Rockwell Automation 20D PowerFlex 700S AC Drives with Phase II Control Reference Manual - Manual (Page 166)

166 Rockwell Automation Publication PFLEX-RM003E-EN-E - January 2011
Chapter 1 Detailed Drive Operation
SLAT Minimum Mode
In SLAT minimum mode, you would typically configure a speed reference that
forces the speed regulator into saturation (the speed reference is slightly above the
speed feedback). In this case the drive would follow the torque reference until
there was a breakage or slippage in the application.
When the drive is following a torque reference (torque mode) in SLAT minimum
mode, either one of two conditions will force the drive into following the speed
reference (speed mode):
• The output of the speed regulator becomes less than the torque reference.
This is the same condition that exists in minimum torque mode.
Or,
• The speed error becomes negative (the speed feedback becomes greater
than the speed reference). This is forced speed mode. By forcing the drive
to enter speed mode, the transition occurs earlier than it would have in the
minimum torque mode, resulting in less velocity overshoot.
Parameters 119 [SLAT ErrorSetpnt] and 120 [SLAT Dwell Time] allow you to
set some hysteresis for turning off the forced speed mode. They are set to 0 as
default so that there is no hysteresis. In SLAT minimum mode, [SLAT
ErrorSetpnt] sets how much less the speed feedback should be than the speed
reference before turning off the forced speed mode. The SLAT dwell time sets
how long the speed error must exceed the SLAT error set point before turning off
the forced speed mode.
At the time that the drive switches from torque mode to forced speed mode, the
speed regulator output is loaded with the internal motor torque reference to
create a smooth transition.
In order for the drive to switch from speed mode to torque mode, forced speed
mode (if active) must first be turned off. Forced speed mode will turn off when
the speed error is greater than the SLAT error set point for the SLAT dwell time.
With default parameter settings, this will occur when the speed error becomes
positive.
When forced speed mode is off, the drive will switch back to torque mode when
the speed regulator output becomes greater than the torque reference. This is the
same condition that exists in minimum torque mode.