# Introduction to Controlled Rectifiers

## Introduction to Controlled Rectfiers

Tags: Controlled Rectifier using SCR, Theory of controlled Rectifier, Full wave controlled Rectifier, Single Phase Full controlled Converter,
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###### INTRODUCTION TO CONTROLLED RECTIFIERS

Controlled rectifiers are line commutated ac to dc power converters which are used to convert a fixed voltage, fixed frequency ac power supply into variable dc output voltage.

Type of input: Fixed voltage, fixed frequency ac power supply.
Type of output: Variable dc output voltage

The input supply fed to a controlled rectifier is ac supply at a fixed rms voltage and at a fixed frequency. We can obtain variable dc output voltage by using controlled rectifiers. By employing phase controlled thyristors in the controlled rectifier circuits we can obtain variable dc output voltage and variable dc (average) output current by varying the trigger angle (phase angle) at which the thyristors are triggered. We obtain a uni-directional and pulsating load current waveform, which has a specific average value.

The thyristors are forward biased during the positive half cycle of input supply and can be turned ON by applying suitable gate trigger pulses at the thyristor gate leads. The thyristor current and the load current begin to flow once the thyristors are triggered (turned ON) say at ωt= α. The load current flows when the thyristors conduct from ωt= α to β . The output voltage across the load follows the input supply voltage through the conducting thyristor. At ωt= β, when the load current falls to zero, the thyristors turn off due to AC line (natural) commutation.

In some bridge controlled rectifier circuits the conducting thyristor turns off, when the other thyristor is (other group of thyristors are) turned ON.

The thyristor remains reverse biased during the negative half cycle of input supply. The type of commutation used in controlled rectifier circuits is referred to AC line commutation or Natural commutation or AC phase commutation.

When the input ac supply voltage reverses and becomes negative during the negative half cycle, the thyristor becomes reverse biased and hence turns off. There are several types of power converters which use ac line commutation. These are referred to as line commutated converters.

###### DIFFERENT TYPES OF SINGLE PHASE CONTROLLED RECTIFIERS

Single Phase Controlled Rectifiers are further subdivided into different types

• Half wave controlled rectifier which uses a single thyristor device (which provides output control only in one half cycle of input ac supply, and it provides low dc output).
• Full wave controlled rectifiers (which provide higher dc output)
• Full wave controlled rectifier using a center tapped transformer (which requires two thyristors).
• Full wave bridge controlled rectifiers (which do not require a center tapped transformer)
• Single phase semi-converter (half controlled bridge converter, using two SCR’s and two diodes, to provide single quadrant operation).
• Single phase full converter (fully controlled bridge converter which requires four SCR’s, to provide two quadrant operations).

Three Phase Controlled Rectifiers are of different types

• Three phase half wave controlled rectifiers.
• Three phase full wave controlled rectifiers.

• Semi converter (half controlled bridge converter).
• Full converter (fully controlled bridge converter).

The circuit diagram of a single phase fully controlled bridge converter is shown in the figure with a highly inductive load and a dc source in the load circuit so that the load current is continuous and ripple free (constant load current operation).

The fully controlled bridge converter consists of four thyristors T1 ,T2 ,T3 and T4 connected in the form of full wave bridge configuration as shown in the figure. Each thyristor is controlled and turned on by its gating signal and naturally turns off when a reverse voltage appears across it. During the positive half cycle when the upper line of the transformer secondary winding is at a positive potential with respect to the lower end the thyristors T1 and T2 are forward biased during the time interval ωt= 0 to π . The thyristors T1 and T2 are triggered simultaneously , the load is connected to the input supply through the conducting thyristors T1 and T2 . The output voltage across the load follows the input supply voltage and hence output voltage V0=Vm sin ωt  . Due to the inductive load T1 and T2 will continue to conduct beyond ωt= π, even though the input voltage becomes negative. T1 and T2 conduct together during the time period  α to (π + α ), for a time duration of radians (conduction angle of each thyristor =180º )

During the negative half cycle of input supply voltage for ωt= π to 2π, the thyristors T3 and T4 are forward biased. T3 and T4 are triggered at ωt= (π + α ) . As soon as the thyristors T3 and T are triggered a reverse voltage appears across the thyristors T1 and T2 and they naturally turn-off and the load current is transferred from T1 and T2 to the thyristors T3 and T4 . The output voltage across the load follows the supply voltage and V0=-Vm sin ωt  during the time period ωt= (π + α ) to (2π + α ) . In the next positive half cycle when T1 and T2 are triggered, T3 and T4 are reverse biased and they turn-off. The figure shows the waveforms of the input supply voltage, the output load voltage, the constant load current with negligible ripple and the input supply current.

