482

To produce the graph in

a value of

3

2 has been used for k

f

. A far worseHDF

would result without this increase in frequency in theDPWM technique.

HDF as a function of the modulation factor with otherwise unchanged

conditions but increased switching frequency f

SW

for theDPWM techniques

To evaluate the quality of the PWM technique a look at the losses of the power

semiconductors apart from the HDF has to be taken. In the CPWM technique all three

inverter phase currents commutate within one period of the fundamental frequency.

Therefore, the switching losses for thesePWM variants, based on theCPWMalgorithm,

are identical and independent of the current phase angle and, thus, independent of the

power factor cos

ϕ

. For the DPWM techniques, however, there is a dependency of the

switching losses on the phase angle of the current, as – depending on the selected

DPWM algorithm – the power semiconductors are not switched for a certain period. To

assess the loss behaviour of the individual variants the switching loss factor (SLF) is

introduced, which is defined as the relation of the losses with a certain modulation

variant to the losses occurringwith aCPWM technique.

CPWM

DPWM

P

P SLF

=

According to the calculations of A.M. Hava et al.

he following relations result for the

variousDPWM techniques, defined section by section:

π

≤ϕ≤

π

π

≤ϕ≤

π

−

π

−≤ϕ≤

π

−

ϕ

+

ϕ ⋅

−

ϕ

−

=

2

6

6

6

6

2

4

sin 5.0

4

cos 3 1

4

sin 5.0

SLF

DPWMMIN

DPWMMIN

DPWMMAX

SLF

SLF

=

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