Allen-Bradley PowerFlex 70 Reference Manual page 163

Open Loop
As the load on an induction motor increases, the rotor speed or shaft speed of the
motor decreases, creating additional slip (and therefore torque) to drive the larger
load. This decrease in motor speed may have adverse effects on the process. If the
[Speed Mode] parameter is set to "Open Loop, " no speed control will be
exercised. Motor speed will be dependent on load changes and the drive will
make no attempt to correct for increasing or decreasing output frequency due to
load.
Slip Compensation
As the load on an induction motor increases, the rotor speed or shaft speed of the
motor decreases, creating additional slip (and therefore torque) to drive the larger
load. This decrease in motor speed may have adverse effects on the process. If
speed control is required to maintain proper process control, the slip
compensation feature of the PowerFlex drives can be enabled by the user to more
accurately regulate the speed of the motor without additional speed transducers.
When the slip compensation mode is selected, the drive calculates an amount to
increase the output frequency to maintain a consistent motor speed independent
of load. The amount of slip compensation to provide is selected in [Slip RPM @
FLA]. During drive commissioning this parameter is set to the RPM that the
motor will slip when operating with Full Load Amps. The user may adjust this
parameter to provide more or less slip.
As mentioned above, induction motors exhibit slip which is the difference
between the stator electrical frequency, or output frequency of the drive, and the
induced rotor frequency.
The slip frequency translates into a slip speed resulting in a reduction in rotor
speed as the load increases on the motor. This can be easily seen by examining
Figure 33.
Figure 33 Rotor Speed with/without Slip Compensation
Slip Compensation
Inactive
Load
Applied
No Load
0.5 p.u. Load
1.0 p.u. Load
1.5 p.u. Load
Slip @
F.L.A.
0
0
Without slip compensation active, as the load increases from no load to 150% of
the motor rating, the rotor speed decreases approximately proportional to the
load.
With slip compensation, the correct amount of slip compensation is added to the
drive output frequency based on motor load. Thus, the rotor speed returns to the
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
Speed Control, Mode, Regulation & Vector Speed Feedback
Slip Compensation Slip Compensation
Active
1.5 p.u. Load
Load
1.0 p.u. Load
Applied
0.5 p.u. Load
Load
Removed
Time
Active
163