User Manual Instruction Manual

Application Notes 13-75
When enabled, flux braking automatically increases the motor flux resulting in an
increase of motor losses. The flux current is only increased when the bus voltage
regulator is active. When the bus voltage regulator is not active, the flux current is
returned to normal. The maximum flux current is equal to rated motor current but may
be further reduced depending on the load level, IT protection, or current limits. In
general, the flux current is not increased when the motor is at or above rated speed. At
higher speeds, field weakening is active and the motor flux current cannot be
increased. As the speed decreases below base speed, the flux current increases until
there is enough voltage margin to run rated motor current.
Because flux braking increases motor losses, the duty cycle used with this method
must be limited. Check with the motor vendor for flux braking or DC braking
application guidelines. You may also want to consider using external motor thermal
protection.
13.28 Flux Up
AC induction motors require flux to be established before controlled torque can be
developed. To build flux in these motors, voltage is applied to them. GV6000 drives
have two methods to flux the motor.
The first method is a normal start. During a normal start, flux is established as the
output voltage and frequency are applied to the motor. While the flux is being built, 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.
The second method is Flux Up Mode. In this mode, DC current is applied to the motor
at a level equal to the lesser of the current limit setting, drive rated current, and drive
DC current rating. The flux up time period is based on the level of flux up current and
the rotor time constant of the motor.
Figure 13.21 – Accel Profile during Normal Start -- No Flux Up
Time
0
Frequency
Reference
Frequency
Rated Flux
Rotor
Stator
Oscillation due
to flux being
established