Rockwell Automation 1395 DC Drive, 800-1250 Hp, FRN 5.XX-9.30 - Manual (Page 56)

Chapter 3
Functional Description
3-23
Feedback Control (Circle 14)
Two current transformers (CT’s) sense armature current flow. The current
feedback is scaled using Motor Armature Full Load Amps (Parameter 611)
and Rated Armature Bridge Current (Parameter 615). Parameter 112 is the
average armature current feedback value. The field current transducer
(FCT) provides field current feedback to the control which is scaled by
Rated Field Motor Current (Parameter 612) and Rated Field Bridge Current
(Parameter 616). The average field current can be read by Parameter 118.
Tach Loss Recovery
When Tach Loss Recovery is selected, it allows the Drive to continue
operation under armature voltage control in the event that the primary
feedback device fails (the primary device can be an encoder or DC tach).
The switchover occurs automatically and does not shut the Drive down.
This feature also provides an option for configuring a tach loss as a
Warning Fault. This is done with Parameter 691 “Tach Switch Sel”. If
Parameter 691 is set to one, the Tach Loss Recovery feature is activated
and Tach Loss becomes a Warning fault. If Parameter 691 is set to zero, a
tach loss causes a soft fault. The default value for Parameter 691 is zero
(tach loss recovery disabled).
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 (new Parameter 688), for a period of time in
excess of 40 msec., an automatic switchover to Armature Volts
Feedback will occur.
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 (689 for Tach Loss K
i
and 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
bit1 = 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