1-8
Functional Description
7000-TD002A-EN-P – September 2007
Flux Control
The function of the flux control block (Figure 1.5) is to determine the
magnetizing component (
I
sd
) of the stator current (
I
s
) needed to
maintain the desired flux profile in the motor. The inputs are
Flux
Feedback
(306) and
Stator Freq
(448) from the motor model,
Speed
Feedback
(289)
and
Torque Reference
(291) from the speed control
block and the measured voltage at the input of the rectifier,
Rec
Input Volt
(696).
The
Flux Feedback
is subtracted from the
Flux Reference
(305) to
determine the
Flux Error
(307), which is the input to the flux PI
regulator. The gains are determined from desired
Flxreg Bandwidth
(97) and motor parameters
T Rotor
(132) and
Lm Rated
(131). The
output of the flux regulator is
FluxCurRegulator
(309). An open loop
estimate of the magnetizing current
FluxCur Feedfwd
(308) is
determined by dividing the
Flux Reference
by parameter
Lm Rated
.
FluxCur Feedfwd
and
FluxCurRegulator
are added to produce
Mtr
Flux CurCmd
(310) which is the magnetizing component of the
stator current command. To calculate the magnetizing current
supplied by the inverter
Inv Flux CurCmd
(312), the current supplied
by the motor filter capacitor in magnetizing is calculated and
subtracted from
Mtr Flux CurCmd
. It should be noted that as the
motor speed increases,
Inv Flux CurCmd
decreases. This is because
as the motor voltage increases more of the magnetizing current
requirement of the motor is met by the capacitor. At resonant point,
Inv Flux CurCmd
is nearly zero and becomes negative at speeds
above resonance.
InvTorque CurCmd
(from Speed Control block)
and
Inv Flux CurCmd
are then passed to the Current Control block to
determine the dc link current reference (
Idc Reference
) and the firing
angles of the two converters (
Alpha Rectifier
and
Alpha Inverter
).
The flux profile in the drive is adjusted by the parameters
Flx Cmd
No Load
(103) and
FlxCmd RatedLoad
(100). Using these
parameters
, Flux Reference
is adjusted linearly with the desired
Torque Reference
. At light loads motor flux
is decreased allowing
reduction in losses while full flux is produced at rated load. The
maximum flux reference is limited to
Flux Cmd Limit
(623). This
limit is dependent on the
Rec Input Volt
and the motor speed (
Speed
Feedback
). If the drive operates at reduced line voltage, then
Flux
Reference
is reduced. Also if the motor is running above the
Base
Speed
, the flux profile is made inversely proportional to the speed of
the motor resulting in the field weakening or the constant power
mode of operation of the drive. This is accompanied by a decrease in
the motor torque capability.
Содержание PowerFlex 7000
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