Rockwell Automation Publication 2198-UM002E-EN-P - February 2018
337
Motor Control Feature Support
Appendix D
Flux Up
AC induction motors require that flux builds in the motor stator before
controlled torque can develop. To build flux, voltage is applied. There are two
methods to flux the motor and three configurable FluxUpControl settings.
With the No Delay setting (normal start), flux is established when the output
voltage and frequency are applied to the motor. While flux is building, the
unpredictable nature of the developed torque can cause the rotor to oscillate
even though acceleration of the load can occur. In the motor, the acceleration
profile does not follow the commanded acceleration profile due to the lack of
developed torque.
Figure 168 - Acceleration Profile during Normal Start - No Flux Up
With the Automatic setting (default) DC current is applied to the motor so
that flux builds before rotation. The flux-up time period is based on the level of
flux-up current and the rotor time constant of the motor. The flux-up current is
not adjustable.
In the Manual setting, DC current is applied to the motor so that flux builds
before rotation. The flux-up time period is determined by the FluxUpTime
attribute. The flux-up current is not adjustable.
Figure 169 - Flux Up Current versus Flux Up Time
0
Stator
Rotor
Frequency
Reference
Rated Flux
Oscillation due to flux
being established.
Time
Fre
qu
en
cy
0
Flux Up Current = Maximum DC Current
Rated Motor Flux
Motor Flux
Rated Flux
Current
Flux Up Time
T1
T2
T3
T4
Fl
ux Up
Cu
rrent