Download - 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
GATE CURRENT WAVEFORMS
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GATE CURRENT WAVEFORMS
VOLTAGE WAVEFORMS FOR 120 MODE
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VOLTAGE WAVEFORMS FOR 120 MODE
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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