Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 192)

192 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 3 Diagnostics and Protection
Slip Compensation When slip compensation mode is selected, the drive automatically adds the
appropriate amount of output frequency to maintain a consistent motor speed
independent of load. During drive commissioning, P621 [Slip RPM at FLA] is
set based on entered motor nameplate information. This parameter can be
adjusted to provide more or less compensation.
See the motor speed compensation figure below for a comparison of operation
with and without slip compensation. This shows that over time, slip
compensation corrects for changes in load (curved lines). In contrast, open loop
operation shows that no correction is made based on load.
Internally, the drive converts the rated slip in RPM to rated slip in frequency. To
more accurately determine the rated slip frequency in hertz, an estimate of flux
current is necessary. This parameter is either a default value based on motor
nameplate data or the autotune value. The drive scales the amount of slip
compensation to the motor rated current. The amount of slip frequency added to
the frequency command is then scaled by the sensed torque current (indirect
measurement of the load) and displayed.
Slip compensation also affects the dynamic speed accuracy (ability to maintain
speed during shock loading) as illustrated in the rotor speed response figure
below. Initially, the motor is operating at some speed and no load. Some time
later, an impact load is applied and the rotor speed decreases as a function of load
and inertia. Finally, the impact load is removed and the rotor speed increases
momentarily until the slip compensation is reduced based on the applied load.
Open Loop
Mode
Slip Compensation
Active
Time
Ro
to
r S
pee
d
1.5 p.u. Load
1.0 p.u. Load
0.5 p.u. Load
0
0
Slip Compensation
Active
Slip Compensation
Active
0.5 p.u. Load
1.0 p.u. Load
1.5 p.u. Load
Load
Applied
Load
Applied
Load
Removed
Slip @
F.L.A.