7.3.6.3 Overmodulation
The "usqref" and "usdref" components permit so-called overmodulation of the DC
link voltage (limitation to hexagon instead of circle). The maximum output voltage
which can be set for each phase angle results from the circle which fits in the voltage
hexagon (see diagram Section "“Circle and hexagon voltages” diagram" on page
128).
By setting the hexagonal modulation to "HEX_PHASE(3)" using P 432 - CON_
CCON_Mode, the length of the vector for the output voltage can be placed in the
range of the DC link voltage (red). As a result only two of the three half-bridges are
switched in each switching interval. The third remains at the upper or lower potential
of the DC link voltage for a period of 60° of the output frequency.
This method has only two thirds of the switching losses of modulation with all three
phases. Disadvantages are higher harmonics of the motor currents and thus less
smooth running at high motor speeds.
Representation of the eight vectors of the three-phase voltage system (3 half-bridges
each with 2 states [2
3
]). The vectors correspond to the DC link voltage U
ZK
and form
a voltage hexagon.
MOOG
ID No.: CB40859-001 Date: 11/2020
MSD Servo Drive - Device Help
128
7 Control
Image 7.18: “Circle and hexagon voltages” diagram
U
N
Mains voltage
U
L
Voltage at inductor
U
u
Inverter voltage
U
zk
DC link voltage
α Phase angle
Legend for “Circle and hexagon voltages” diagram
ID
Index Name
Unit Description
431
0
CON_CCON_VLimit
%
Voltage limit for first current controller
432
0
CON_CCON_Mode
Select current control / limitation mode
Table 7.9: “Advanced torque control - Overmodulation” parameters
