Thyristor Power Electronics, 3 The Power Thyristor · Thyristor operation in an ac circuit...

© Festo Didactic 86363-00 53

When you have completed this exercise, you will know what a thyristor is, and how it operates. You will be familiar with the operation of the thyristor in ac circuits with either a resistive or inductive load.

The Discussion of this exercise covers the following points:

The thyristor

Thyristor operation in a dc circuit

Thyristor operation in an ac circuit (resistive load)

Thyristor operation in an ac circuit (resistive-inductive load)

Applications

The thyristor

The thyristor, also called silicon-controlled rectifier (SCR), is a semiconductor that allows electrical current to flow in one direction only. Figure 29 shows typical thyristors used in low-power, medium-power, and high-power applications.

Figure 29. Typical thyristors for low-power, medium-power, and high-power applications.

Figure 30 shows the construction, terminals, and schematic symbol of a thyristor. The thyristor is composed of four layers of P- and N-type material. It has three terminals: an anode (A), a cathode (K), and a gate (G).

The Power Thyristor

Exercise 3

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

(c) High-power (b) Medium-power (a) Low-power

Exercise 3 – The Power Thyristor Discussion

54 © Festo Didactic 86363-00

The gate (G) is used to initiate the conduction (i.e., to turn the thyristor on). When the thyristor is conducting, it operates like a diode, allowing current to flow from its anode (A) to its cathode (K).

Figure 30. Construction, terminals, and schematic symbol of a thyristor.


The operation of a thyristor in a dc circuit is described below:

When no voltage is present across the anode and the cathode, the thyristor is in the “off” (blocked) state. Therefore, the thyristor acts like an open switch, and no current flows through the thyristor, as Figure 31 shows.

Figure 31. When no voltage is present across the anode and the cathode, the thyristor acts as an open switch. Therefore, no current flows through the thyristor.

The arrowhead points toward the cathode, i.e., in the direction of

conventional current flow

A K

G

A

G

Switch open

K

No voltage

Anode (A)

Cathode (K)

Gate (G)

P- and N-typesemiconductor

junctions

A K

G

P N P N

(c) Schematic symbol(a) Construction (b) Terminals



When a voltage is present across the anode and the cathode, and the voltage at

the anode is lower than the voltage at the cathode (i.e., when voltage is negative), the thyristor is reverse biased. Therefore, the thyristor acts like an open switch, and no current flows through the thyristor, as Figure 32 shows.

Figure 32. When the voltage at the anode is lower than the voltage at the cathode (i.e., when

voltage is negative), the thyristor acts as an open switch: no current flows through it.

When a voltage is present across the anode and the cathode, and the voltage at the anode is higher than the voltage at the cathode (i.e., when voltage is positive), the thyristor is forward biased, as Figure 33 shows. However, the thyristor remains in the blocked state as long as there is no current flowing from

the gate to the cathode (i.e., when ).

Figure 33. When the voltage at the anode is higher than the voltage at the cathode (i.e., when

voltage is positive), the thyristor is forward biased but it acts as an open switch as long as there is no current flowing from the gate to the cathode.

A K

G

A

Forward voltage

Reverse voltage

Switch open

K

G

Switch open



As soon as a current of value sufficient to initiate conduction flows from the gate

to the cathode (gate current ), even just for an instant, the forward biased thyristor is fired, i.e., it passes from the “off” (blocked) state to the “on” (conducting) state. The thyristor acts as a closed switch, thereby allowing current to flow from the anode to the cathode (anode-to-cathode current, or

main current ), as Figure 34 shows.

Figure 34. As soon as a current of sufficient value flows from the gate to the cathode

(current ), the forward biased thyristor turns on and acts as a closed switch, allowing

current to flow from the anode to the cathode (current ).

Thereafter, the thyristor continues to conduct as long as current remains higher than a minimum value, called the holding current . Once the thyristor

has entered into conduction, gate current no longer has any control on the conduction. (Thus, removing current does not interrupt the conduction.)

