UNIT-8 INVERTERS 11/27/2018

Topic details INTRODUCTION TYPES OF INVERTERS SINGLEPHASE HALF BRIDGE INVERTER PERFORMANCE PARAMETERS SINGLEPHASE FULLBRIDGE INVERTER AMPLITUDE CONTROL OF SINGLEPHASE INVERTERS 11/27/2018

INTRODUCTION Inverters 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. 11/27/2018

Voltage Source Inverter (VSI): Input voltage remains constant TYPES OF INVERTERS Voltage Source Inverter (VSI): Input voltage remains constant Current Source Inverter (CSI): Input current remains constant 11/27/2018

VSI WITH VARIABLE DC LINK DC 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. 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Consists of 2 choppers, 3-wire DC source Transistors switched on and off alternately Need to isolate the gate signal for Q1 (upper device) Each provides opposite polarity of Vs/2 across the load 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER CONTN., SINGLE PHASE HALF BRIDGE INVERTER Q1 on, Q2 off, vo = Vs/2 Peak Reverse Voltage of Q2 = Vs 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER CONTN., SINGLE PHASE HALF BRIDGE INVERTER Q1 off, Q2 on, vo = -Vs/2 Peak Reverse Voltage of Q2 = Vs 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Waveforms with resistive load SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Fourier Series of the instantaneous output voltage SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER HALF BRIDGE INERTER SINGLE PHASE HALF BRIDGE INVERTER + Vs/2 - + Vs/2 - Turn off Q1 at t = To/2 Current falls to 0 via D2, L, Vs/2 lower 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER HALF BRIDGE INERTER SINGLE PHASE HALF BRIDGE INVERTER + Vs/2 - + Vs/2 - Turn off Q2 at t = To Current falls to 0 via D1, L, Vs/2 upper 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Load Current for a highly inductive load SINGLE PHASE HALF BRIDGE INVERTER Transistors are only switched on for a quarter-cycle, or 90 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Performance Parameters SINGLE PHASE HALF BRIDGE INVERTER Harmonic factor of the nth harmonic (HFn) for n>1 Von = rms value of the nth harmonic component V01 = rms value of the fundamental component 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Performance Parameters (continued) SINGLE PHASE HALF BRIDGE INVERTER Distortion Factor (DF) Indicates the amount of HD that remains in a particular waveform after the harmonics have been subjected to second-order attenuation. for n>1 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Single-phase full-bridge inverter SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Operational Details SINGLE PHASE HALF BRIDGE INVERTER Consists of 4 choppers and a 3-wire DC source Q1-Q2 and Q3-Q4 switched on and off alternately Need to isolate the gate signal for Q1 and Q3 (upper) Each pair provide opposite polarity of Vsacross the load 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER OPER SINGLE PHASE HALF BRIDGE INVERTER + Vs - Q1-Q2 on, Q3-Q4 off, vo = Vs 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Operational Details-Contn., - Vs + Q3-Q4 on, Q1-Q2 off, vo = -Vs 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Load current for a highly inductive load SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER The 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. 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER 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. 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER 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. 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control Production of PWM waveform using reference sinewave: Comparator determines instants at which waveforms cross in order to produce switching waveform PWM output waveform tracks amplitude and frequency of reference sinewave 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER 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 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 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER Natural (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 implementation Optimised PWM - PWM waveform are constructed based on certain performance criteria, e.g. THD. Harmonic elimination/minimisation PWM PWM waveforms are constructed to eliminate some undesirable harmonics from the output waveform spectra. Highly mathematical in nature Space-vector modulation (SVM) A simple technique based on volt-second that is normally used with three-phase inverter motordrive 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control SINGLE PHASE HALF BRIDGE INVERTER 11/27/2018 11/27/2018

SINGLE PHASE HALF BRIDGE INVERTER Amplitude & Harmonic Control The square wave output can be produced using a comparator to compare the triangle wave with the sine wave. 11/27/2018 11/27/2018

THANK YOU 11/27/2018