exp 4 familiarization with the tfu

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EEK 471 LAB 4

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Universiti Sains MalaysiaElectrical Engineering DepartmentAdvanced Power Electronic Laboratory EEK471

Familiarization with the Thyristor Firing Unit

OBJECTIVE

To learn how to use the Thyristor Firing Unit.

INTRODUCTION

The buck chopper

The Thyristor Firing Unit is a module which detects when a sinusoidal voltage passes throughzero and, following an adjustable delay, provides a firing pulse. This pulse is designed to triggerthat is to cause a thyristor to conduct when the anode-cathode voltage polarity is positive. Thedelay is set by a voltage applied to the ANGLE CONTROL INPUT of the module. The Thyristor

Firing Unit is built around a microcontroller which is infact a microcomputer in a single integratedcircuit. Therefore it is fully digital and very accurate. Figure 1 shows a simple single-phase half-wave controlled rectifier built with a Thyristor Firing Unit and a Power Thyristors module (theoperation of this module is dealt within section 3 of this manual). Notice that the DC SOURCEshown in Figure 1 is part of the Thyristor Firing Unit.

Figure 1: A single-phase half-wave controlled rectifier.

Figure 2 shows the waveforms related to the circuit shown in Figure 1. The time interval t represents the delay from the zero-crossing of the sinusoidal voltage waveform to the start of thepulse which triggers the thyristor. Usually, this delay is expressed as an angle and is called the


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angle of retard, or firing angle, . It is defined as the angular intenval, expressed in degrees, bywhich the starting point of conduction is delayed in relation to the starting point of the same circuitif the thyristors were replaced by diodes. The delay variables and t are related by the equation:

= (t / T) x 360

where T is the period of the sinusoidal supply voltage, expressed in the same units as t. When

the Thyristor Firing Unit is used to control thyristors in a circuit powered by a source whosefrequency is 50 Hz, T becomes 20ms.

Figure 2: Signal generated by a single-phase half-wave controlled rectifier.

EQUIPMENTS

EMS 8821 Enclosure Power Supply

EMS 8840-0A PE Power Supply

EMS 8841-2A Power Thyristor Module

EMS 9030-30 Thyristor Firing Unit

EMS 8412-05 Lab-Volt DC Voltmeter/Ammeter

Textronic Oscilloscope

Connection Leads

WARNINGS

The voltages and currents that are used during this lab are larger and rated at 240VACLine-to-Neutralwith current as high as 20 amps (or higher if circuits are improperly connected). Pleasetake the proper precautions and use your head before touching any circuitry. NEVERchange anycircuit connections while the power supply is turned on. Ask the demonstrator to check yourconnections before turning on the switches. And follow the rating of voltmeters and ammetersgiven to prevent equipments from damaged.


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Experiment: Part IEffect of the conversion functions on the relationship between the firing angle

and the voltage at the ANGLE CONTROL INPUT

1. In this part, you will use the circuit shown in Figure 1a to observe the effect theconversion functions have on the relationship between the firing angle and the voltage atthe ANGLE CONTROL INPUT. Connect the modules as shown in Figure 1a. Note thatthe DC SOURCE is part of the Thyristor Firing Unit.

Figure 1a: Circuit used to observe the effect of the conversion functions of theThyristor Firing Unit.

2. Make the following settings on the Thyristor Firing Unit

Setting 1

ANGLE CONTROL COMPLEMENT 0ANGLE CONTROL ARC COSINE 0DC SOURCE MinimumFIRING CONTROL MODE 1~

3. Make sure that the main power switch of the Power Supply is set to the 0 (OFF) position.Set the main power switch to 1 (ON) position. On the Thyristor Firing Unit, vary thesetting of the DC SOURCE control knob while observing the FIRING ANGLE display andthe voltage at the ANGLE CONTROL INPUT on the oscilloscope screen. Describe whathappens when the voltage at the ANGLE CONTROL INPUT varies.

4. On the Thyristor Firing Unit, vary the setting of the DC SOURCE control knob tosuccessively obtain each of the voltages given in Table 1a. For each voltage, read thefiring angle indicated on the Thyristor Firing Unit, and then note it in the appropriate rowof the FIRING ANGLE column of Table 1a.