During the time period ωt= α to π, the input supply voltage Vs and the input supply current Is are both positive and the power flows from the supply to the load. The converter operates in the rectification mode during ωt= α to π .

During the time period ωt= π to (π + α ) , the input supply voltage Vs is negative and the input supply current Is is positive and there will be reverse power flow from the load circuit to the input supply. The converter operates in the inversion mode during the time period ωt= π to (π + α ) and the load energy is fed back to the input source.

The single phase full converter is extensively used in industrial applications up to about 15kW of output power. Depending on the value of trigger angle α , the average output voltage may be either positive or negative and two quadrant operations is possible.

###### TO DERIVE AN EXPRESSION FOR THE AVERAGE (DC) OUTPUT VOLTAGE

The average (dc) output voltage can be determined by using the expression

The output voltage waveform consists of two output pulses during the input supply time period between 0 & 2π radians . In the continuous load current operation of a single phase full converter (assuming constant load current) each thyristor conduct for π radians (180º) after it is triggered. When thyristors T1 and T2 are triggered at ωt= α T1 and T2 conduct from α to (π + α ) and the output voltage follows the input supply voltage. Therefore output voltage V0=Vm sin ω for ωt= α to (π + α ).

Hence the average or dc output voltage can be calculated as

Therefore Vdcn=Vn= cos α ; for a single phase full converter assuming continuous and constant load current operation.

###### CONTROL CHARACTERISTIC OF SINGLE PHASE FULL CONVERTER

The dc output control characteristic can be obtained by plotting the average or dc output voltage Vdc versus the trigger angle α

For a single phase full converter the average dc output voltage is given by the equation

Figure.2 Control Characteristics

We notice from the control characteristic that by varying the trigger angle α we can vary the output dc voltage across the load. Thus it is possible to control the dc output voltage by changing the trigger angle α . For trigger angle α in the range of 0 to 90 degrees (ie: 0≤ α ≤90º) ,Vdc is positive and the average dc load current Idc is also positive. The average or dc output power Pdc is positive; hence the circuit operates as a controlled rectifier to convert ac supply voltage into dc output power which is fed to the load.

For trigger angle α > 90º,cos α becomes negative and as a result the average dc output voltage Vdc becomes negative, but the load current flows in the same positive direction i.e.,Idc is positive . Hence the output power becomes negative. This means that the power flows from the load circuit to the input ac source. This is referred to as line commutated inverter operation. During the inverter mode operation for α > 90º the load energy can be fed back from the load circuit to the input ac source

###### TWO QUADRANT OPERATION OF A SINGLE PHASE FULL CONVERTER

Figure: 3 Voltage Vs Current Characteristics

The above figure shows the two regions of single phase full converter operation in the Vdc versus Idc plane. In the first quadrant when the trigger angle α < 90º,Vdc and Idc are both positive and the converter operates as a controlled rectifier and converts the ac input power into dc output power. The power flows from the input source to the load circuit. This is the normal controlled rectifier operation where Pdc is positive.

When the trigger angle is increased above 90ºVdc becomes negative but Idc is positive and the average output power (dc output power) Pdc becomes negative and the power flows from the load circuit to the input source. The operation occurs in the fourth quadrant where Vdc is negative and Idc is positive. The converter operates as a line commutated inverter.

###### TO DERIVE AN EXPRESSION FOR THE RMS VALUE OF THE OUTPUT VOLTAGE

The rms value of the output voltage is calculated as

The single phase full converter gives two output voltage pulses during the input supply time period and hence the single phase full converter is referred to as a two pulse converter. The rms output voltage can be calculated as

Hence the rms output voltage is same as the rms input supply voltage

The rms thyristor current can be calculated as

Each thyristor conducts for π radians radians or 180º in a single phase full converter operating at continuous and constant load current.

Therefore rms value of the thyristor current is calculated as

The average thyristor current can be calculated as

###### APPLICATIONS OF PHASE CONTROLLED RECTIFIERS

• DC motor control in steel mills, paper and textile mills employing dc motor drives.
• AC fed traction system using dc traction motor.
• Electro-chemical and electro-metallurgical processes.
• Magnet power supplies.
• Reactor controls.
• Portable hand tool drives.
• Variable speed industrial drives.
• Battery charges.
• High voltage DC transmission.
• Uninterruptible power supply systems (UPS).

###### Circuit Diagram

Figure: Single phase Full Converter with Free Wheeling Diode

Figure: Single phase Full Converter without Free Wheeling Diode