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THE ANALYSIS OF THE THREE PHASE RECTIFIER WITH SEVERAL PULSES

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					THE ANALYSIS OF THE THREE PHASE RECTIFIER WITH SEVERAL PULSES

                                              Dã nilãCe      sc
                                                        rbule u, University of Craiova
                                              e-mail:dcerbule@electronics.ucv.ro
                                              Craiova.Ro-1100,Romania




Abstract-This paper describes o
complete analysis of a thre-phase bridge
rectifier with resistive load, using
Matlab/Simulink enviroments.
The work present the analysis of the
wave forms of the three-phase
controlled rectifier in normal function
regime.

 Key words: rectifier, bridge, SCR,
diode, LIGB1.                                              Fig.1. Three-phase bridge
                                                           rectifier.
1. INTRODUCTION
                                                           The medium value of the
        The rectifier depicted in fig. 1 is                rectified tension is given by:
formed by 6 thyristors and 2 diodes with                   U oα = U o cos α .      (1)
a three phase power source, in witch we
have 4 branches, 3 of them only with                                         πm
diodes and one branch with diodes. The                                    sin
branch on witch the diodes are                             U o = 2U 2         m .        (2).
connected is connected to the power                                         π
source null.                                                                m
      2. FUNCTION
      The regulation of the load tension                    witch U2 is the tension of
is made by modifying the driving angle                     phase.
of the thyristors.
      Depending on the driving angle of                 The rectified tension pulses are
the thyristors we have 3 functioning              given by fragments of the line tension.
domains:                                          That’s because only the devices that
                                                  have the greater line tension are
      Domain I of functioning, in witch           commuting.
the driving angle ∝ < α < 30. In this                   If we note with 1 logic the
domain we have a conduction state only            conduction state and with 0 logic the
for thyristors. The rectifier is a three          blocking state, the conducting and
phase in bridge, full driven, RTP6, in            commuting sequence is given by the
witch we have 6 pulses on a period.               step matrix:
      Devices    →1 2 3 4 5 6 7 8                           7     00011000
     Tact 1 ↓    11000000                                   8     00010010
          2      01100000                                   9     00001100
          3      00110000                                   10    0 0 0 01 0 0 1
         4       00011000                                   11    1 0 0 00 1 0 0
         5       000 01100                                  12    00000101
         6       100 00100
                                                        The rectified tension has
      The rectified tension contains                   different values, depending of
only n=km order components (multiple                   the way the rectified tension is
of m phases),                                          obtained from the line tension or
                                                       fragments of the phase tension.
witch
                                                                3. 3
ud = U do{1 +                                            ud =        . 2 .U L {cos α +
                                                                 π
            cos[kmωt − (km + 1)α ] 
                                                                                                1/ 2
                                   −                    1          1         2 cos 2α   
                                                     (6n 1) 2 + (6n 1) 2 − (6n 1)(6n 1) 
                                                ∞
   ∞                                                                                                  .
+ ∑ cos kπ 
                    km + 1
                                     }      + ∑         −          +         +        −              }.
  k =1     + cos[kmωt − (km − 1)α ]          n =1
                                                    sin( 6nωt + v6 n
           
                     km − 1         
                                     
                                             witch

                                                                   cos(6n + 1)α cos(6n − 1)α
                                                                               −
                                                       nπ             6n + 1        6n − 1 .
      Domain II.       If we angle the       ν 6n   =−    + tg −1.
                                                        6          sin(6n + 1)α sin(6n − 1)α
driving, 30<∝<90 we have time                                                  −
intervals in witch alternatively conducts                             6n + 1       6n − 1
2 thyristors and one thyristor and a
diode.                                             Domain III. If we angle the
      The waveform of the load tension       driving, 90o<∝<150o .
is given by fragments of the line tension
(when thyristors are conducting)
alternating with fragments of the phase
tension (when a thyristor and a diode are
conducting).
      In the same way as in the domain
I, we can write the step matrix, witch is:


     Devices → 1 2 3 4 5 6 7 8
      Tact 1 ↓ 1 1 0 0 0 0 0 0
           2   10000001
           3   01100000                                Fig.2.     The      rectifier     full
           4   01000010                                controler.
           5   00110000
           6   00100001
       If we can say that the circuit in fig              II we have a driving angle
2 is full rectifier, then the circuit in fig 1            equal with π/m = 60°. The
is a half rectifier.                                      conduction interval is equal
                                                          with a step of the step matrix.
      As for the AC converters,
presented in fig 3, related to the devices              The steps have equal length.
type we can have AC converters full              From the Fourier series developing
driven or half driven. Depending on the          results that the first non-zero harmonic,
type of the devices we have 3                    for domain I is f1=pf = 300Hz and for
functioning domains.                             the domain II, the frequency of the first
                                                 harmonics depends of the fragments of
                                                 the line or phase tension witch
                                                 approximates the load tension
                                                        - f1 = 300Hz – when the
                                                            thyristors are conducting (we
                                                            have RTP6)
                                                        - f1 = 300Hz or f1 = 150Hz
                                                            when a thyristor and a diode
                                                            are conducting
                                                 The optimal filter to make smooth the
      Fig.3. AC converter.                       output tension should be projected in the
                                                 domain 150Hz-300Hz, so it cannot
       At a rectifier, for driving a DC          fulfill.
electric motor, we have to follow:
       - the regulation of the medium            CONCLUSION: In the case of using the
          value of the rectified tension,        rectifier with several pulses, in witch a
          in order to ensure the couple          branch of the bridge uses the null of the
          of the running machine;                electrical network, in order to have an
                                                 optimal filter we have to use, in
      -   the harmonics content, in order        functioning each domain separatelly
          to analyze the way in witch the
          electrical network is loaded           We can say that we have 3 functioning
          with harmonics from the                domains for C input converters because
          rectifier functioning and how          the output differs: Dc for rectifier and
          the electrical DC engine is            AC for AC converters.
          loaded with harmonics witch
          drive to the overheating and           REFERENCES
          functioning limitation.                1. Kelemen A., power Electronics
                                                     Bucharest 1983
 If we compare the two step matrix               2. Cerbulescu D., power Static
given by the relations 1 and 2, we can               converters, Vol 1, Craiova 1997
deduce:                                          3. Suceveanu I,D. Brevet OSIM nr
      - for the domain I each device                 83111/1983.
          drives a time interval equal           4. Cerbulescu D., Three-Phase Half-
          with 2π/m = 120° (m= number            Wave AC Controllers. Sintes 9, 4, June
          of phases) and for the domain          1998