User's Manual

PMAC2 User Manual
Setting Up PMAC2 for Direct PWM Control 15
+
+
Magnetization
Current
Ix77
Torque Command
from Position/
Velocity Servo
Direct Current
Command
Quadrature
Current Command
dq
to
abc
DAC
I
q
I
d
I
b
I
a
DAC
PI
PI
V
b
V
a
PWM
PWM
I
a
I
b
Phase A Current Loop
Phase B Current Loop
PWM
V
c
Balance Loop
v
c
v
b
v
a
+
-
-
-
-
-
-
-
'
'
'
'
PMAC
A
MPLIFIER
E
Q
Figure 8 PMAC/PMAC2 Commutation with Analog Current Loop
Digital Loops in the Field Frame
In an system with a digital current loop, it is possible to close the current loops in the field frame (not all
such systems do, however). Instead of transforming the current commands from field frame to stator
frame before closing the loop, the actual current measurements are transformed from stator frame to field
frame. In the field frame, a direct-current loop is closed, and a quadrature current loop is closed. This
produces a direct-voltage command, and a quadrature-voltage command; these are transformed back into
the stator frame to become phase-voltage commands, which are implemented as PWM values.
The direct and quadrature current values are DC quantities, even as the motor is rotating and the field is
slipping. Therefore, the high-frequency limitations of the servo loop are irrelevant. This provides
significantly improved high-speed performance from identical motors and power electronics.
PWM
ADC
PWM
PI
PI
P
Magnetization
Current
Ix77
Torque Co mman d
from Position/
Velocity Servo
or 'O' Command
Direct Current
Command
Quadrature
Current Command
Direct Current Loop
Quadrature Current Loop
P
+
+
+
+
-
-
-
-
ADC
dq
to
abc
abc
to
dq
PWM
V
a
V
d
v
a
I
a
I
d
I
d
I
q
i
a
V
b
v
b
I
b
I
q
i
b
V
c
V
q
v
c
E
Θ
Figure 9 PMAC2 Commutation with Digital Current Loop
PMAC2 has a PI (proportional-integral) digital current loop. There is only one set of gains, which serves
for both the direct current loop and the quadrature current loop. Tuning is best done on the direct current
loop, because this will generate no torque, and therefore no movement. The current-loop autotuner in the
PMAC Executive program uses the direct current loop to tune. This is valid even for permanent-magnet
brushless motors where no direct current will be used in the actual application. It is important to
remember that current loop performance is not load-dependent, so the motor does not need to be attached
to the load during the tuning process (for position/velocity loop tuning, the load does need to be attached).