0195136047.pdf

770 BASIC CONTROL SYSTEMS

CS

Figure 16.1.18Symbol of a self-commutated semiconductor switch.

A common symbol for theself-commutated semiconductor switchis shown in Figure 16.1.18.

The control signal (either voltage or current) is denoted by CS, and the diode gives the direction

in which the switch can conduct current. GTOs, power transistors, and MOSFETs are classified

as self-commutated semiconductor devices because they can be turned off by their respective

control signals: GTOs by a gate pulse, a power transistor by a base drive, and a MOSFET by

a gate-to-source voltage. A thyristor, on the other hand, is a naturally commutated device that

cannot be turned off by its gate signal. A thyristor combined with a forced commutation circuit

behaves like a self-commutated semiconductor device, however. The self-commutation capability

makes its turnoff independent of the polarity of the source voltage, the load voltage, or the nature

of load. Self-commutated semiconductor switches are suitable for applications for converters fed

from a dc source, such as inverters and choppers.

Figure 16.1.19 shows a three-phase voltage-source inverter circuit, along with the corre-

sponding voltage and current waveforms. The motor connected to terminalsA, B,andCcan have

wye or delta connection. Operating as a six-step inverter, the inverter generates a cycle of line

or phase voltage in six steps. The following Fourier-series expressions describe the voltagesvAB

andvAN:

vAB=

2

√

3

π

Vd

[

sin

(

ωt+

π

6

)

+

1

5

sin

(

5 ωt−

π

6

)

+

1

7

sin

(

7 ωt+

π

6

)

+...

]

(16.1.46)

vAN=

2

π

Vd

[

sinωt+

1

5

sin 5ωt+

1

7

sin 7 ωt+···

]

(16.1.47)

The rms value of the fundamental component of the phase voltagevANis given by

S 4

iA A

S 1

iC 1

O Vd

+

−

(a)

iC 4

D 1

D 4

S 6

B

S 3

iC 3

iC 6

D 3

D 6

S 2

C

S 5

iC 5

iC 2

D 5

D 2

Figure 16.1.19Three-phase voltage-source inverter circuit with corresponding voltage

and current waveforms.