Tài liệu HIGH PERFORMANCE DRIVES_Chapter1 ppt

HIGH PERFORMANCE DRIVES

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 E Levi, 2001

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2.4.2. Voltage source inverter (VSI)

Variable voltage, variable frequency operation of induction machines is realised utilising autonomous

inverters, in conjunction with a rectifier and a DC link circuit. The voltage source inverter (VSI) is the

most frequently applied power supply source for speed control of induction motors. It can be operated in

six-step mode or in PWM mode. Six-step operation will be considered first.

Three-phase VSI contains three inverter legs. Input voltage for a three-phase VSI is provided by a three￾phase (or single-phase) bridge rectifier with capacitor placed at the output. The capacitor provides

smoothing of the DC voltage and, for sufficiently large capacitance, DC voltage at the rectifier output

approaches a constant value. It will therefore be assumed that inverter input voltage is constant in all the

subsequent analysis.

Power circuit of a voltage source inverter is shown in Fig. 2.13. As the inverter itself controls only the

frequency of the output voltage when operated with switching frequency equal to output fundamental

frequency (six-step mode), a controllable rectifier must be used in order to provide control of the output

voltage magnitude (output voltage magnitude is proportional to the input DC voltage). Each switch in

the inverter circuit is composed of two back-to-back connected semiconductor devices. One of these two

is a controllable switch, while the other one is a diode. The three inverter legs are controlled in such a

way that leg voltages constitute three-phase system of square-wave voltages. This means that, assuming

that upper transistor in leg A is fired at time instant zero, firing of upper transistor in leg B will take

place after 120 degrees, while firing of the upper transistor in leg C will be delayed for another 120

degrees. The conduction of each of the six semiconductor switches is again 180 degrees so that at any

time three out of six switches are on and the remaining three switches are off. The resulting output

voltage waveforms for line-to-line voltages are quasi-square waves, with two 60 degrees zero intervals

and two 120 degrees intervals in which line-to-line voltage equals plus and minus DC voltage,

respectively. VSI operated in the 180 degrees conduction mode is therefore usually called six-step

inverter. Leg voltages of the inverter are given in Fig. 2.14 with respect to the negative pole of the DC

link. Line-to-line voltages applied to the induction machine are obtained directly from leg voltages as

p

C

VDC A BC

n

IM

Rectifier and inverter control

Fig. 2.13 - Three-phase voltage source inverter (VSI) fed induction motor drive.