Danaher Motion SuperDrive ACS24 Series, Руководство пользователя

Страница: 62 / 99

Configuration & control of the AC SuperDrive
26.04.2005
6.12 SPWM-
SYM
6.12.1 B
ASICS
The principle of SPWM-sym (Sinusoidal Pulse Width Modulation with summarization) is shown in
Figure 29 .
Figure 29. Simplified scheme of SPWM-sym.
The reference voltages, Ud Reference and Uq Reference, computed by the Current Control are
first normalised with the ratio between the nominal DC bus voltage (36, 48 or 80 V) and the
measured DC bus voltage. The voltages are then transformed to three phase quantities by the
use of the transformation angle. The three phase reference voltages (Uu Reference, Uv
Reference and Uw Reference) are symmetries according to
Figure 30 before the dead-band
compensation and the minimum on/off time check is done. The dead-band compensation is
necessary in order to avoid disturbances in the motor currents. Especially third harmonics with
relatively high amplitude will otherwise occur in the currents. The min on/off time check is done in
order to protect the MOSFETs, i.e. a too short time between the turn-on and turn-off of a
MOSFET may give permanent damage to the device. Finally the reference voltages are
compared with a triangular reference wave and the intersection gives the switching instants. The
frequency of the triangular carrier wave determines the PWM switching frequency. Nominally the
switching frequency is equal to 8 kHz but with the parameter SPWM Switching Frequency it is
possible to change it to 12 or 16 kHz even when the inverter is enabled (see
Table 24 ). However,
a higher switching frequency means higher losses in the inverter and thereby the rating of the
ACS is decreased. The output from the DSP is the MOSFET gate pulses and the pulses are
generated with the necessary dead-band between the turn-off and the turn-on of MOSFETs in
each phase leg of the inverter.
Index Sub-Index Name Description
2070h 19 SPWM Switching Frequency 8, 12 or 16 kHz.
Table 24. Parameter SPWM Switching Frequency in the SPWM-sym block.
The symmetrisation of the reference voltages is shown in Figure 30 . With the symmetrisation it is
possible to improve the utilization of the SPWM method, i.e. the range of the linear relationship
between the reference voltages and the realized line-to-line voltages is extended compared to
traditional SPWM. The symmetrisation method is comparable to the method where a third
harmonic is added. However, symmetrisation is to prefer in a digital implementation since the
harmonic content as well as the generated voltage time area is optimized during each PWM
switching period.
Item No.
89Y05120A
User's Guide 1.0
62