PWM TECHNIQUES

• The output voltage of the inverter needs to be varied as per load requirement.

• Whenever the input DC varied, the output voltage can change.

• Hence these variation needs to be compensated.

• Incase of motor drives the ratio of voltage to frequency (v/f) is maintained constant.

• Similarly ,in UPS the output voltage of inverter is to be regulated.

• The PWM techniques is mainly used for voltage control..

• These techniques are most efficient and they control the drives of the switching devices.

• 1.single pulse width modulation• 2.multiple pulse width modulation.• 3.sinusoidal pulse width modulation.• 4.modified sinusoidal pulse width modulation.• 5.phase displacement control.

Single pulse modulation

• This modulation give quasi-square wave output.

II. PWM METHODSA. Sine PWM (2)

VA0

VB0

VC0

VAB

VBC

VCA

t

Fig. 7 Waveforms of three-phase sine PWM inverter.

Three-phase sine PWM waveformsvtri vcontrol_A vcontrol_B vcontrol_C

where, VAB = VA0 – VB0

VBC = VB0 – VC0

VCA = VC0 – VA0

When vcontrol > vtri, VA0 = Vdc/2

When vcontrol < vtri, VA0 = -Vdc/2

Frequency of vtri = fs

Frequency of vcontrol = f1

Frequency of vtri and vcontrol

where, fs = PWM frequency

f1 = Fundamental frequency

Inverter output voltage

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II. PWM METHODSA. Sine PWM (3)

Amplitude modulation ratio (ma)

A01A0

10

Vofcomponentfrequecnylfundamenta:)(Vwhere,

,2/

)(

dc

A

tri

controla V

Vofvaluepeakvofamplitudevofamplitudepeakm

Frequency modulation ratio (mf)

frequencylfundamentafandfrequencyPWMfwhere,, 1s1

ff

m sf

mf should be an odd integer

if mf is not an integer, there may exist sunhamonics at output voltage

if mf is not odd, DC component may exist and even harmonics are present at output voltage

mf should be a multiple of 3 for three-phase PWM inverter

An odd multiple of 3 and even harmonics are suppressed

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Sinusoidal Pulse width modulationThe switches in the voltage source inverter (See Fig. 1)can be turned on and off as required. In thesimplest approach, the top switch is turned on If turned on and off only once in each cycle, asquare wave waveform results. However, if turned on several times in a cycle an improved harmonicprofile may be achieved.

• In the most straightforward implementation, generation of the desired output voltage is achieved by comparing the desired reference waveform (modulating signal) with a high-frequency triangular ‘carrier’ wave as depicted schematically in Fig.2. Depending on whether the signal voltage is larger or smaller than the carrier waveform, either the positive or negative dc bus voltage isapplied at the output.

• Note that over the period of one triangle wave, the average voltage appliedto the load is proportional to the amplitude of the signal (assumed constant) during this period. The resulting chopped square waveform contains a replica of the desired waveform in its low frequency components, with the higher frequency components being at frequencies of an close to the carrier frequency.

• Notice that the root mean square value of the ac voltage waveform is still equal to the dc bus voltage, and hence the total harmonic distortion is not affected by the PWM process.

• The harmonic components are merely shifted into the higher frequency range and are automaticallyfiltered due to inductances in the ac system.

• When the modulating signal is a sinusoid of amplitude Am, and the amplitude of the triangular carrier is Ac, the ratio m=Am/Ac is known as the modulation index.

• Note that controlling the modulation index there for controls the amplitude of the applied output voltage. With a sufficiently high carrier frequency

Fig 2: Principal of Pulse Width Modulation