As soon as current becomes lower than the holding current , the conduction is interrupted and the thyristor returns to the blocked state, as Figure 35 shows. The conduction can be reinitiated by making an appropriate current flow from the gate to the cathode.

a The value of the holding current is very low compared to the nominal maximum value of current .

Figure 35. As soon as current becomes lower than the holding current , the conduction is interrupted and the thyristor becomes like an open switch.

Forward voltage

A K

G

Switch closes

Switch opens

A K

G



In summary, two requirements must be met to turn a thyristor on:

The voltage applied to the anode must be higher than the voltage applied to the cathode (i.e., the thyristor must be forward biased).

A current of sufficient value must flow from the gate to the cathode (a brief pulse of current is sufficient).

The gate current required to initiate the conduction is very small compared to the main current that can flow through the thyristor. In high-power industrial applications, thyristors can often conduct currents as high as 4 kA under voltages up to 4 kV. Higher currents can be conducted under voltages lower than 4 kV, while operation at higher voltages is possible at currents lower than 4 kA.

The power gain of a thyristor is the ratio of the amount of power controlled by the thyristor to the amount of power in the gate (control) signal. The power gain can be as high as 1 000 000. This means that a gate signal of 1 watt (1 W) can be used to switch (control) power levels of up to 1 megawatt (1 MW).

a Other popular switching devices are the metal-oxide-semiconductor field-effect transistor (MOSFET) and the insulated-gate bipolar transistor (IGBT). The advantage of MOSFETs and IGBTs over the thyristor is that they are not only able of initiating the flow of current but of interrupting it by using a low-level switching signal. The thyristor does not have this turnoff ability, since the gate loses control on the conduction as soon as the thyristor is turned on. However, thyristors can efficiently switch power at levels that are much higher than those at which MOSFETs and IGBTs can operate.



Figure 36. Application example: high-voltage thyristor valves are used for long distance transmission of power from a hydroelectric power plant located in the province of Manitoba, Canada. Each tower in the picture consists of an arrangement of many high-voltage, light-triggered thyristors and is more than 20 m high.


Figure 37 shows a circuit consisting of a thyristor connected between an

ac voltage source and a resistive load (resistor ). A rectangular pulse of current having a value sufficient to trigger conduction flows from the gate to the cathode of the thyristor whenever the phase angle is 0°.

When the phase angle reaches 0°, the source voltage becomes positive and the thyristor becomes forward biased. Since a pulse of

current (control current ) flows through the gate of the thyristor at this instant, the thyristor turns on immediately, allowing current (current ) to flow from the anode to the cathode. The value of current depends on the source voltage and the resistance of the load.



The thyristor continues to conduct as long as current remains higher

than the holding current . When the phase angle approaches 180°, current becomes lower than the holding current , the thyristor stops conducting current (turns off), and the load current and voltage become null. Therefore, the conduction angle of the thyristor is a little less than 180° and the waveforms of the load current and voltage follow the source voltage waveform during this interval.

During the negative half of the source voltage (i.e., between phase angles 180° and 0°), the thyristor stays off since it is reverse biased. Therefore, the load current and voltage are null.

The above sequence of events repeats every cycle of the source voltage waveform. The load voltage and current waveforms thus consist of a pulse of positive polarity every cycle of the source voltage waveform. Consequently, the load voltage and current have a non-null average (dc) value. This circuit is in fact a single-phase, half-wave rectifier similar to the diode single-phase, half-wave rectifier studied in Exercise 1 of this manual. However, using thyristors instead of diodes in rectifiers allows the value of the rectifier output voltage to be varied, as will be discussed later in this manual.



Figure 37. The thyristor turns on when the source voltage becomes positive and turns off

when current (i.e., the load current ) becomes lower than the holding current .

Phase angle (°)0 90 180 270

Load

voltage 0 90 180 270

Phase angle (°)0 90 180 270

Load current

0 90 180 270

Phase angle (°)0 90 180 270

Source

voltage

0 90 180 270

Load

(b) Waveforms of the circuit voltages and current

The thyristor turns on

becomes lower

than : the thyristorturns off

0

0

0

Phase angle (°)0 90 180 270 0 90 180 270

Thyristor gate

current 0

(a) Thyristor connected between an ac voltage source and a resistive load. A pulse of

current ( ) flows through the gate at the beginning of every cycle of the ac power source voltage.

becomes lower

than : the thyristor turns off




Figure 38 shows a circuit consisting of a thyristor connected between an ac voltage source and a resistive-inductive load (resistor in series with

inductor ). A rectangular pulse of current having a value sufficient to trigger conduction flows from the gate to the cathode of the thyristor whenever the phase angle is 0°.

