Seminar on Three Phase Bridge Inverter

A presentation onThree Phase Bridge InverterRudra Kumar Mishra Contents

Introduction of InverterThree Phase Bridge InverterThree Phase 180 ModeEquivalent Circuit & WaveformsThree Phase 120 modeEquivalent Circuit & WaveformsDifferences between 180 & 120 modeApplications

Rudra Kumar MishraInverter IntroductionA device that converts dc power into ac power at desired output voltage and frequency is called an inverter.The output voltage can be fixed or variable at a fixed or variable frequency.A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant.Rudra Kumar MishraThree Phase Bridge InverterA basic three phase inverter is a six step bridge inverter. It uses a minimum of six Thyristor.

For one cycle of 360 degree each step would be of 60 degree interval for a six step inverter. This means that Thyristors would be gated at regular intervals of 60 degree in proper sequence so that a 3 phase ac voltage is synthesized at the output terminals of a six step inverterRudra Kumar Mishra180 Mode inverterAC supply contains positive voltage as well as negative voltage.For each phase we require 2 Thyristors. 1 for Positive cycle ( for 180 Duration) 1 for negative cycle ( for 180 Duration )Similarly for three phase we require 6 Thyristors. 3 for positive cycle ( for 180 Duration) 3 for negative cycle ( for 180 Duration)Rudra Kumar MishraCircuit Diagram & waveforms

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Rudra Kumar Mishra7Voltage calculation for each 0 -60 interval

General circuitEquivalent circuit for 0-60Calculation for phase voltages:Z of an and z of cn are parallel. So resultant will be z/2. By using voltage divider we get,

Rudra Kumar MishraStep Thyristors conductingThyristors connected to +ve dc terminalThyristors connected to ve dc terminalVaoVboVco1.5,6,11,56Vs/3-2Vs/3Vs/32.6,1,212,62Vs/3-Vs/3-Vs/33.1,2,31,32Vs/3Vs/3-2Vs/34.2,3,432,4-Vs/32Vs/3-Vs/35.3,4,53,54-2Vs/3Vs/3Vs/36.4,5,654,6-Vs/3-Vs/32Vs/3Voltage Values for different intervals for 180 modeRudra Kumar Mishra120 Mode Inverter In this each Thyristor will conduct only for 120. For 120 to 180 no Thyristor will conduct.After the conduction of 120 each Thyristor gets the 60 off duration interval.

Ex: T1- 0 to 120 ( conduction occurs ) 120 to 180 ( No conduction occur )

Rudra Kumar MishraCircuit Diagram & waveforms

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Rudra Kumar MishraVoltage Values for different intervals for 180 modeStep Thyristors conductingThyristors connected to +ve dc terminalThyristors connected to ve dc terminalVaoVboVco1.6,116Vs/2-Vs/202.1,212Vs/20-Vs/23.2,3320Vs/2-Vs/24.3,434-Vs/2Vs/205.4,554-Vs/20Vs/26.5,6560-Vs/2Vs/2Rudra Kumar MishraThe phase voltages & Line Voltages equations:

Phase VoltagesLine VoltagesFor n = 3, Cos (3/6) = 0. This indicates that the triplen harmonics are absent from the line voltages. This voltages are independent of the nature of load which can be any combination of resistance, inductance and capacitance. Rudra Kumar MishraComparison180 MODE120 MODEPhase voltage is continuous.Phase voltage is discontinuous.Line voltage is discontinuous.Line voltage is continuous.Efficiency = 66.67%Efficiency = 50%No delay is provided between Switching ON and Switching OFF of Thyristors in same arm. 60 delay is provided between Switching ON and Switching OFF of Thyristors in same arm.Short circuit condition of an arm may arise.No chance of Short circuit condition of an arm.

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Induction Heating

UPS

HVDCRudra Kumar MishraThank YouRudra Kumar Mishra