Table 1a: Firing angle versus the voltage at the ANGLE CONTROL INPUT

ANGLE CONTROL INPUT VOLTAGE (V) -10 -8 -5 -2 0 +2 +5 +8 +10FIRING ANGLE ()


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5. Plot the relationship between the firing angle and the voltage at the ANGLE CONTROLINPUT in Figure 1b.

Figure 1b: Relationship between the firing angle and the voltage at the ANGLE CONTROLINPUT when the COUPLEMENT and ARC COSINE ANGLE CONTROL push buttons

are set to the O position.

6. Describe the relationship between the firing angle and the voltage at the ANGLECONTROL INPUT when the COMPLEMENT and ARC COSINE ANGLE CONTROL pushbuttons are set to the O position.

7. Repeat steps 4, 5 and 6 of the procedure using different setting of the Thyristor FiringUnit as listed below.

a) Setting 2


b) Setting 3



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c) Setting 4


8. Set the main power switch to the 0 position and turn OFF the power supply. Remove allleads, cables and probes.

Experiment: Part IIObservation of the firing control signals in the single-phase (1~)

and three-phase (3~) modes

1. In this part, you will use the circuit shown in Figure 2a to observe the synchronizationsignal and the firing control signals generated by the Thyristor Firing Unit in the singleand three phase firing control modes. You will vary the firing angle to observe the effect ithas on the phase relationship between the synchronization signal and the firing controlsignals. Connect the modules as shown in Figure 2a.

Figure 2a: Circuit for observing the firing control signals

2. Set e1 scale to 600V and make the following settings on the Thyristor Firing Unit


3. On the oscilloscope, set the input coupling of each channet to the ground position andthen place the trace of each channel in the middle of the screen. Then set the inputcoupling of each channel to the DC position.

4. Make sure that the main power switch of the Power Supply is set to the 0 (OFF) position.Set the main power switch to 1 (ON) position. The oscilloscope should now display thesynchronization signal applied to the Thyristor Firing Unit ( line to neutral sinusoidalvoltage ) and firing control signal 1.


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5. On the Thyristor Firing Unit, slowly vary the setting of the DC SOURCE control knob tovary the firing angle while observing firing control signal 1 on the oscilloscope screen.Describe how the phase relationship between the synchronization signal and firingcontrol signal 1 varies as the firing angle is varied.

6. On the Thyristor Firing Unit, set the DC SOURCE control knob so that the firing angledisplay indicates 135. Measure the time between the positive slope zero crossing of theSynchronization signal and the rising edge of firing control signal 1. Convert this time intoa firing angle using desired equation. Note the result in the space provided below.

Firing angle (measured) = __________

Compare the measured firing angle with the firing angle indicated on the Thyristor FiringUnit. Are they nearly equal? You can repeat this step for various firing angles.

7. On the Thyristor Firing Unit, set the DC SOURCE control knob so that the firing angledisplay indicates 45. Sketch the waveforms of the synchronization signal and firingcontrol signal 1 in Figure 2b. Using channel 2 of the oscilloscope, successively observefiring control signals 2, 4, and 5 on FIRING CONTROL INPUTS 2, 4, and 5 of the PowerThyristors module. Sketch the waveforms of these firing control signals in Figure 2b.

Figure 2b: Waveforms of the synchronizalion signal and the firing control signals in thesingle phase mode.


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8. Make the following settings on the Thyristor Firing Unit

FIRING CONTROL MODE 3~

Connect FIRING CONTROL INPUT 1 of the Power Thyristors module to channel 2 inputof the oscilloscope. Sketch the waveforms of the synchronization signal and firing controlsignal 1 in Figure 2c. Using channel 2 of the oscilloscope, successively observe firingcontrol signals 2, 3, 4, 5, and 6 on FIRING CONTROL INPUTS 2, 3, 4, 5, and 6 of thePower Thyristors module. Sketch the waveforms of these firing control signals in Figure2c. From the firing control signals sketched in Figure2c, determine the order in which thethyristors are fired.

Figure 2c: Waveforms of the synchronization signal and the firing control signals in thethree phase mode.

9. Set the main power switch to the 0 position and turn OFF the power supply. Remove allleads, cables and probes.


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REVIEW QUESTIONS

1. What is the function of the Thyristor Firing Unit?

2. Enumerate the five sections of the Thyristor Firing Unit.

3. What is the range over which the firing angle can be set?

4. What is the function of the Synchronization Input?

5. Describe the options of the Thyristor Firing Unit.

exp 4 familiarization with the tfu

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