three phase half wave controlled rectifier experiment

Half Wave Rectifier Circuit Diagram & Working Principle

What is a Half Wave Rectifier?

Half wave rectifier theory, half wave rectifier capacitor filter, half wave rectifier formula, ripple factor of half wave rectifier, efficiency of half wave rectifier, rms value of half wave rectifier, peak inverse voltage of half wave rectifier, form factor of half wave rectifier, output dc voltage, applications of half wave rectifier, advantages of half wave rectifier, disadvantages of half wave rectifier, 3 phase half wave rectifier.

A typical configuration of a three-phase half wave rectifier supplying a purely resistive load is shown below. Here, each phase of the transformer is considered as an individual alternating source. The simulation and measurement of voltages are as shown in the circuit below. Here we have connected an individual voltmeter across each source as well as across the load.

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Lessons from life, 3 phase half wave controlled rectifier (2 circuits) – detailed explanation.

25/04/2022 Engineer ELECTRONICS 0

Learn about the working principle of 3 phase controlled half-wave rectifier circuit (2 circuit diagrams). How to generate pulses to fire thyristors in a controlled rectifier circuit.

Working principle of the control circuit

A three-phase half-wave controlled rectifier is a circuit that uses three thyristors (or SCRs) to convert AC voltage to DC voltage. Only one SCR can conduct at a time.

For example, when V1 is maximum, SCR1 is forward biased; SCR2, SCR3 is reverse biased. So if SCR1 is fired, then SCR1 will conduct, and SCR2, SCR3 turn off. Similarly for SCR2 and SCR3, voltage V2 and V3 should respectively be greater than other two phase voltages.

• 3 phase half wave controlled rectifier

The input phase voltages V1, V2, and V3 have the same amplitude and frequency with 120◦ phase shifts. The control signal must be synchronized with the input phase voltage. In Psim software, we design a control circuit that generates control pulses.

Control circuit designed on Psim software

The control circuit uses a voltage sensing block to convert the input phase voltage to a smaller voltage that can be measured. And move the electrical zero degree angle of the control pulse to a new position where the thyristor can conduct immediately when the thyristor is fired.

After that, this signal will pass through a comparator block to generate a digital signal. This digital signal puts into Block Angle Control. When the signal level is high, the alpha block will wait for a time t (t = alpha) and then output a signal to activate the thyristor.

3 phase half wave controlled rectifier (2 circuits)

1. controlled rectifier with resistive load.

The 3 phase controlled rectifier uses 3 SCRs instead of diodes. The circuit below uses a load of resistor R = 50 Ohm and an alpha trigger angle of 90.

Three phase half wave controlled rectifier with R load

Working principle:

+ When the phase voltage V1 is maximum, Thy1 is forward biased. If there is a control pulse G1 placed on the gate terminal of Thy1, then Thy1 will conduct, so output voltage Vo = V1.

Unlike the uncontrollable rectifier circuit, when V2 > V1, Thy1 will continue to conduct (Vo = V1). Because when the phase voltage V2 is the largest, the control pulse G2 is not available, so Thy2 is still OFF; Thy1 continues to conduct.

Until V1 < 0, Thy1 is reverse biased, so it stops conducting (Vo = 0, Io = 0).

+ When there is a control pulse G2 put into the gate terminal of Thy2, Thyristor 2 will conduct. The principle of operation is the same for Thyristor3.

=> Output voltage waveform is identical to the input phase voltage corresponding to the thyristor conducting at the time. Vout is discontinuous for a resistive load. The average value of output voltage: (Vmp = √ 2. V RMS )

2. Three phase half wave controlled rectifier with RL load

Three phase controlled rectifier with RL load

– Working principle:

+ When V1 > V2 > V3: If there is a control pulse G1, thyristor1 will conduct (Vo = V1 > 0).

+ When V1 < 0: Thy1 is reverse biased, but it continues to conduct. Since the load RL generates energy, the current flows in the same direction as the previous current, which causes thy1 to continue in the conduction state. The output voltage is equal to the source voltage, so it has a negative value Vo = V1 < 0, Io > 0.

+ When the load runs out of energy, thy1 stops conducting (Io = 0, Vo = 0). The output current is interrupted. If L has a sufficiently large value or the excitation angle is small, the output current can be continuous.

The disadvantage of a 3 phase half wave controlled rectifier with an RL load is that the output voltage has parts less than zero (Vo < 0).

+ The principle of operation is similar for thy1 and thy2.

>>> Related posts:

Half wave rectifier using scr and diode (8 circuits)

3 phase bridge rectifier circuit (4 circuits)

3 phase half wave rectifier circuit (4 circuits)

>>> See also: Half wave controlled rectifier working & waveforms – Engineering Funda

• 3 phase controlled rectifier
• three phase controlled rectifier
• three phase half wave controlled rectifier

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Experiment No.7 Three-Phase half wave Uncontrolled Rectifier

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Half Wave Controlled Rectifier or Converter – with R & RL Load

The process of conversion of ac power into dc power is called Rectification. The device used for the rectification process is called Rectifier. In normal rectifiers, which includes P-N junction semiconductor diodes, the output voltage obtained is fixed in amplitude by the amplitude of ac input voltage.

