Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 31)

DESCRIPTION OF OPERATION 5-13
1557-UM050F-EN-P –June 2013
Flux Control for Synchronous Motor
Most of the magnetization for a synchronous motor is supplied by the rotor field winding, unlike an
induction motor where all of the magnetizing current is supplied through the stator. However, control of
the motor flux through the field current is very slow because of the large time constant of the dc field
winding and the current and voltage limitations of the field supply. To obtain sufficiently fast response
from the flux regulator the magnetizing current is split into transient and steady state components, with
the steady state component supplied through the rotor and the transient component through the stator.
The additions to the flux control required for synchronous machines are shown in the block diagram
below. A low pass filter controlled by parameter If Cmd Bandwidth is used to extract the steady state
component of Isd Command which becomes I Field Command, the current reference for the field supply.
The transient component of Isd Command is obtained by subtracting I Field Command from the original
Isd Command and multiplying by the ratio of L magnetizing to Lmd to provide the same gain for the
steady state and transient components. The portion of the motor filter capacitor current supplied by the
drive is then added to determine Ix Command, which is the magnetizing component of the dc link current
command.
Parameter Icd Command Gain determines how the motor filter capacitor current is split between the
motor and the drive. When this parameter is set to its minimum value of 0.0, all the capacitor current is
supplied by the drive. The line current is higher than the motor current and the motor operates at
approximately unity power factor. When this parameter is set to its maximum value of 1.0, all the
capacitor current is supplied by the motor. The line current is less than the motor current and the motor
operates at a lagging power factor with reduced field current. Increasing Icd Command Gain improves
the input power factor of the drive, but also increases the coupling between flux and torque and causes
the stability of the flux control to deteriorate.
-
Icd command
gain (P105)
Icd command
(V311)
Isd command
(V310)
If cmd bandwidth
(P104)
LOW PASS L magnetizing
(p131)
Lmd
(P135)
Ix
command
(V312)
I field
command
(V314)
+ + +
+ -
Flux Control for Synchronous Motor