Rockwell Automation 20A PowerFlex 70EC/700VC - Manual (Page 50)

46 Flux Up
Flux Up AC induction motors require flux to be established before controlled torque can be
developed. To build flux, voltage is applied. There are two methods to flux the
motor.
The first method is during a normal start. Flux is established as the output voltage
and frequency are applied to the motor. While the flux is being established, the
unpredictable nature of the developed torque may cause the rotor to oscillate even
though acceleration of the load may occur. In the motor, the acceleration profile
may not follow the commanded acceleration profile due to the lack of developed
torque.
Figure 5 Accel Profile during Normal Start - No Flux Up
The second method is Flux Up Mode. In this mode, DC current is applied to the
motor so that the flux is established 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 user adjustable.
Figure 6 Flux Up Current versus Flux Up Time
[Flux Up Time]
Once rated flux is reached in the motor, normal operation begins and the desired
acceleration profile is achieved.
Figure 7 Rated Flux Reached
70EC 700VC 700H
✔ ✔
Time
0
Frequency
Reference
Fr
eque
nc
y
Rated Flux
Rotor
Stator
Oscillation due
to flux being
established
Flux Up Time
Flux Up Current = Maximum DC Current
Rated Motor Flux
Motor Flux
Rated Flux
Current
T1
T2
T3
T4
Flux Up Current
Time
Ir Voltage - SVC
Greater of IR Voltage or
Voltage Boost - V/Hz
Flux Up Normal
Operation
Flux Up
Voltage
Motor Flux
Stator Voltage
Rotor Speed
Motor Flux
Stator Freq