KEB COMBIVERT F5 Reference Manual page 152

LM - Motor Data Parameters
LM24
Field Weakening
Corner
152
The field weakening corner determines at which speed the peak torque limit
starts being reduced. It is necessary to reduce the peak torque limit of the
motor since the drive's ability to force current into the motor is limited by the
applied voltage as rated speed is reached.
If the drive tries to demand more torque than the motor can produce given the
available voltage and actual motor speed, it is possible that the breakdown
torque of the motor will be exceeded and as a result the motor will appear to
stall and run at less than desired speed.
Generally this phenomenon can be identified as the car reaches contract
speed momentarily but then drops to a lower speed or the car speed stalls
at some speed lower than contract speed. Monitor the Modulation Grade in
the Diagnostic Screen #2 or DG.10. If the value is reaching 100% or higher,
the voltage limit is being reached; this may be caused from a sag in the line
voltage. As a result the peak torque command must be further limited in order
to maintain control of the motor.
The solution is simply to reduce the value of LM24 to about 60% of synchronous
speed (720 rpm for a 1,200 rpm motor). A setting of 45% of synchronous
speed should be used as the practical lower limit of this parameter.
Motor Torque vs. Speed
350
250
150
100
50
0
0 600 900
Motor Speed (rpm)
This parameter is not applicable to PM Synchronous motors and therefore
will not appear.
Rated Torque
Peak Torque
Limited Pk Torque
1200 1800 2400