When the phase angle reaches 0°, the source voltage becomes positive and the thyristor becomes forward biased. Since a pulse of

current ( ) flows through the gate of the thyristor at this instant, the thyristor turns on immediately, allowing current to flow from the anode to

the cathode (current ). The value of current depends on the source voltage and the impedance of the load.

The thyristor conducts current during the whole positive half of the

source voltage (i.e., between 0° and 180°), since current remains higher than holding current during this interval. This is because load inductor , which opposes any changes in current by storing energy in the form of a magnetic field, causes the load current waveform to have an elongated, flattened shape that spreads out beyond 180°.

When the source voltage waveform crosses zero and becomes negative (i.e., when the phase angle is 180°), the thyristor continues to conduct current (i.e., it stays on) even if it is reverse biased because the energy stored in the inductor keeps the value of current higher than the

holding current . The load voltage thus changes polarity (it becomes negative).

The inductor discharges the energy stored in its magnetic field at a rate determined by the time constant of the load. When the inductor has discharged most of its stored energy, current becomes lower than the

holding current , the thyristor stops conducting current (turns off), and the load current and voltage become null.

During the rest of the negative half of the source voltage (i.e., between the phase angle when the thyristor stops conducting and 0°), the thyristor stays off since it is reversed biased. Therefore, the load current and voltage are null.

The load voltage waveform thus consists of a pulse of positive polarity and a shorter pulse of negative polarity every cycle of the source voltage waveform.

The above sequence of events repeats every cycle of the source voltage waveform. The conduction angle (greater than 180°) of the thyristor is greater than that observed (a little less than 180°) when the load is purely resistive. Thus, the addition of an inductor to the resistive load causes the thyristor to turn off during the negative half of the ac source voltage waveform, and thus, increases the conduction angle of the thyristor beyond 180°.



Figure 38. The thyristor turns on when the source voltage becomes positive and it stays on

even when crosses zero and reverses polarity (becomes negative) because the inductor extends the conduction angle of the thyristor beyond 180°.

Phase angle (°)

Load

voltage

Phase angle (°)

Load current

Phase angle (°)0 90 180 270

Source

voltage 0 90 180 270

Load

(b) Waveforms of the circuit voltages and current

The thyristor turns on

0

0

0

(a) Thyristor connected between an ac voltage source and an inductive load. A pulse of

current ( ) flows through the gate at the beginning of every cycle of the ac power source voltage.

Phase angle (°)

Thyristor gate

current 0

becomes lower


becomes lower


0 90 180 270 0 90 180 270

0 90 180 270 0 90 180 270

0 90 180 270 0 90 180 270



Applications

Thyristors are used in numerous applications to achieve switching or control of large amounts of electrical power. Low-power thyristors are commonly used in home dimmers to control the intensity of lighting in each room of a house, as Figure 39 shows.

Figure 39. Home dimmer used to control the intensity of lighting in each room of a house.

Thyristors are an essential component of solid-state relays (SSRs). Solid-state relays are widely used to switch ac power in a variety of applications. For example, they can be used to turn the heating element on and off in furnaces and industrial ovens. They can also be used to switch capacitor banks in static var compensators (SVC).

Figure 40. Solid-state relays with a thyristor output are used for switching ac power in a variety of applications.



Thyristors are used to improve performance and energy savings in modern domestic appliances like washing machines and water heaters.

Figure 41. Thyristors are used in energy-efficient washing machines to limit the current drawn when starting the machine, improve speed control and braking, and limit mechanical shocks and noise.

Figure 42. Thyristors are used to closely and accurately regulate the temperature of water at the output of tankless water heating devices.



