Thyristor Three-Phase, Six-Pulse Converter

OBJECTIVE

1. To study the operation of the three-phase, six-pulse rectifier/inverter.
2. To plot the static transfer function of the three-phase, six-pulse rectifier/inverter, and compare this with the theoretical curve.

DISCUSSION

The three-phase, six-pulse thyristor converter, or rectifier inverter, shown in Figure 1, is used in power electronics. This type of circuit gives the highest and most regular output voltage with the least amount of ripple. It can function as a rectifier or, when connected to a correctly polarized dc source, as an inverter.

This is the static transfer function of a three-phase, six-pulse converter, and is valid only when the on-time of the thyristors is equal to 120°. That is, when the series inductor is large enough to ensure continuous conduction.

Figure 1: Three-phase, six-pulse converter with resistive load.

Figure 2: Waveforms for the three-phase, six-pulse converter.

With respect to the three-phase restifier/inverter, the six-pulse circuit has the following differences:

1. the average value of E_D is twice as great
2. the transfer of active power is two times greater with the same value of current
3. the ripple frequency of E₀ is 360 Hz instead of 180 Hz
4. the average value of current I_A , I_B , I_C is zero.

This last difference is important because it prevents saturation of the transformers supplying the converter. According to Kirchhoff’s current law, I_A = I₁ - I₄. We can therefore draw the curve of I_A as shown in Figure 2. I_A changes polarity each half cycle. being equal to I₁ than -I₄ , and then repeating again. I_A flows for 240° or two-third of the cycle.

Three-phase bridge using three thyristors and three diodes

A three-phase bridge can be made using three thyristors and three diodes. Figure 3 shows an example of such circuit.

Figure 3: Three-phase bridge using three common-cathode thyristors.

The free-wheeling diode D₄ is necessary to ensure that the circuit be able to turn off an inductive load. Without this diode, when the gate pulses are stopped, the current may never drop to zero and one thyristor may continue to conduct.The freewheeling diode also relives the thyristors from freewheeling duty, allowing the use of lower power thyristors.

One advantage of the circuit in Figure 3 is that the firing control circuit can be simplified since the cathode of the three thyristors are at a common potential. This circuit is of lower cost than a three-phase six-thyristor bridge of comparable power, and both allow control of power from 0 to 100%. Unlike the six-thyristor bridge, however, this bridge cannot be used to make a line-commutated inverter.

Procedure summary

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

In the second part of this exercise, you will operate the three-phase, six pulse converter in both the rectifier and the inverter modes. You will plot the static transfer function of the converter, and compare it to the theoretical curve.

next Thyristor Three-Phase, Six-Pulse Converter (PROCEDURE)

Thyristor Single-Phase Bridge Rectifier/Inverter (PROCEDURE)

Setting up the equipment

(1) Install the Power Supply, the Enclosure / Power Supply, the DC Motor / Generator, the Four-Pole Squirrel-Cage Induction Motor, the Resistive Load, the Smoothing inductors, the DC Voltmeter/Ammeter, the AC Ammeter, the Three-Phase Wattmeter/Varmeter, the Temdem Rheostates, the Power Thyristors, and the Power Diodes modules in the Mobile Workstation.

Note: Align the brushes of the DC Motor / Generator in the neutral position by centering the metal tab on the red mark (on casing).

(2) Install the Thyristor Firing Unit and and the Current/Voltage Isolators in the Enclosure / Power Supply.

Note: Before installing the Thyristor Firing Unit, make sure that switches SW1 and SW2 (located on the printed circuit board) are in the 0 position.

(3) Make sure that the main power switch of the Power Supply is set to the 0 (OFF) position. Set the voltage control knob to 0. Connect the Power Supply to a three-phase wall receptacle.

(4) Plug the Enclosure / Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the 1 (ON) position.

(5) On the Power Supply, set the 24-V ac power switch to the 1 (ON) position.

(6) Make sure that the toggle switches on the Power Thyristors and the Resistive Load modules are all set to the 0 (open) position.

Controlled bridge supplying a passive load

(7) Set up the circuit of Figure 12 using the resistive load Z (a). To simplify connecting the thyristors, set both interconnection switches on the Power Thyristors module to the 1 position.

Figure 12: Thyristor bridge circuit.

LINE VOLTAGE (Vac)	I1ac (A)	I2dc (A)	I1 (A)	E1dc (V)	e1 (V)	Z1(a)	Z1(a)
120	2.5	2.5	10	150	300	R=60Ω	R=60Ω, L=0.2H (3Adc max)
220	1.5	1.5	5	300	600	R=220Ω	R=220Ω, L=0.8H (1.5Adc max)
240	1.5	1.5	5	300	600	R=240Ω	R=240Ω, L=0.8H (1.5Adc max)

(8) Make the following setting:

On the Power Supply
    Voltage Selector . . . . . . . . . . . . . . . . . . . . . . . 4-N
On the Thyristor Firing Unit
    ANGLE CONTROL COMPLEMENT . . . . . . . . . . . . . . .  0
    ANGLE CONTROL ARC COSINE . . . . . . . . . . . . . . . .  0
    FIRING CONTROL MODE . . . . . . . . . . . . . . . . . . . . 1~
    DC SOURCE . . . . . . . . . . . . . . . . . . . . . . . . . . . MIN.
On the Oscilloscope
    Channel-1 Sensitivity . . . . . . . .  5 V/DIV. (DC coupled)
    Channel-2 Sensitivity . . . . . . . .  2 V/DIV. (DC coupled)
    Time Base . . . . . . .  . . . . . . . . . . . . . . . .  5 ms/DIV.
    Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  LINE

Figure 13: Voltage and current waveforms (α = 45°).

