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Voltage source PWM inverter for PMSM supply
Chapter 3 VOLTAGE SOURCE PWM INVERTER FOR PMSM SUPPLY
3.1 Introduction
The block scheme of an adjustable speed drive (ASD) commonly used in industrial
applications to supply three-phase AC motor is presented in Fig. 3.1.
Three-phase
grid
Rectifier
DC link
filter
Inverter
AC motor
Choke
Figure 3.1 Basic scheme of adjustable speed AC motor system.
An ASD is supplied from three or single phase grid. It consists of a diode rectifier, DC
link filter and an inverter. The rectifier converts supply AC voltage into DC voltage.
The DC voltage is filtered by a capacitor in the DC link. The inverter converts the DC
to an variable voltage, variable frequency AC for motor speed or (torque/current)
control.
The rectifier section of an ASD, called the front end, is responsible for generating
current harmonics into the power supply system. Therefore, to reduce the total harmonic
distortion (THD) of phase current it is necessary to add additional choke inductances.
There are generally to way how insert choke inductances (see Fig. 3.2).
a)
b)
L F
D 1
D3
D5
D 1
D3
D5
L F
L F
C F
U DC
C F
U DC
L F
D 2
D4
D6
D 2
D4
D6
L F
Fig. 3.2 Three phase diode rectifier with smoothing choke: a) at the input b) at the DC link side.
By adding a choke inductance at the input of rectifier gives the significant harmonic
reduction. Some drive manufactures are starting to include this choke inductance in the
DC link of the drive, providing the same harmonic current reduction benefit.
Regarding to power electronics standards IEEE Std 519 it is recommended that
production of harmonics should be less than 5%. It is a trend to replace the diode
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