yeni-2_Power_Electronic_Experiments by shitingting

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									          Doğuş University
Department of Control and Automation
             Engineerig


            CONT 311
     Power Electronics Circuits
              2012-2013 Fall




     Lecturer: Assist. Phd. Mustafa Doğan

     Laboratory Assistant: Kübra Tural
Experiment 2: Single Phase Full Wave Rectifier with
               Thyristors

Preliminary Work:

   1. What is wave figure of the network voltage? What is the line frequency in
      Turkey? Why it is?
   2. What is the effective and average value of voltage which is line voltage?
      Write the formula of this transformation.
   3. What does the alpha angle mean in experiment? Explain.
   4. What is the difference between currents of half wave and full wave
      rectifiers? Why it exists, explain.
   5. What is the function of a freewheeling diode? Explain why it is used.




Aim:

    Analyzing the operation and behaviors of the single phase rectifiers with
thyristor. In this experiment, full wave thyristor rectifiers (as single phase ) will
be analyzed with ohmic and inductive loads. The rectifier circuits are controlled
with thyristors in this experiment. This means, load voltages can be changed, by
controlling the turn on timings of the thyristors. Therefore, thyristor rectifiers are
also called as controlled rectifiers.

Theory:

    Rectifiers with diodes can produce constant output voltage. If the industrial
application necessitates an adjustable voltage instead of a constant voltage, we
can not use rectifiers with diodes in that case. In this type of applications, phase
controlled thyristors take the place of diodes. The output voltage of the thyristor
can be controlled by changing the delay and triggering angle of the thyristor.
Thyristor starts conducting by a current pulse applied on the gate terminal,
however, while its voltage is negative, if the current decreases below a certain
value it stops conducting. In AC systems, the voltage and current naturally drops
to negative but in DC systems this is not the case. Therefore, we can not use
thyristors in DC systems.
    As the phase controlled systems are simple, efficient and comparatively
cheaper, they are used in industrial applications, especially in adjustable fast
driver systems, in a wide interval of a few kW to MW levels. Thyristor rectifiers,
as rectifiers with diodes, will be analyzed in a classification of single phase, tri-
phase and full wave, half wave rectifiers.

Single Phase Half Wave Thyristor Rectifier:

    As it is seen in figure 2.1, there is a thyristor as a diode in the circuit.


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                 Figure 2.1 Rectifier with half wave thyristor

    Phase control is provided by making thyristor conducting in any instance of
the positive phase of the input voltage. After this point, thyristor’s voltage
remains zero and it continues conducting until the current decreases. If the load
is an ohmic load, the waveform of the thyristor current and voltages remains
same and no voltage problem occurs. However, as it is the case in dioded half
wave rectifier, at inductive load, the current lags and the thyristor will be off
after a while. Until that time a negative current will flow which will lead to a
voltage drop on the load. In figure 2.2, we can see the waveforms observed in
the output of half wave rectifier for different triggering angles.




  Figure 2.2. The output current and voltage waveforms with 0˚, 60˚ , 90˚
              triggering angles and with ohmic and inductive loads.

   Half wave rectifier circuits are not preffered in industry because they have
lower frequency components and higher swings.

    The average and effective values of output voltage can be calculated as they
are calculated in rectifiers with diodes.




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Single Phase Full Wave (Bridge) Rectifier:

    In figure 2.3 we can see half controlled and full controlled bridge rectifier.
While the full controlled rectifier is composed of 4 thyristors, half controlled
rectifier is composed of 2 thyristors and 2 diodes. In half controlled rectifier,
voltage and current is always positive, which means it is a rectifier working with
single side. However, in full controlled rectifier, if free wheeling diode is not used,
voltage may drop to negative. But current is always positive in this type as it is
in half controlled rectifier.

   This full controlled system also works with positive current and positive-
negative voltage, that is, with two sides.




       Figure 2.3 Single phase, half controlled and full controlled rectifier

    In figure 2.4, a typical output voltage waveform and conducting intervals of
thyristor and diodes are given for a half controlled rectifier under inductive load.




                    Figure 2.4 Half controlled bridge rectifier



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    As it is seen in the figure, T1-D2 switches conduct the positive phase, and
T2-D1 switches conduct the negative phase of the voltage. When voltage comes
to the end of positive phase and passes into negative phase, the duty of the T1-
D2 couple ends.

    As voltage passes into negative phase, D2 cuts off while D1 starts conducting
with a positive voltage. At this point, because the T2 is not triggered yet and
load current is still over zero, T1 thyristor remains in conduction until T2 thyristor
is triggered. When       wt     ,   T2 is triggered and T2-D1 couple starts
conducting. The average and effective value of the voltage is as below.




    In full controlled rectifier, we can use thyristors instead of diodes and every
switch on the circuit is controlled, so, it is called full controlled rectifier. According
to a full controlled circuit seen in figure 2.3, T1-T4 and T2-T3 thyristors work
together as couples. At the instant of wt   , T1 and T4 are triggered and they
remain in conducting until wt   instant. As T2 and T3 thyristors will not be
triggered until wt     instant, if the load is inductive, T1 and T4 thyristors will
not stop conducting and continue carrying the load current. So, they will continue
to conduct the negative voltage towards the output until the T2 and T3 diodes
start conducting. At the moment of wt   , T1 and T4 thyristors will be triggered
and start conducting. This situation together with thyristors’ conduction periods
is shown in figure 2.5.




                                        Figure 2.5


     In figure 2.5, it is seen that the load current is discontinuous. Therefore,
there are time intervals in which none of the switches in the circuit is conducting.
In case of a continuous load currents, current will be conducted by T1-T4
thyristors until other thyristor couple is triggered. As a result, average voltage
will decrease while the continuity of the current is provided. In the period from 

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to  , voltage and current are positive, which means power flows from network
to load. We call the situation as circuit is in rectification mode. In the negative
part of the voltage after  , current is again positive. Power flows from load to
network. This time, we call the situation as circuit operates in inverter mode.

    If the current is continuous, average and efficient voltage for full controlled
bridge rectifier can be calculated with the formulas below.




2.1 Single Phase Full Controlled Rectifier (with Inductive Load)




                                   Figure 2.6




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                                   Figure 2.7


EXPERIMENTAL PROCEDURE:

    Connect a 50 ohm resistance and 100 mH inductance to the output of the
circuit seen in figure 2.6. Adjust the triggering angle of the thyristor to 0 degree
and draw the waveforms of the load voltage and current.

    Observe and draw the voltages by adjusting the thyristor triggering angles to
30, 60, 90, 120 and 150 degrees. Measure and take note of the voltage and
currents for each angles.




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