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

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

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

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

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

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.

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

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

1-Phase Half Bridge Inverter

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

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

1-Phase Full Bridge Inverter

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

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.