Danaher Motion SuperDrive ACS24 Series, Руководство пользователя

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26.04.2005
Configuration & control of the AC SuperDrive
Figure 30. Symmetrisation of the reference voltages.
The maximum line-to-line voltage that can be generated with SPWM-sym without over
modulation is U
line-to-line
= Udc/
√
2
≈
0.707
⋅
Udc. Compared to traditional SPWM (U
line-to-line
=
Udc/2/
√
2
⋅√
3
≈
0.612
⋅
Udc) the voltage is 15.4% higher but compared to square-wave modulation
(U
line-to-line
=
√
2/
π⋅
Udc
⋅√
3
≈
0.80
⋅
Udc) it is 9.3% lower. However the SPWM-sym block utilizes over
modulation so the maximum voltage that is possible to generate is close to that of square-wave
modulation.
Over modulation will occur above the base speed since the motor voltage then is high. The result
of over modulation is that there will be less switching in the inverter since the reference voltages
are above the triangular carrier wave during a number of PWM switching periods, i.e. the
MOSFETs are mainly switching during the zero crossings of the reference voltages. The use of
over modulation will increase the harmonic content in the motor currents mainly by adding 5
th
and
7
th
harmonics. This will increase the harmonic losses in the motor. However, since a higher
voltage can be generated, the total losses will be lower than if no over modulation was used. It is
because a lower fundamental current is required for a certain torque and it is the R
⋅
I
2
losses in
the motor caused by the fundamental part of the current that dominates compared to the
increased harmonic losses.
An example of the resulting currents at no over modulation and over modulation can be seen in
Figure 31 . The measurements are from a no load situation which means that there is only
magnetization current. Therefore the harmonics at over modulation are rather high compared to
the fundamental current.
User's Guide 1.0 Item No.
89Y05120A
63