three phase inverters

8/10/2019 Three Phase Inverters

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Three-phase inverters are used for high-power

applications such as ac motor drives, inductionheating, uninterruptive power supplies

A three-phase inverter circuit changes DC inputvoltage to a three-phase variable frequency, variable

voltage output The input DC voltage can be from a DC source or a

rectified AC voltage

A three-phase bridge inverter can be constructed bycombining three single-phase half-bridge inverters.


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THREE PHASE INVERTER


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THREE PHASE LOADS


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It consists of six power-switches with six associatedfreewheeling diodes

The switches are opened and closed periodically in theproper sequence to produce the desired outputwaveform

The rate of switching determines the output frequencyof the inverter

Basically, there are two possible schemes of gating thedevices

In one scheme each device (switch) conducts for 180and in the other scheme, each device conducts for 120

But in both these schemes, gating signals are appliedand removed at 60 intervals of the output voltagewaveform.


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180-Conduction Mode with Resistive Load

In this control scheme, each switch conducts for a period of 180or half-cycle electrical.

Switches are triggered in sequence of their numbers with aninterval of 60.

At a time, three switches (one from each leg) conduct. Thus, twoswitches of the same leg are prevented from conductingsimultaneously.

One complete cycle is divided into six modes, each of 60intervals.

Switch pair in each leg, i.e. S1 S4, S3 S6, and S5 S2 are turned-onwith a time interval of 180

It means that switch S1 conducts for 180 and switch S4 for thenext 180 of a cycle

Switches, in the upper group, i.e. S1, S3, S5 conduct at an intervalof 120

It means that if S1 is fired at 0, then-S3 must be triggered at

120 and S5 at 240. Same is true for lower group of switches.


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Operation Table

Sl.

No

Interval Incoming

SCR

Conducting

Pair

Outgoing

SCR

1 I T1 T5, T6, T1 T4

2 II T2

T6

, T1

, T2

T5

3 III T3 T1, T2, T3 T6

4 IV T4 T2, T3, T4 T1

5 V T5 T3, T4, T5 T2

6 VI T6 T4, T5, T6 T3


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GATE CURRENT WAVEFORMS


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VOLTAGE WAVEFORMS FOR 180 MODE


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The following points can be noted from the wave formsand the operating Table,

Each switch conducts for a period of 180

Switches are triggered in the sequence 1, 2, 3, 4, 5 and 6 Phase shift between triggering the two adjacent switches

is 60.

From table, it is observed that in every step of 60

duration, only three switches are conducting-two fromupper group and one from the lower group and vice-versa

The output voltage waveforms (EAB, EBC, ECA) are quasi-

square-wave with a peak-value of Edc The three-line voltages are mutually phase-shifted by

120.


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The three-phase-voltages EAN,EBN, and ECNare six-stepwaves, with step heights Edc/3 and 2Edc/3.

Line voltage EABis leading the phase-voltage EANby 30.

Phase voltages EAN, EBNand ECNhave also been drawn forstarconnected resistive load.

For a star-connected load, the line-to-neutral voltages must

be determined to find the line or phase currents.


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There are three modes of operation in a half-cycle andthe equivalent circuits are shown below for a star-connected load


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During interval I for


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During interval II,

During interval III,


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EBCand ECAcan be found by phase shifting EABby 120

and 240 respectively

The line-to-line RMS voltage can be found from


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The RMS nth component of the line voltage is

which, for n = 1, gives the fundamental line voltage

The RMS value of line-to neutral voltages can be found

from the line voltage

8 C d i M d i h RL L d


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180 Conduction Mode with RL-Load

With resistive loads, the diodes across the switch haveno functions. If the load is inductive, then the current

in each arm of the load will be delayed to its voltage asshown below


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When switch S1 is triggered, S4 is turned-off but, because theload current cannot reverse, the only path for this current isthrough diode D1. Hence, the load phase is connected to thepositive end of the d.c. source but, until the load currentreverses at t1, switch S1 will not take up conduction

Similar arguments apply in the reverse half-cycle at instant t2

For a star-connected load, the phase voltage is EAN= EAB/3with a delay of 30. The line current ILfor an RL load is given by

For a delta-connected load, the phase currents can be obtaineddirectly from the line-to-line voltages. Once the phase currentsare known, the line currents can be determined


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O ti T bl


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Operation Table

Sl.

No

Interval Incoming

SCR

Conducting

Pair

Outgoing

SCR

1 I T1 T6, T1 T5

2 II T2 T1, T2 T6

3 III T3 T2, T3 T1

4 IV T4 T3, T4 T2

5 V T5 T4, T5 T3

6 VI T6 T5, T6 T4



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Following points can be noted from the waveforms and theoperating Table

The base drives of two switches in the same-half-bridge havean inherent dead band of 60. Hence, there is no possibility ofcross conduction or shoot-through fault.

Conduction period for each switch is 120.

The phase-shift between the triggering of every two adjacentswitches is 60.

Three line voltages, EABEBCand ECAare six-step waves, withstep heights Edc/2 and Edc

The three-line voltages are mutually phase shifted by 120.

The three-phase voltages EAN, EBNand ECNare quasi-square-

waves with peak values of Edc/2. They are also mutually phase-shifted by 120

The line-voltage EABis leading the phase-voltage EANby 30

From operating table two switches conduct at a time, onefrom the upper group and the other from the lower group.


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The equivalent circuits for a starconnected load are


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During interval I, for

Switches S1 and S6 conduct

During interval II,for Switches S1 and S2 conduct


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During interval III,for

Switches S2 and S3 conduct

The line-to-neutral voltages can be expressed inFourier-series as

C i f T C d ti M d


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Comparison of Two Conduction Modes

In 180 mode conduction, when gate signal Ig1is removed toturn-off switch S1 at t = 180, gating signal Ig4is simultaneouslyapplied to turn-on switch S4 inthe same leg

In practice, a commutation interval must exist between theremoval of Ig and application of Ig for proper and reliableoperation of the inverter circuit

Since enough time may not be provided for the commutation of

switch and two switches in series may simultaneously conduct,resulting into short circuit of the source by these switches

This problem is overcome considerably in 120 mode inverter

In this inverter, there is a 60 interval between the turning-off ofS1 and turning-on of S4

During this 60 interval, switch S1 can be commutated safely.Thus, enough time is made available for the outgoing switch tocommutate before the switch in series is turned-on

Therefore, commutation is more reliable and the possibility of

two series switches conducting simultaneously is much less.


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The second important difference is in terms of utility ofdevices

The comparison of the two patterns is done using a figureof merit termed as utility factor (UF) and is defined as

UF= Po/ PT where Pois the rated output power of the inverter and PTis

the measure of total power handling capability of thedevices employed in the inverter, and is defined asPT= N VDRM. Irms

where N= number of thyristors, VDRM= repetitive peak forward-voltage

Irms = rated RMS forward current For the purpose of comparison of the two control schemes,

it is usual to calculate the utility factor for ideal resistiveload


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Inverter with 180 -Conduction

For star-connected load, if R is the resistance perphase, then

Device rating will be chosen such that the RMScurrent flowing through it will be equal to its RMScurrent rating Irms

Therefore, under the rated conditions, the RMS valueof load phase current Ipis given by,


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Load power Po can also be written as

For this load, the repetitive peak forward voltage ratingof the device should be equal to Edc. Thus,


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Inverter with 120 Conduction

For star-connected load, with a per phase resistance ofR, output power Po will be given by

Three-phase six-level diode-clamped inverter structure.


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p p


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Eleven-level wye-configured cascaded inverter.


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y g

three phase inverters

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