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Single Phase Bridge VSI
Inverter in Power-Electronics refers to a class of power conversion circuits that operate from a dc
voltage source or a dc current source and convert it into a symmetric ac voltage or current. Itdoes reverse of what ac-to-dc converter does. The input to the inverter is a direct dc source or
dc source derived from an ac source. For example, the primary source of input power may beutility ac voltage supply that is converted to dc by an ac - dc rectifier with filter capacitor and
then inverted back to ac using an inverter. Here, the final ac output may be of a differentfrequency and magnitude than the input ac of the utility supply.
If the input dc is a voltage source, the inverter is called a Voltage Source Inverter (VSI). One cansimilarly think of a Current Source Inverter (CSI), where the input to the circuit is a current
source. The VSI circuit has direct control over output (ac) voltage whereas the CSI directly
controls output (ac) current.
The simplest dc voltage source for a VSI may be a battery bank or a solar photovoltaic cells
stack. An ac voltage supply, after rectification into dc can also serve as a dc voltage source. Avoltage source is called stiff, if the source voltage magnitude does not depend on load connectedto it. All voltage source inverters assume stiff voltage supply at the input.
Output of voltage waveforms of ideal inverters should be sinusoidal. However practical inverterwaveforms are non sinusoidal and contain certain harmonics. For low and medium power
applications square wave or quasi square wave voltages are acceptable.
A variable voltage can be obtained by varying the input dc voltage and maintaining the gain of
the inverter constant. On the other hand, if the dc input voltage is fixed then variable output
voltage can be obtained by varying the gain of the inverter. This can be accomplished by Pulse
Width Modulation-PWM control within the inverter. PWM means the width of the square pulsein positive and negative halves can be adjusted according to the rms of the output required. The
inverter gain may be defined as ratio of the ac output (rms) voltage to dc input voltage. In Square
Wave PWM technique the output ac rms voltage is fixed when input dc voltage is fixed.
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Fig 4(a) shows the power circuit diagram for single phase bridge voltage source inverter. In this
four switches (in 2 legs) are used to generate the ac waveform at the output. Any semiconductorswitch like IGBT, MOSFET or BJT can be used. Four switches are sufficient for resistive load
because load current io is in phase with output voltage vo. However this is not true in case of RLload where the io is not in phase with vo and diodes connected in anti-parallel with switch will
allow the conduction of the current when the main switch is turned off. These diodes are called
as Feedback Diodes since the energy is fed back to the dc source.
Fig 4(a) IGBT based Single phase bridge voltage source inverter power circuit diagram.
Square Wave PWM
In full bridge inverter, when T1, T2 conduct the output voltage is Vs and when T3, T4 conducts
the output voltage is -Vs. The switches T1, T2 conducts for period of 0
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PWM techniques for voltage control of Single Phase Inverters
The following PWM techniques are used for controlling the output ac rms voltage and frequency
in an inverter:
Single-Pulse-Width-Modulation
Multiple-Pulse-Width-Modulation Sinusoidal-Pulse-Width-Modulation (SPWM)
Single-Pulse-Width-Modulation
In single pulse width modulation control, there is only one pulse per half cycle and the outputrms voltage is changed by varying the width of the pulse. The gating signals and output voltages
of single pulse-width modulation are shown in fig 4(b). The gating signals are generated bycomparing the rectangular control signal of amplitude Vc with triangular carrier signal Vcar. The
frequency of the control signal determines the fundamental frequency of ac output voltage. Theamplitude modulation index is defined as:
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The rms ac output voltage
where
By varying the control signal amplitude Vc from 0 to Vcar the pulse width ton can be modified
from 0 secs to T/2 secs and the rms output voltage Vo from 0 to Vs.
In multiple PWM control, instead of having a single pulse per half cycle, there will be multiple
number of pulses per half cycle, all of them being of equal width.
Sinusoidal-Pulse-Width-Modulation (SPWM)
In sinusoidal pulse width modulation there are multiple pulses per half-cycle and the width of the
each pulse is varied with respect to the sine wave magnitude. Fig 4(c) shows the gating signalsand output voltage of SPWM with unipolar switching. In this scheme, the switches in the two
legs of the full-bridge inverter are not switched simultaneously, as in the bi-polar scheme.
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Fig 4(b) Gating signals and output voltage of Single pulse-width modulation
In this unipolar scheme the legs A and B of the full-bridge inverter are controlled separately by
comparing carrier triangular wave vcar with control sinusoidal signal vc and -vc respectively. ThisSPWM is generally used in industrial applications. The number of pulses per half-cycle depends
upon the ratio of the frequency of carrier signal (fc) to the modulating sinusoidal signal. The
frequency of control signal or the modulating signal sets the inverter output frequency (fo) and the
peak magnitude of control signal controls the modulation index ma which in turn controls the rmsoutput voltage. The area of each pulse corresponds approximately to the area under the sine wave
between the adjacent midpoints of off periods on the gating signals. Ifton is the width of nth
pulse, the rms output voltage can be determined by:
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The amplitude modulation index is defined as:
where, = peak magnitude of control signal (modulating sine wave)
= peak magnitude of carrier signal (triangular signal)
Thefrequency modulation ratio is defined as:
where, = frequency of control signal (sine signal)
= frequency of carrier signal (triangular signal)
Applications
Uninterruptible Power Supply (UPS),
Adjustable Speed Drives (ASD) for ac motors, Electronic frequency changer circuits used in induction heating, welding etc.,
HVDC transmission at lower power levels
Renewable Energy such as solar, fuel cell to AC conversion Electronic Ballast and Compact Fluorescent lamps
Active Filters for power quality improvement Custom power devices: DSTACTCOM, DVR, UPQC,
FACTS: STATCOM, SSSC, UPFC, etc.
Sinusoidal Pulse Width Modulation (SPWM)
Apply multiple gate signals to the IGBTs T1, T2 in positive half-cycle and T3, T4 in negativehalf-cycle. The gate signals for T1, T2 are generated by comparing the high frequency (fcar)
triangular carrier signal vcar1 with sine control signal vc1 of frequency fo. Similarly, the gate signals
for T3, T4 are generated by comparing thevcar2 (=vcar1)
with sine control signalvc2 = -vc1
offrequency fo. The peak magnitude of control sine signal controls the modulation index ma which
in turn controls the rms output voltage.
Observe the waveforms across each element of the power circuit with R and RL loads.
The output voltage and current can be observed for different values ofRo, Lo, Vdc, Vc1, Vc2, ma, mf orfo.
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Main Circuit Diagram
Circuit Diagram With Voltmeters And Ammeters At Various Points
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Waveforms
For R Load
R = 10
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For RL Load.
R = 1 , L = 10 mH
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