1. Dr. Unnikrishnan P.C. Professor, EEE
EE305 Power Electronics
Dr. Unnikrishnan P.C.
Professor, EEE

2. Inverters
DC to AC converter-converts a DC voltage into an AC of variable voltage and variable frequency.
Different switches used.
Applications
Where the input available is battery or solar panel to drive AC appliances (eg: mobile video, audio and computing equipments)
UPS, Variable frequency drives

3. Classification of inverters
Based on
Number of phases
Single phase inverter
Three phase inverter
Type of source
Voltage Source Inverter (VSI)
Single Phase Half Bridge Inverter
Single Phase full bridge Inverter
Current Source Inverter (CSI)
Capacitor commutated CSI
Auto Sequential commutated Inverter(ASCI)
Type of devices
Thyristor and fully controlled switches

4. Classification of inverters……..
Circuit configuration
Half bridge inverter
Full bridge inverter
Waveform of output voltage
Square wave inverter
Quasi square wave inverter
Sine wave inverter
Method of control
Switched Mode PWM
Resonant inverters
Method of commutation of conducting devices
Line commutated thyristor inverter
Forced commutated thyristor inverter-Series and parallel inverter

5. Voltage Source Inverter-Block Diagram
Terminal voltage almost constant
DC supply with low internal impedance
Eg: Battery bank

6. Current Source Inverter-Block Diagram
DC power supply of high internal impedance used
Can deliver constant current.
Stiffness of output current maintained by connecting a series inductance of large value in this line.

7. Comparison Sl no Voltage Source Inverter (VSI)
Current Source Inverter (CSI) 1
Inverter fed by a stiff voltage source
Inverter fed by a stiff Current source 2
DC source has low internal impedance
DC source has high internal impedance 3
Half bridge ,full bridge ,square wave and pulse width modulated inverters fall under VSI
Capacitor commutated Current source inverter and auto sequential commutated inverters(ASCI) fall under CSI 4
Need feedback diodes
No feedback diodes required

8. Comparison ….. Sl no Voltage Source Inverter (VSI)
Current Source Inverter (CSI) 5
Used in variable speed drives ,standby power supply, high voltage DC transmission systems
In high power drives, three phase induction motor drives ,lagging VAR compensators etc. 6
Good control over the inverter output voltage,frequency and reduced harmonics when PWM technique is implemented
Inherent short circuit fault protection 7
Commutation circuit complicated and bulky
Simple

9. 1-Phase Half Bridge Inverter

10. 1-Phase Half Bridge Inverter
Assumption: Each thyristor conducts for the duration its gate pulse is present.
During 0 < t < T/2 , thyristor T1 conducts and the load voltage is VS/2
During T/2 < t < T , thyristor T2 conducts and the load voltage is -VS/2
Load voltage is alternating of amplitude VS/2 and frequency 1/T
By controlling T, frequency of output can be changed

11. 1-Phase Half Bridge Inverter …..
For a resistive load, the load voltage and load current will be in phase.
For other loads feedback diodes D1 and D2 will conduct when the main thyristors are turned off
Precaution: Simultaneous turning On of T1 & T2 will short circuit the source
Drawback: Needs a three-wire dc supply

12. 1-Phase Full Bridge Inverter

13. 1-Phase Full Bridge Inverter ……
Assumption: Each thyristor conducts for the duration its gate pulse is present.
During 0 < t < T/2 , thyristor T1 & T2 conducts and the load voltage is VS
During T/2 < t < T , thyristor T3 & T4 conducts and the load voltage is -VS
Load voltage is alternating of amplitude VS and 4 times power compared to half bridge inverter
By controlling T, frequency of output can be changed
Precaution: Simultaneous turning On of SCRs in the same branch will short circuit the source

14. 1-Phase Full Bridge Inverter ……
For a resistive load, the load voltage and load current will be in phase.
For other loads feedback diodes D1 , D2 , D3 and D4 will conduct when the main thyristors are turned off
Precaution: Simultaneous turning On of thyristors in the same branch will short circuit the source.