Rockwell Automation 20D PowerFlex 700S AC Drives with Phase II Control Reference Manual - Manual (Page 126)

126 Rockwell Automation Publication PFLEX-RM003E-EN-E - January 2011
Chapter 1 Detailed Drive Operation
Slip Compensation Note : Slip compensation is only active when the motor feedback is set to
sensorless mode or parameter 485 [Motor Ctrl Mode] = 3 “V/Hz.”
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 700S drive can be enabled to more
accurately regulate the speed of the motor without speed feedback.
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.
Slip Compensation Configuration
Slip compensation is enabled by setting parameter 153 [Control Options], bit 19
“SlipComp En” = 1.
The amount of slip compensation to provide is set in parameter 98 [Slip RPM @
FLA]. If desired, the user may adjust parameter 98 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 seen
by examining
Figure 21 below.
Figure 21 - Rotor Speed With/Without Slip Compensation
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