Rockwell Automation 1395 Digital DC Drive FRN 5-10.10/9.30 - Manual (Page 115)

Chapter 5
Functional Description
5-29
When enabled, the Tach Loss feature operates as follows:
Velocity feedback from an encoder or tach is compared against velocity
feedback derived from armature voltage. When the magnitude
(absolute value) of the difference between the two feedbacks exceeds
the Tach Loss Window (Parameter #688), for a period of time in
excess of 40 msec., an automatic switchover to Armature Volts
Feedback will occur.
NOTE: This statement is true for version 5.01 Firmware. For Firmware
versions greater than 5.01 refer to parameters 731 and 732 for Tach
Loss/Switch over algorithm.
When a loss of feedback is detected in Tach Loss Recovery mode, the
following action is automatically taken by the drive:
1. Change the velocity loop K
p
and K
i
gains to new values that will
provide stable operation under armature voltage control. These gains
are supplied by two parameters (Parameter 689 for Tach Loss K
i
and
Parameter 690 for Tach Loss K
p
).
2. Freeze the field flux to the value present at the time of the Tach Loss.
3. Issue a Tach Loss warning via the Logic Status Word (Parameter 100
bit 1 = 0, bit 0 = 1) and the VP Fault Word (Parameter 101 bit 0 = 1).
4. Change the Feedback Device parameter to the Armature Voltage
Feedback value (set Parameter 621 to 1).
5. The forward and reverse speed limits are set to the speed value at the
time of the Tach Loss.
These changes remain in effect until a “Clear Faults” command is issued,
or until the feedback device selection is changed back to its original value.
From this point on, the drive will continue running in Armature Voltage
Feedback mode. If you were to make the original feedback device
functional again, it would be possible to switch back to it using one of two
methods:
1. Change the value of the feedback device parameter (Parameter 621)
to the original feedback type. NOTE: If the drive is running while
this change is made, the drive will check to make certain the feedback
from the primary feedback device is within the tach loss window. If it
is, the switch will be honored and the parameter values will be
restored to their previous values. If not, the switchover will not be
allowed.
2. Issue a “Clear Faults” command to the Drive. The fault will be
cleared, and the Drive will be reset to it’s previous feedback device,
gains and field flux level. Note that this command is only honored
when the Drive is not running.