 2 “TachLoss Rst”.
Setting parameter 153 [Control Options], bit 16 “Auto Tach Sw”, to a value of
“1” enables the automatic switching of speed feedback devices when a failure is
detected. Only automatic switching from the primary device specified in
parameter 222 [Mtr Fdbk Sel Pri] to the alternate device specific in parameter
223 [Mtr Fdbk Sel Alt] is available. With the exception of “Sensorless” mode,
switching from the alternate to the primary device must be done manually by
setting parameter 151 [Logic Command], bit 2 “TackLoss Rst” from “0” to “1.”
ATTENTION: Even though the encoder loss ride through function
attempts to automatically switch feedback devices with minimum
disturbance, a system disturbance will occur. In some cases feedback
device degradation may occur before complete failure takes place. The
user must determine if the encoder loss ride through function is
appropriate for each application.