So in order to obtain controllable variable dc output, controlled rectifiers are used. Controlled rectifiers incorporate phase-controlled thyristors instead of diodes to obtain the variable output. The controlled rectifiers can be single-phase or three-phase. Based on control over output voltage, there are two types of controlled rectifiers,

Half Wave Controlled Rectifier and, Full Wave Controlled Rectifier. (adsbygoogle = window.adsbygoogle || []).push({});

In this article, let us learn about half-wave controlled rectifiers. In practice, the performance of controlled rectifiers gets affected by the type of load (resistive or inductive, or capacitive) to which it is supplying power. So, let us see the operation of half-wave controlled rectifiers with various types of loads.

Single-phase Half Wave Controlled Rectifier with R Load :

The circuit diagram for a single-phase half-wave controlled rectifier with resistive load is shown below. The circuit is energized from transformer secondary and an SCR is included between load and secondary. The ac input is given to the primary of the transformer. In the below circuit load is assumed to be purely resistive.

Working of Single-phase Half Wave Controlled Rectifier with R Load :

We know that thyristor is a unidirectional device, allowing the flow of current only in one direction. When the thyristor is forward-biased i.e., anode terminal is positive with respect to the cathode terminal, and the gate terminal is not triggered, there will be no conduction due to the reverse biasing of the inner junction of SCR. Hence, the entire supply voltage appears across the SCR.

If the gate terminal of SCR is triggered with an adequate duration pulse, then the width of the depletion layer (reverse biased inner junction) starts reducing and finally disappears and SCR starts conducting. As soon as the conduction starts, the voltage across SCR drops to a very small value and the supply voltage appears across the load.

The magnitude of the conduction current depends upon the instant when it is triggered i.e., firing angle ‘α’, and the load resistance R. Since the circuit does not contain any energy storing elements, the load current will be in phase with voltage and becomes zero instantaneously with the voltage at zero crossing (at ωt = π rad/sec).

During the negative half cycle of supply voltage, the SCR is reverse-biased, so there will be no conduction even if the gate is triggered. Hence, the entire supply voltage appears across SCR. The waveforms for the above circuit operation with resistive load are illustrated below.

In the above waveforms, the load current and voltage are zero from 0 to α. When SCR is triggered by giving gate signal at α. The entire supply voltage except for drop across SCR will be applied across the load (from ωt = α to ωt = π). At ωt = π, the phase reversal takes place and the negative half-cycle of the input supply will start.

Due to the negative half-cycle, the SCR will be reverse biased and will be turned OFF at ωt = π. From ωt = π to ωt = 2π, the load current and voltage will be zero. Again when the positive half cycle starts i.e., from ωt = 2π, SCR will be forward biased but it will not be switched ON until it is triggered i.e. until ωt = (2π + α).

Calculation of Average Load Voltage and Current with R load :

From the above waveform, the load voltage appears only from α to π in the period of 2π during the first cycle of the input supply. From this, the average value of load voltage and the load current is derived as,

Therefore, from the above equation, the average output voltage across the load can be varied by varying the firing angle α. The maximum output voltage across the load is obtained when firing angle α = 0.

Single-phase Half Wave Controlled Rectifier with RL Load :

When the load is complex i.e., when the load contains any energy storage elements like inductor (RL load), the situation becomes different from that of pure resistive load. Here the load current doesn’t follow the waveform of the load voltage (load current will be out of phase with load voltage). The below figure illustrates the half-wave rectifier with RL (resistive-inductive) load.

Working of Single-phase Half Wave Controlled Rectifier with RL Load :

At ωt = α, when SCR is triggered, it starts conducting. Now, the SCR acts as a closed switch due to which the voltage across SCR decreases and becomes equal to ON state drop. The load voltage will now become equal to the supply voltage; whereas, the load current increases at a slow rate because of the presence of an inductor.

At ωt = π, the supply voltage becomes zero but the load current will not become zero instantaneously because of the load inductor. The load current after ωt = π slowly decreases due to which the load voltage still follows the supply voltage even in the negative cycle. At some angle ωt = π + β (extinction angle), the load current becomes zero and hence the load voltage. The above process repeats even in the next cycle.

Calculation of Average Load Voltage and Current with RL load :

The average value of load voltage and load current when half-wave controlled rectifier supplying power to an inductive load is derived as,.

Working of Single-phase Half Wave Controlled Rectifier with Freewheeling Diode :

From the above rectifier operation with inductive load. We notice that, the load inductor stores energy in the initial part of the positive half-cycle of the applied voltage and maintains the conduction of current during the negative half-cycle by releasing the stored energy for a substantial period.

