479

•

The operating voltage occurring at the fuse terminals after it has blown must

not behigher than the fuse nominal voltage.

•

The voltage of the arc or the switching voltage must not be higher than the

voltage rating of thepower semiconductor.

•

The fuse turn-off integer I²t must be lower than that of the mains (rectifier

diodes or SCRs.

•

When using filter circuitry, check that the short circuit current is high enough to

blow the fuse.

The company SIBA, for example, offers design software for fuses in inverters as an aid

for the selectionof the right fuse.

The quality of the output current, as well as the switching losses of the inverter power

semiconductors, is determined to a large degree by the modulation algorithm used. A

standard modulation algorithm for a multitude of applications is pulse width modulation

PWM, of which there are various implementations. It is based on the voltage-time area

congruence concept. Chapter

ooked at the sine-trianglemodulation (SPWM) in

the context of the definition of themodulation factor. This concept, just like space vector

PWM (SVPWM) and third harmonic injection PWM (THIPWM), belongs to the

continuous PWM (CPWM) group. Thismeans that turning on and off occurs continually

within each period of the carrier wave. Another group of PWM algorithms is classed as

discontinuous PWM (DPWM). Here, some of the switches are not operated for a certain

time within the carrier period. Purely by numbers it can be derived from this that the

CPWM process will automatically lead to higher switching losses than DPWM. On the

other hand, DPWMwill render higher distortionof the output current thanwill CPWM.

The groupof theCPWMalgorithms include:

•

(Sinusoidal PWM)

This concept samples a sinewave reference signal with a triangular signal of a

higher frequency. Each crossing of the triangular signal with the sine wave

reference signal leads to a switching command given to the power switches

(chapte

.

•

(SpaceVector PWM)

This process is used in the majority of all applications. The expected output

voltage vector V

*

is calculated in phase and amplitude by mathematical and

time-based averaging of two neighbouring voltage vectors and the two zero

vectors.

hows this with an example. The reference voltage vector

V

*

is emulated during the sampling time T

S

by the two active vectors V

1

and V

2

aswell as the two zero vectorsV

0

andV

7

. Vector V

1

is active for the time t

1

and

vector V

2

for the time t

2

. The two zero vectors are then used for the remaining

sampling time

2 1 S 0

t t

T t

−− =

.

•

(ThirdHarmonic InjectionPWM)

This technique, which is based on SPWM or SVPWM, tries to eliminate the

third harmonic in the output signal by overlapping the fundamental of the

reference signal with the thirdharmonic of it.

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