Thyristors are also an essential component of soft starters for three-phase induction motors. They are used to gradually increase the motor speed and obtain smooth and reliable acceleration of the motor and its load.

Figure 43. Three-phase soft starter for induction motors.

Figure 44. Soft starters are used to start and stop pump motors smoothly.

In high-power applications, thyristors are used for power generation and transmission. They are used in rectifiers to control the exciting field of ac generators in large electrical power plants. They are also used as converters in HVDC (high-voltage, direct-current) power transmission systems. Figure 36, for example, shows high-voltage thyristor valves used for long distance transmission of power from a hydroelectric power plant. These valves consist of a multitude of series-connected light-triggered thyristors that are stacked vertically and cooled by means of deionized water.

Exercise 3 – The Power Thyristor Procedure Outline


The Procedure is divided into the following sections:

Set up and connections




High voltages are present in this laboratory exercise. Do not make or modify any

banana jack connections with the power on unless otherwise specified.

Set up and connections

In this part of the exercise, you will set up and connect the equipment.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the exercise.

Install the equipment in the Workstation.

2. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply.

Connect the Low Power Input of the Power Thyristors module to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on.

3. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer.

Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer.

4. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position, then connect the Power Supply to a three-phase ac power outlet.

Make sure that the main power switch of the Four-Quadrant Dynamometer/Power Supply is set to O (off), then connect its Power Input to an ac power outlet.

Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Power Supply. This connects the internal power supply of the module to the Power Supply terminals on the front panel.

Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on).

PROCEDURE OUTLINE

PROCEDURE

Exercise 3 – The Power Thyristor Procedure


5. Turn the host computer on, then start the LVDAC-EMS software.

In the LVDAM EMS Start-Up window, make sure that the Data Acquisition

and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Thyristor Bridge Control functions for the Data Acquisition and Control Interface are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then

click the OK button to close the LVDAM EMS Start-Up window.


In this part of the exercise, you will study the operation of a thyristor in a dc circuit. You will determine the conditions to be met for the thyristor to turn on.

6. Set up the circuit shown in Figure 45. In this circuit, is a positive dc voltage source implemented with the Four-Quadrant Dynamometer/Power Supply. E1, E2, E3, and I1 are voltage and current inputs of the Data Acquisition and Control Interface (DACI). The thyristor is one of the thyristors in the Power

Thyristors module. Load resistor is implemented with the Resistive Load module.

Figure 45. DC circuit with a thyristor in reverse bias condition.

7. Connect the Digital Outputs of the Data Acquisition and Control Interface to the Firing Control Inputs of the Power Thyristors module using the provided cable with DB9 connectors. This connection provides the signals that control the thyristors in the Power Thyristors module. The firing control signals are provided by the Data Acquisition and Control Interface and controlled in the Thyristor Control window of the LVDAC-EMS software.

Also, perform the following connections to be able to observe the control

signal applied to thyristor : connect Firing Control Input 1 of the Power Thyristors module to Analog Input 1 of the DACI, using a 2 mm lead; connect the common (white) terminal of the Firing Control Inputs on the Power

400

Power Thyristors module

Firing control signals from the digital outputs of the DACI



Thyristors module to one of the two analog common (white) terminals on the DACI using a 2 mm lead.

a The signals applied to the Firing Control Inputs of the Power Thyristors module control the corresponding power thyristors. For instance, the signal applied to Firing Control Input 1 of the Power Thyristors module controls thyristor , the signal applied to Firing Control Input 2 controls thyristor , and so on.

8. In LVDAC-EMS, open the Thyristor Control window, and make the following settings:

Set the Function parameter to Thyristor Single-Phase Half-Wave Rectifier. This setting is required to allow the control of the power thyristor.

Set the parameter to Off. This disables the firing control signal of thyristor . Therefore, the current flowing from the gate to the

cathode (current ) of the thyristor is null (zero).

Leave all other parameters set to their default values.

Start the Thyristor Single-Phase Half-Wave Rectifier function by clicking the Start/Stop button or by setting the Status parameter to Started.

9. In LVDAC-EMS, open the Metering window. Make sure that meters E1, E2, E3, and I1 are enabled. Set these meters to display dc (direct current) values.

