yeni-2 power electronic experiments

8/11/2019 Yeni-2 Power Electronic Experiments

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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



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Experiment 2: Single Phase Full Wave Rectifier withThyristors

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 withthyristor. In this experiment, full wave thyristor rectifiers (as single phase ) willbe analyzed with ohmic and inductive loads. The rectifier circuits are controlledwith thyristors in this experiment. This means, load voltages can be changed, by

controlling the turn on timings of the thyristors. Therefore, thyristor rectifiers arealso called as controlled rectifiers.

Theory:

Rectifiers with diodes can produce constant output voltage. If the industrialapplication necessitates an adjustable voltage instead of a constant voltage, wecan not use rectifiers with diodes in that case. In this type of applications, phasecontrolled thyristors take the place of diodes. The output voltage of the thyristorcan 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 certainvalue it stops conducting. In AC systems, the voltage and current naturally dropsto negative but in DC systems this is not the case. Therefore, we can not usethyristors in DC systems.

As the phase controlled systems are simple, efficient and comparativelycheaper, they are used in industrial applications, especially in adjustable fastdriver 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 Thyristo r Recti f ier:

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 ofthe positive phase of the input voltage. After this point, thyristor’s voltageremains zero and it continues conducting until the current decreases. If the loadis 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 halfwave rectifier, at inductive load, the current lags and the thyristor will be offafter a while. Until that time a negative current will flow which will lead to avoltage drop on the load. In figure 2.2, we can see the waveforms observed inthe 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 havelower frequency components and higher swings.

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



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Sing le Phase Full Wave (Bri dg e) Rectif ier:

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 controlledrectifier 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 withsingle 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 isin 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 ofthyristor 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, andT2-D1 switches conduct the negative phase of the voltage. When voltage comesto 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 conductingwith a positive voltage. At this point, because the T2 is not triggered yet andload current is still over zero, T1 thyristor remains in conduction until T2 thyristor

is triggered. When t w , 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 everyswitch on the circuit is controlled, so, it is called full controlled rectifier. Accordingto a full controlled circuit seen in figure 2.3, T1-T4 and T2-T3 thyristors work

together as couples. At the instant oft

w , T1 and T4 are triggered and they

remain in conducting untilt

w instant. As T2 and T3 thyristors will not be

triggered untilt

w instant, if the load is inductive, T1 and T4 thyristors will

not stop conducting and continue carrying the load current. So, they will continueto conduct the negative voltage towards the output until the T2 and T3 diodes

start conducting. At the moment of t w

, T1 and T4 thyristors will be triggeredand start conducting. This situation together with thyristors’ conduction periodsis 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 voltagewill 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 negativepart 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 controlledbridge 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 thecircuit seen in figure 2.6. Adjust the triggering angle of the thyristor to 0 degreeand draw the waveforms of the load voltage and current.

Observe and draw the voltages by adjusting the thyristor triggering angles to30, 60, 90, 120 and 150 degrees. Measure and take note of the voltage and

currents for each angles.

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