1. UNIT-8
INVERTERS
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2. Topic details INTRODUCTION TYPES OF INVERTERS
SINGLEPHASE HALF BRIDGE INVERTER
PERFORMANCE PARAMETERS
SINGLEPHASE FULLBRIDGE INVERTER
AMPLITUDE CONTROL OF SINGLEPHASE INVERTERS
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3. 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.
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4. 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
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5. 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.
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6. 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
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7. SINGLE PHASE HALF BRIDGE INVERTER
CONTN.,
SINGLE PHASE HALF BRIDGE INVERTER
Q1 on, Q2 off, vo = Vs/2
Peak Reverse Voltage of Q2 = Vs
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8. SINGLE PHASE HALF BRIDGE INVERTER
CONTN.,
SINGLE PHASE HALF BRIDGE INVERTER
Q1 off, Q2 on, vo = -Vs/2
Peak Reverse Voltage of Q2 = Vs
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9. SINGLE PHASE HALF BRIDGE INVERTER
Waveforms with resistive load
SINGLE PHASE HALF BRIDGE INVERTER
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10. SINGLE PHASE HALF BRIDGE INVERTER
Fourier Series of the instantaneous output voltage
SINGLE PHASE HALF BRIDGE INVERTER
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11. 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
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12. 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
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13. 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
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14. 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
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15. 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
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16. SINGLE PHASE HALF BRIDGE INVERTER
Single-phase full-bridge inverter
SINGLE PHASE HALF BRIDGE INVERTER
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17. 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
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18. SINGLE PHASE HALF BRIDGE INVERTER
OPER
SINGLE PHASE HALF BRIDGE INVERTER
+ Vs -
Q1-Q2 on, Q3-Q4 off, vo = Vs
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19. SINGLE PHASE HALF BRIDGE INVERTER
Operational Details-Contn.,
- Vs +
Q3-Q4 on, Q1-Q2 off, vo = -Vs
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20. SINGLE PHASE HALF BRIDGE INVERTER
Load current for a highly inductive load
SINGLE PHASE HALF BRIDGE INVERTER
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21. 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.
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22. 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.
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23. 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.
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24. SINGLE PHASE HALF BRIDGE INVERTER
Amplitude & Harmonic Control
SINGLE PHASE HALF BRIDGE INVERTER
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25. 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
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26. 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
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27. 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
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28. SINGLE PHASE HALF BRIDGE INVERTER
Amplitude & Harmonic Control
SINGLE PHASE HALF BRIDGE INVERTER
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29. 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.
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30. THANK YOU
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