Generation Of A Variable Voltage By Pulse Width Modulation - Siemens SINAMICS G130 Engineering Manual

If we consider all three phases, there are a total of 2
states in the motor can be defined by voltage phasors.
Switching states of the
inverter
V
1
V
2
V
3
V
4
V
5
V
6
V
7
V
8
If, for example, phase L1 is connected to the positive DC link voltage, and phases L2 and L3 to the negative voltage
so as to produce switching state V
designated phase I. The length of this phasor is determined by the DC link voltage.
Representation of resultant motor voltages as phasor
If the switching state changes from V
change in potential at terminal L2. The length of the phasor remains unchanged.
In the same way, the relevant voltage phasors are produced by switching combinations V
combinations V and V produce the same potential at all motor terminals. These two combinations therefore produce
7 8
voltage phasors of "zero" length (zero voltage phasor).

Generation of a variable voltage by pulse width modulation

Voltage and frequency must be specified in a suitable way for a certain operating state of the motor, characterized by
speed and torque. Ideally, this corresponds to control of the voltage vector V
rotation ωt = 2 π
f and adjusted absolute value. This is achieved through modulation of the actual settable voltage
* *
space vectors (pulse width modulation). In this way, the momentary value V
actual settable voltage space vectors and the voltage zero.
The solid angle is set directly by varying the ratio of the ON durations (pulse width) of adjacent voltage vectors, the
desired absolute value by varying the ON duration of the zero voltage vector. This method of generating gating
signals is called space vector modulation SVM. Space vector modulation provides sine-modulated pulse patterns.
Fundamental Principles and System Description
³
= 8 switching states in the inverter, and the effect of these
Phase Phase
L1 L2
+ -
+ +
- +
- +
- -
+ -
+ +
- -
, the resultant voltage phasor points in the direction of motor phase L1 and is
1
to V , then the voltage phasor rotates clockwise by an angle of 60°el due to the
1 2
Engineering Information
Phase
L3
-
-
-
+
+
+
+
-
on a circular path with the speed of
(ωt)
is formed by pulses of the adjacent,
(ωt)
SINAMICS Engineering Manual - May 2008
© Siemens AG
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