Disable meter E4. Select the Continuous Refresh mode by clicking the Continuous Refresh button.

a When any one of the functions in the Thyristor Control window is in use,

meter E4 in the Metering window must be disabled for the Continuous Refresh

mode to be operational. This is because input E4 of the Data Acquisition and

Control Interface is dedicated to the thyristor control function for the

synchronization of the thyristor firing signals.

10. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings:

Set the Function parameter to Voltage Source (+). This setting makes the internal power source operate as a positive voltage source.

Set the Voltage parameter to 100 V by entering 100 in the field next to this parameter or by using the control knob in the lower left corner of the window. This sets the Voltage Source (+) to 100 V.

Start the Voltage Source (+) function by clicking the Start/Stop button or by setting the Status parameter to Started.



11. According to the voltages and the current indicated by the meters in the

Metering window, is the thyristor on or, in other words, does a current ( ) flow from the anode to the cathode of the thyristor? Briefly explain why.

12. Start the Oscilloscope and display the firing signal of thyristor [Analog Input 1 (AI-1) of the DACI]. Notice that this signal is at low level (0 V) because there is no current flowing through the gate of thyristor .

a The firing signal is at low level when its voltage is approximately 0 V. It is at high level when its voltage is 4 V approximately.

13. In the Thyristor Control window, set the parameter to On while observing the firing signal of thyristor on the Oscilloscope. Notice that the firing signal passes from 0 V to about 4 V, thereby allowing a current whose value is sufficient to initiate conduction to flow from the gate to the cathode

(current ) of the thyristor. According to the meters, is the thyristor on? Why?

14. In the Thyristor Control window, set the parameter to Off.

In the Four-Quadrant Dynamometer/Power Supply window, stop the Voltage Source (+) function by clicking the Start/Stop button or by setting the Status parameter to Stopped.

Reverse the connections at the thyristor terminals as shown in Figure 46. This reverses the thyristor orientation in your circuit, and thus reverses the polarity of the voltage applied to thyristor.



Figure 46. DC circuit with a thyristor in forward bias condition.

15. In the Four-Quadrant Dynamometer/Power Supply window, start the Voltage Source (+) function by clicking the Start/Stop button or by setting the Status parameter to Started.

According to the voltages and the current indicated by the meters, is the thyristor on? Why?

16. In the Thyristor Control window, set the parameter to On while observing the Oscilloscope and the meters. According to the voltages and the current indicated by the meters, is the thyristor on or, in other words, does a

current ( ) flow from the anode to the cathode of the thyristor? Briefly explain.

400





In the Thyristor Control window, set the parameter to Off to disable the firing control signal of thyristor . Does the state (on or off) of the thyristor change? Explain.

In the Thyristor Control window, leave parameter set to Off. By using the Voltage control knob in the Four-Quadrant Dynamometer/Power Supply window, slowly decrease the dc source output voltage in order to

progressively decrease current (indicated by meter I1 in the Metering window). Observe that when current reaches some minimum value, it suddenly drops to zero while the voltage across the thyristor decreases to a low value (E2 falls to about 1 V). Explain why.

17. Determine the value of the holding current by performing the steps below.

In the Four-Quadrant Dynamometer/Power Supply window, set the Voltage parameter to 100 V.

In the Thyristor Control window, set the parameter to On to

enable the firing control signal of thyristor . Observe that thyristor is on.

Set the parameter to Off to disable the firing control signal of

thyristor . Observe that thyristor remains on because current is higher than the holding current .

Slowly decrease the value of current (displayed by meter I1) by progressively decreasing the dc source output voltage and determine

the value of current just before it suddenly drops to zero. (Use the buttons of the Voltage control knob to decrease the voltage very slowly, when necessary). Record your result below.

Holding current mA

18. In the Thyristor Control window, stop the Thyristor Single-Phase Half-Wave Rectifier function by clicking the Start/Stop button or by setting the Status parameter to Stopped.

In the Four-Quadrant Dynamometer/Power Supply window, stop the Voltage Source (+) function.



Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Dynamometer. This disconnects the internal power supply of the module from the Power Supply terminals on the front panel. Close the Four-Quadrant Dynamometer/Power Supply window and the Metering window.

Turn the Four-Quadrant Dynamometer/Power Supply off.

a The Four-Quadrant Dynamometer/Power Supply will not be used for the rest of this exercise.


In this part of the exercise, you will study the operation of a thyristor in an ac circuit with a resistive load. You will observe the waveforms of the circuit voltages and current using an oscilloscope. You will determine the conduction angle of the thyristor.

19. Set up the circuit shown in Figure 47. In this circuit, is a single-phase ac power source obtained by using the line 1 (L1) and neutral (N) terminals of the three-phase ac power source in the Power Supply (Model 8823). E1, E2, E4, and I1 are voltage and current inputs of the DACI. The thyristor ( )

is one of the thyristors in the Power Thyristors module. Load resistor is implemented with the Resistive Load module. The resistance value to be used for this resistor depends on your local ac power network voltage (see table in the diagram).

a Input E4 of the DACI is used for synchronization of the firing signal of

thyristor . This input must be connected across the ac voltage source

terminals, as Figure 47 shows. Input E4 cannot be used to measure or

observe the ac source voltage. This explains why another voltage input (E1) is

connected in parallel with input E4.

20. On the Power Supply, turn the three-phase ac power source on by setting the corresponding switch to I (on).

21. In the Thyristor Control window of LVDAC-EMS, make the following settings:

Set the Function parameter to Thyristor Single-Phase Half-Wave Rectifier. This setting is required to allow the control of power

thyristor .

Set the parameter to Active. This makes the firing signal of

thyristor depend on the Firing Angle Control and Firing Angle parameters.

Make sure that the Firing Angle Control parameter is set to Knob. This allows the firing angle to be controlled manually.



Make sure that the Firing Angle parameter is set to 0°. This sets the

firing angle of thyristor to 0°.

Leave all other parameters set to their default values.

Start the Thyristor Single-Phase Half-Wave Rectifier function by clicking the Start/Stop button or by setting the Status parameter to Started.

Local ac power network

( ) Voltage

(V)

Frequency

(Hz)

120 60 1200

220 50 2200

240 50 2400

220 60 2200

Figure 47. Thyristor operation in an ac circuit (resistive load).

22. On the Oscilloscope, display the source voltage (E1), the firing signal of

thyristor [Analog Input 1 (AI-1) of the DACI], the load current (I1), and the load voltage (E2) on channels 1, 2, 3, and 4, respectively. Set the time base to display at least one cycle of the source voltage waveform. In the Oscilloscope Settings panel, enable the acquisition filtering function.





Observe the relationship between the waveforms of the firing signal, load

current and load voltage , and the waveform of the source voltage . Notice that each time the source voltage crosses zero and becomes positive, the firing signal momentarily goes from low to high level (i.e., from 0 V to about 4 V). Does the thyristor enter into conduction at this instant? Explain.

Notice that the waveforms of the load current and voltage have the same shape as the source voltage waveform during most of the positive half of each cycle. Why do the load voltage and current continue to follow the source voltage waveform after the firing signal returns to the low level (0 V)?

Do the waveforms of the load current and voltage return to zero a little before the end of the positive half of each cycle of the source voltage waveform, i.e., a little before the source voltage crosses zero and becomes negative? Why?

During the negative half of the source voltage, are the load current and voltage null? Why?



23. Evaluate the holding current and record your result below.

Holding current mA

24. Evaluate the angle of conduction of the thyristor. Is this angle a little less than 180°?

25. Is the thyristor circuit of Figure 47 similar to the diode single-phase half-wave rectifier studied in Exercise 1 of this manual? Explain briefly.

26. On the Power Supply, turn the three-phase ac power source off by setting the corresponding switch to O (off).

In the Thyristor Control window, stop the Thyristor Single-Phase Half-Wave Rectifier function by clicking the Start/Stop button or by setting the Status parameter to Stopped.


