DC to AC ConverterMohd Shawal Bin JadinFaculty of Electrical & Electronic [email protected]
BEE4223 Power Electronics & Drives Systems
OverviewIntroduction Principle of operationPerformance parameterThe half-bridge inverter Pulse Width Modulation (PWM)PWM GenerationPWM HarmonicsCurrent Source InverterIntroduction to three-phase inverterConclusions
Learning outcomesAt the end of the lecture, student should be able to:State the operation and characteristics of Inverter. Discuss the performance parameters and use different technique for analyzing and designing of DC to AC Converter.
IntroductionInverters are circuits that converts dc input voltage to a symmetric ac output voltage by which both magnitude and frequency can be controlled.Applications :adjustable speed ac motor drives, uninterruptible power supplies (UPS), and ac appliances run from an automobile battery.
Types of inverterVoltage Source Inverter (VSI):
Current Source Inverter (CSI)
Voltage source inverter (VSI) with variable DC linkDC link voltage is varied by a DC-to DC converter or controlled rectifier.Generate square wave output voltage.Output voltage amplitude is varied as DC link is varied.
Frequency of output voltage is varied by changing the frequency of the square wave pulses.
Voltage source inverter (VSI) with variable DC linkAdvantages:simple waveform generationReliable
Disadvantages:Extra conversion stagePoor harmonics
VSI with fixed DC linkDC voltage is held constant.
Output voltage amplitude and frequency are varied simultaneously using PWM technique.
Good harmonic control, but at the expense of complex waveform generation
Square Wave InverterSquare wave inverter can be simplifying justified with a switching scheme of full bridge converter.An square wave ac output voltage is synthesized from a dc input by closing and opening the switches in an appropriate sequence. The output voltage can be +Vdc, -Vdc, or zero, depending on which switches are closed.
Operation of simple square-wave inverter Parallel diode is used when the current in the switch is negative Diode will reverse-biased when current is positive in the switch
SQUARE-WAVE INVERTERSEQUIVALENT CIRCUIT
when S1-S2 turn ON & S3-S4 OFF for t1 < t < t2
when S1-S2 turn off& S3-S4 On for t2 < t < t3
ExampleA square-wave inverter has a dc source of 125V, an output frequency of 60 Hz, and R-L series load with R = 20 Ohm and L = 20 mH. DetermineAn expression for load currentRms load current andAverage source current
Fourier Series Analysis for Square Wave InverterFourier series method is often the most practical way to analyze load current and to compute power absorbed by load.
Fourier Series Analysis for Square Wave Inverter
The quality of ac output voltage or current can be expressed by total Harmonic Distortion (THD)
Example 1Consider a square wave inverter with Vdc=100V, R=10, L=25mH, and f=60Hz. DetermineFundamental output voltageTHD for output voltage and current and power absorbed by load
Example 2Fundamental output voltage
THDv and THDI
Example 2nfn (Hz)Vn (V)Zn ()In (A)Pn (W)160127.313.79.27429.3318042.3301.4210530025.548.20.531.4742018.266.70.270.37954014.185.40.170.14
Example 1 (cont)
Amplitude & Harmonic ControlThe amplitude of the fundamental frequency for a square-wave output from the full-bridge inverter is determined by the dc input voltage. A controlled output can be produced by modifying the switching scheme. This output voltage can be controlled by adjusting the interval on each side of the pulse where the output is zero.Harmonic also can be eliminated by choosing a value of which make the sine terms go to zero.
Example 2Design an inverter that will supply the series R-L load of R=10, L=25mH with a fundamental frequency of 60Hz and current amplitude of 9.27A and THD less than 10%. A variable source is available.
Example 2 (cont)The dominant harmonic current is for n = 3 (third harmonic), so the switching scheme must eliminate the third harmonic.
Example 1 (cont)
Example 1 (cont)nfn (Hz)Vn (V)Zn ()In (A)160127.313.79.2731800300530025.548.20.53742018.266.70.279540085.40
Try thisFor the full-bridge inverter:Given : Dc source = 125 V; Load (R-L in series) R = 10 and L = 20 mH- switching frequency = 60 Hz.
Determine to produce output with an amplitude 100V at fundamental frequency.Determine the THD of the load current.
Pulse-Width Modulated Output
In square wave inverters, maximum output voltage is achievable.However there in NO control in harmonics and output voltage magnitude. i.e the harmonics are always at three, five, seven etc times the fundamental frequency.Hence the cut-off frequency of the low pass filter is somewhat fixed. The filter size is dictated by the VA ratings of the inverter.To reduce filter size, the PWM switching scheme can be utilized.In this technique, the harmonics are pushed to higher frequencies. Thus the cut-off frequency of the filter is increased. Hence the filter components (i.e. L and C) sizes are reduced. The trade off for this flexibility is complexity in the switching waveforms.
Pulse Width Modulation (PWM)
Pulse Width Modulation (PWM)Triangulation method (Natural sampling)Amplitudes of the triangular wave (carrier) and sine wave (modulating) are compared to obtain PWM waveform. Simple analogue comparator can be used.Basically an analogue method. Its digital version, known as REGULAR sampling is widely used in industry.
Pulse Width Modulation (PWM)Production of PWM waveform using reference sinewave:
Comparator determines instants at which waveforms cross in order to produce switching waveformPWM output waveform tracks amplitude and frequency of reference sinewave
Pulse Width Modulation (PWM)As switching frequency is increased, switching loss becomes issue Implementation by ICs which essentially contain tables of pre-calculated values of switching angles covering range of output frequencies As computational speeds of ICs increase, it is now possible to calculate required firing angles in real time in order to optimise strategy for harmonic elimination, and control, further improving inverter performance
Pulse Width Modulation (PWM) typesNatural (sinusoidal) sampling (as shown on previous slide)Problems with analogue circuitry, e.g. Drift, sensitivity etc.Regular sampling - simplified version of natural sampling that results in simple digital implementationOptimised PWM - PWM waveform are constructed based on certain performance criteria, e.g. THD.Harmonic elimination/minimisation PWMPWM waveforms are constructed to eliminate some undesirable harmonics from the output waveform spectra.Highly mathematical in natureSpace-vector modulation (SVM)A simple technique based on volt-second that is normally used with three-phase inverter motordrive
PULSE WIDTH MODULATION IN UNIPOLAR INVERTERS
The square wave output can be produced using a comparator to compare the triangle wave with the sine wave.
Also known as the inverter leg.Basic building block for full bridge, three phase and higher order inverters.G is the centre point.Both capacitors have the same value. Thus the DC link is equally spilt into two.The top and bottom switch has to be complementary, i.e. If the top switch is closed (on), the bottom must be off, and vice-versa.
Shoot through fault andDead-timeIn practical, a dead time as shown below is required to avoid shoot-through faults, i.e. short circuit across the DC rail.Dead time creates low frequency envelope. Low frequency harmonics emerged.This is the main source of distortion for high-quality sine wave inverter.
Introduction to Three-phase inverterEach leg (Red, Yellow, Blue) is delayed by 120 degrees.A three-phase inverter with star connected load is shown below
Three phase inverter waveforms
SummaryHave examined operation of inverters as means of producing variable-frequency, variable voltage AC source from DC supply PWM provides amplitude control of the fundamental output frequency although the harmonics have large amplitudes, they occur at high frequency and are filtered easily. Considered voltage-sourced and current-sourced inverters which operate from DC supplies which approximate constant voltage source Introduced pulse-width-modulated inverter