Delta Tau GEO BRICK LV User Manual
Turbo PMAC User Manual
Setting Up Turbo PMAC-Based Commutation and/or Current Loop
93
A current vector in the stator that is parallel to the rotor field induces current in the rotor that changes the
magnetic field strength of the rotor (when the stator and rotor field are rotating relative to each other).
This component of the stator current is known as direct current. For an induction motor, this is required
to create a rotor magnetic field. For a permanent-magnet brushless motor, the rotor magnets always
produce a field, so direct current is not required, although it can be used to modify the magnetic field
strength. On Turbo PMAC, parameter Ixx77 for Motor xx determines the magnitude of the direct current.
magnetic field strength of the rotor (when the stator and rotor field are rotating relative to each other).
This component of the stator current is known as direct current. For an induction motor, this is required
to create a rotor magnetic field. For a permanent-magnet brushless motor, the rotor magnets always
produce a field, so direct current is not required, although it can be used to modify the magnetic field
strength. On Turbo PMAC, parameter Ixx77 for Motor xx determines the magnitude of the direct current.
Analog Loops in the Stator Frame
In an amplifier with an analog current loop, the closure of the loops on the stator windings must be closed
in the stator frame, because the current measurements are in the stator frame, and analog circuitry has no
practical way to transform these. In such a system, the current commands must be transformed from the
field frame in which they are calculated to the stator frame, and converted to voltage levels representing
the individual stator phase current commands. These are compared to other voltage levels representing
the actual stator phase current measurements.
in the stator frame, because the current measurements are in the stator frame, and analog circuitry has no
practical way to transform these. In such a system, the current commands must be transformed from the
field frame in which they are calculated to the stator frame, and converted to voltage levels representing
the individual stator phase current commands. These are compared to other voltage levels representing
the actual stator phase current measurements.
As the motor is rotating, and/or the field is slipping, these current values, command and actual, are AC
quantities. Overall loop gain, and therefore system performance, is reduced at high frequencies (high
speeds). The back EMF phase voltage, which acts as a disturbance to the current loop, is also an AC
quantity. The current loop integral gain or lag filter, which is supposed to overcome disturbances, falls
well behind at high frequencies.
quantities. Overall loop gain, and therefore system performance, is reduced at high frequencies (high
speeds). The back EMF phase voltage, which acts as a disturbance to the current loop, is also an AC
quantity. The current loop integral gain or lag filter, which is supposed to overcome disturbances, falls
well behind at high frequencies.
PMAC / PMAC2 Commutation with Analog Current Loop
PWM
PI
PWM
PI
Phase A Current Loop
Phase B Current Loop
PWM
DAC
DAC
V
b
-
-
-
+
+
-
+
-
-
-
I
a
V
a
V
c
I
b
V
b
V
a
Balance Loop
E
Torque Command
from Position/
Velocity Servo
I
a
’
I
b
’
dq
to
abc
Quadrature
Current Command
I
q
’
I
d
’
Magnetization
Current
Ix77
Direct Current
Command
SG
Ix78
1/s
PMAC
Amplifier
+
+
Θ
r
Θ
e
DC
AC
V
c
Digital Loops in the Field Frame
In a 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 the current commands being transformed from field frame to stator
frame before the loop is closed, 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.
such systems do, however). Instead of the current commands being transformed from field frame to stator
frame before the loop is closed, 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.
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.