Thyristor Three Phase, Six Pulse Controller

Thyristor Three Phase, Six Pulse ControllerBy Caitlin Barnes, Ashley Van Niekerk and Ashlee Vuckovic!OverviewBackgroundTheoretical DerivationFiring Angle controlSimulationDemonstrationConclusionReferences

Theoretical DerivationWorking out VoIoRipple VoHarmonic DistortionectBackground

What is a Thyristors?

A thyristor or Silicon Controlled Rectifier (SCR) is essentially is a four-layer semiconducter device that consists of an alternating P type and N type material (PNPN) as seen below:

It consists of an Anode, Gate and Cathode, they are made to operate at high voltage. It requires a gate signal to turn it ON, the controlled part of the name and once ON it behaves like a rectifying diode, the rectifier part of the name. In fact the circuit symbol for the thyristor suggests that this device acts like a controlled rectifying diode.

4Background

How does it work?Thyristors similar to diodes only conduct when the voltage across the device becomes forward, however unlike a diode a current must be conduct at the gate for the thyristor to operate . For example:

When the cathode is negatively charged relative to the anode, no current flows until a pulse is applied to the gate. Then the SCR begins to conduct, and continues to conduct until the voltage between the cathode and anode is reversed or reduced below a certain threshold value. Using this type of thyristor, large amounts of power can be switched or controlled using a small triggering current or voltage.

In this simple single phase example the thyristor is only conducting when there is a positive current from the anode to the cathode and when there is current in the form of a pulse at the gate of the thyristor

SCR begins to conduct, and continues to conduct until the voltage between the cathode and anode is reversed or reduced below a certain

5Fire Angle ControlThe Thyristor Firing Unit detects when the AC waveform passes through zero and a firing pulse is inputted into the gate of the thyristor according to the adjustable delayThis delay is expressed as an angle

Reference point used by the firing unit

The Thyristor Firing Unit is a module which detects when a sinusoidal voltagepasses through zero and, following an adjustable delay, provides a firing pulse. Thispulse is designed to trigger, that is to cause, a thyristor to conduct when the anodecathodevoltage polarity is positive. The delay is set by a voltage applied to theANGLE CONTROL INPUT of the module. The Thyristor Firing Unit is built arounda microcontroller which is in fact a microcomputer in a single integrated circuit.Therefore, it is fully digital and very accurate. Figure 2-1 shows a simple singlephase,half-wave controlled rectifier built with a Thyristor Firing Unit and a PowerThyristors module (the operation of this module is dealt with in section 3 of thismanual). Notice that the DC SOURCE shown in Figure 2-1 is part of the ThyristorFiring Unit.6Fire Angle Control (Model 9030-3 Thyristor Firing Unit)

The unit is capable of working with both single phase and 3 phase inputs

Angle control The input voltage to the terminal can be varied by the user. The unit outputs a firing angle of between 0-180Synchronization The terminal synchronizes its input by plugging in two separate phases 1 and 2 respectively.The reference point in each phase is found and the microcontroller will output a current pulse accordingto the set delayIn this section, one finds a small screen which displays the firing angle, expressedin degrees. The firing angle can vary between 0 and 180( (in practice it cannotexceed 179.3() in steps of 0.7(. The FIRING ANGLE display flashes on and offwhen there is no synchronization signal applied at the SYNC. INPUT.

ANGLE CONTROL sectionThis section allows the firing angle of thyristors to be controlled. It is divided in twosubsections. The INPUT terminal receives a voltage which can vary from10 to +10 V to make the firing angle vary between 0 and 180(. The COMPLEMENTand ARC COSINE push buttons allow the relationship between the firingangle and the voltage applied at the INPUT to be selected.When neither the COMPLEMENT nor the ARC COSINE push button is depressed,the firing angle varies linearly from 0 to 179.3( when the INPUT voltage varies from10 to +10 V.7Fire Angle Control (Model 9030-3 Thyristor Firing Unit)

Fire Angle ControlVoltage Output

Fire Angle Control

During the red highlighted parts (