In this part of the exercise, you will study the operation of a thyristor in an ac circuit with a resistive-inductive load. You will observe the waveforms of the circuit voltages and current using an oscilloscope. You will determine the conduction angle of the thyristor. You will compare your results to those previously obtained with a purely resistive load.

27. Connect a load inductor in series with the load resistor as shown in Figure 48. Load inductor is implemented by using the inductors in the Filtering Inductors/Capacitors module.



Local ac power network

( )

(mH) Voltage

(V)

Frequency

(Hz)

120 60 57 50

220 50 210 66

240 50 229 66

220 60 210 66

Figure 48. Thyristor operation in an ac circuit (resistive-inductive load).

28. On the Power Supply, turn the three-phase ac power source on by setting the corresponding switch to I (on).

29. In the Thyristor Control window of LVDAC-EMS, start the Thyristor Single-Phase Half-Wave Rectifier function by clicking the Start/Stop button or by setting the Status parameter to Started.

30. Observe the source voltage (E1), the firing signal of thyristor (Analog Input 1 (AI-1) of the DACI), the load current (I1), and the load voltage (E2) on channels 1, 2, 3, and 4, respectively, using the Oscilloscope. Set the time base to display at least one cycle of the source voltage waveform.





Observe the relationship between the waveforms of the firing signal, load

current , and load voltage , and the waveform of the source voltage . Does the thyristor enter into conduction each time the source voltage crosses zero and becomes positive? Explain.

Is the waveform of the load current identical to the waveform of the source voltage waveform? Why?

When the source voltage crosses zero and becomes negative, does the thyristor continue to conduct current for some period of time even if it is reverse biased? Why?

Why does the thyristor stop conducting current (i.e., turns off) a certain time after the source voltage crosses zero and becomes negative? Explain.

Exercise 3 – The Power Thyristor Conclusion


Describe the waveform of the load voltage with respect to the source voltage waveform.

Once they have returned to zero, do the load current and voltage stay null for the rest of the negative half of the source voltage waveform? Why?

31. Evaluate the angle of conduction of the thyristor. Is this angle greater than that obtained in the previous section of this exercise for a purely resistive load? Why?

32. On the Power Supply, turn the three-phase ac power source off by setting the corresponding switch to O (off). Close LVDAC-EMS. Disconnect all leads and return them to their storage location.

In this exercise, you studied the operation of thyristors. You learned that thyristors act like high-speed switches, allowing current to flow in one direction only. They have a control terminal called a gate, which is used to initiate the conduction. You learned that thyristors can have a power gain as high as 1 000 000, which implies that a gate signal of 1 watt can be used to switch power of up to 1 megawatt. You studied the operation of thyristors in ac circuits with resistive and resistive-inductive loads. The firing signal was a pulse applied to the thyristor gate at the beginning of every cycle of the source voltage. You learned that when the load is purely resistive, the load voltage and current waveforms consist of a pulse of positive polarity every cycle of the source voltage waveform. The conduction angle of the thyristor is a little less than 180°. You learned that the addition of an inductor to the load increases the conduction angle beyond 180°, because the energy stored in the inductor keeps the thyristor current higher than the holding current for a certain time interval after the thyristor becomes reverse biased.

CONCLUSION

Exercise 3 – The Power Thyristor Review Questions


1. Briefly describe the construction of a thyristor and explain the purpose of each thyristor terminal.

2. State the two requirements to meet in order to turn a thyristor on.

3. Once a thyristor has entered into conduction, does removing the gate current interrupt the conduction? When does the thyristor stop conducting?

4. Is the gate current required to initiate the conduction very small compared to the main current that can flow through a thyristor? Give an example of the magnitude of current that thyristors can conduct in high-power industrial applications.

REVIEW QUESTIONS

Exercise 3 – The Power Thyristor Review Questions


5. Consider a simple circuit consisting of a thyristor connected between an ac voltage source and a resistive load. A pulse of current having a value sufficient to trigger conduction flows from the gate to the cathode of the thyristor whenever the phase angle is 0°. Briefly explain how the circuit operates and describe the waveforms of the load current and voltage with respect to the source voltage waveform.

Thyristor Power Electronics, 3 The Power Thyristor · Thyristor operation in an ac circuit...

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