The time of conduction depends upon the value of load inductance. Thus supply positive voltage appears across the load and thus even negative voltage reaches the load. In order to stop the negative voltage wave to reach the load or to prevent the instantaneous value of the load voltage to become negative. A free-wheeling or flywheel diode (FD) is connected across the load as shown below.

By using a free-wheeling diode only positive voltage reaches the load. If the circuit is connected to the ac supply, the diode or power device is likely to damage when the switch is closed and opened due to sparks established. To overcome this situation a freewheel diode must be connected across the load.

At ωt = α, the thyristor is triggered and the supply voltage appears across the load from ωt = α to ωt = π. With a freewheeling diode, the thyristor will not be able to conduct beyond π. At ωt = π the supply voltage, as well as the load voltage, becomes zero, but the load current does not become zero instantaneously because of the presence of an inductor.

At ωt = π the thyristor becomes reverse biased, whereas the freewheeling diode becomes forward biased. Hence after ωt = π, the load current will be transferred from thyristor to the freewheeling diode. During the negative half cycle of supply voltage, the freewheeling diode action takes place and no power will be returned to the source.

In practice, half-wave controlled rectifiers are not generally used, since they cannot produce continuous load current and a large ripple will be present in the output voltage.

Related Articles :

• Single Phase Semi Converter With RL Load & Freewheeling Diode
• What is Half-Bridge Inverter? – Circuit Diagram & Working
• Load Commutated Chopper – Working, Advantages & Disadvantages
• Working of Step Down Chopper – with R and RL Load
• Single Phase AC Voltage Controller – With R & RL Load Operation

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Electrical Concepts

Tricky but Easy Electrical Engineering!

Single Phase Half Wave Controlled Rectifier

Single Phase Half Wave Controlled Rectifier, as the name suggests, is a rectifier circuit which converts AC input into DC output only for positive half cycle of the AC input supply. The word “controlled” means that, we can change the starting point of load current by controlling the firing angle of SCR . These words might seem a lot technical. But firing of SCR simply means, the SCR turn ON at certain point of time when it is forward biased.

Single Phase Half Wave Controlled Rectifier Circuit:

A Single Phase Half Wave Controlled Rectifier circuit consists of SCR / thyristor, an AC voltage source and load. The load may be purely resistive , Inductive or a combination of resistance and inductance. For simplicity, we will consider a resistive load. A simple circuit diagram of Single Phase Half Wave Controlled Rectifier is shown in figure below.

v 0 = Load output voltage

i 0 = Load current

V T = Voltage across the thyristor T

Following points must be kept in mind while discussing controlled rectifier:

• The necessary condition for turn ON of SCR is that, it should be forward biased and gate signal must be applied. In other words, an SCR will only get turned ON when it is forward biased and fired or gated.
• SCR will only turn off when current through it reaches below holding current and reverse voltage is applied for a time period more than the SCR turn off time.

Well, let us go ahead with the above points in mind. Let us assume that thyristor T is fired at a firing angle of α. This means when wt = α, gate signal will be applied and SCR will start conducting. Refer the figure below.

Thyristor T is forward biased for the positive half cycle of supply voltage. The load output voltage is zero till SCR is fired. Once SCR is fired at an angle of α, SCR starts conducting. But as soon as the supply voltage becomes zero at ωt = π, the load current will become zero and after ωt = π, SCR is reversed biased. Thus thyristor T will turn off at ωt = π and will remain in OFF condition till it is fired again at ωt = (2π+α).

Therefore, the load output voltage and current for one complete cycle of input supply voltage may be written as

v 0 = V m Sinωt for α≤ωt≤ π

i 0 = V m Sinωt / R for for α≤ωt≤ π

Calculation of Average Load Output Voltage:

As we know that, average value of any function f(x) cab be calculated using the formula

Let us now calculate the average value of output voltage for Single Phase Half Wave Controlled Rectifier.

From the expression of average output voltage, it can be seen that, by changing firing angle α, we can change the average output voltage. The average output voltage is maximum when firing angle is zero and it is minimum when firing angle α = π. This is the reason, it is called phase controlled rectifier.

Average load current for Single Phase Half Wave Controlled Rectifier can easily be calculated by dividing the average load output voltage by load resistance R.

Let us now calculate the root mean square (rms) value of load voltage.

RMS value of load current can be calculated by dividing the rms load voltage by resistance R. This means,

RMS Load Current I 0rms = RMS Load Voltage / R

Input volt ampere can be calculated as

Input Volt Ampere

= RMS Supply Voltage x RMS Load Current

= V s xI 0rms

7 thoughts on “Single Phase Half Wave Controlled Rectifier”

very informative guide on 1-phase HW controlled rectifier. Great work.

The equation of single phase half-wave controlled rectifier with Inductive load with freewheeling diode.

really very much informative and simply the best……

Which of the following is used in single phase half wave controlled rectifier A diode B Scr C transistor D none of these

Thank you so much

आपने बहुत ही अच्छा समझाया है

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