(9) On the power Supply, make sure that the voltage control knob is set to the 0 position then set the main power switch to 1 (ON). Set the voltage control knob so that voltage indicated by the Power Supply voltmeter is equal to 90 % of the nominal line-to-neutral voltage.

On the Thyristor Firing Unit, set the FIRING ANGLE to 45°. Sketch the voltage and current waveforms in Figure 13.

Fill in the first row of Table 1.

LOAD Z1	OUTPUT VOLTAGE E1 dc	OUTPUT CURRENT I1 dc	OUTPUT POWER P0 = E1 × I1	CONDUCTION ANGLE
	V	A	W	degrees
(a) Resistive
(b) Inductive

Table 1: Measurements for controlled bridge (α = 45°).

On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 (OFF).

(10) Change the load in the circuit to the inductive load Z₁ (b).

On the Power Supply, set the main power switch to 1 (ON). Set the voltage control knob so that the voltage indicated by the Power Supply voltmeter is equal to 90 % of the nominal line-to-neutral voltage. Sketch the voltage and current waveforms in figure 13.

Fill in the second row of Table 1.
On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 (OFF).
(11) Add a free-wheeling diode to the circuit, as shown in Figure 14. On the Power Supply, set the main power switch to 1 (ON), and set the voltage control knob to 90(%)

Figure 14: Thyristor bridge with free-wheeling diode.

LOAD Z1	OUTPUT VOLTAGE E1 dc	OUTPUT CURRENT I1 dc	OUTPUT POWER P0 = E1 × I1
	V	A	W
(b) Inductive

Table 2: Measurements for controlled bridge with free-wheeling diode.

Single-phase bridge with two thyristors and two diodes 

(12) Set up the circuit of Figure 15.

Figure 15: Bridge rectifier wit two thyristors on common ac line.

LINE VOLTAGE (Vac)	I1ac (A)	I2dc (A)	I1 (A)	E1dc (V)	e1 (V)	Z1(a)
120	2.5	2.5	10	150	300	R=60Ω, L=0.2H (3Adc max)
220	1.5	1.5	5	300	600	R=220Ω, L=0.8H (1.5Adc max)
240	1.5	1.5	5	300	600	R=240Ω, L=0.8H (1.5Adc max)

On the Power Supply; set the main power switch to 1 (ON), and set the voltage control knob so that the voltage indicated by the Power Supply voltmeter is equal to 90 % of the nominal line-to-neutral voltage. Vary the firing angle and observe the waveforms. Then set the firing angle to 45° and fill in the first row of Table 3.

CONFIGURATION	OUTPUT VOLTAGE E1 dc	OUTPUT CURRENT I1 dc	OUTPUT POWER P0 = E1 × I1
	V	A	W
Two thyristors on common ac line
Common-cathode thyristors

Table 3: Measurements for bridge rectifiers with two thyristors and two diodes (α = 45°).

Compare the voltage waveform at the output of this bridge to those obtained with the thyristor bridges of Figure 12 and 14.

On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 (OFF).

(13) Set up the circuit of Figure 16.

Figure 16: Bridge rectifier with common-cathode thyristors and free-wheeling diode.

On the Power Supply, set the main power switch to 1 (ON), and set voltage control knob so that the voltage indicated by the Power Supply voltmeter is equal to 90 % of the nominal line-to-neutral voltage. Vary firing angle and observe the waveforms. Then set the firing angle to 45° and fill in the second row to Table 3.

Compare the voltage waveform at the output of this bridge to those obtained with the bridge of Figure 15.

(14) On the Power Thyristors, connect the FIRING CONTROL INPUTS DISABLE jack to + 5 V jack on the Enclosure / Power Supply. This disables the gate pulses to all of the thyristors, and shuts off current to the load. What happens? . . . . . .

(15) On the Power Thyristors, disconnect the FIRING CONTROL INPUTS DISABLE jack from the + 5 V jack on the ENCLOSURE / Power Supply.

On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 (OFF).

Remove the free-wheeling diode from the circuit. Then, on the Power Supply, set the main power switch to 1 (ON), and  set the voltage control knob to 90(%).

Could this bridge operate without the free-whiling diode? Explain. . . . . . . . . . . . .

ON the Power Thyristors, connect the FIRING CONTROL INPUTS DISABLE jack to + 5 V jack on the Enclosure / Power Supply. This disables the gate pulses to all of the thyristors. Disconnect then reconnect the plug at the FIRING CONTROL INPUTS DISABLE jack several times. Explain what you observe. . . . . . . . . . . .

On the Power Supply, set the voltage control to 0 then set the main power switch to 0 (OFF).

(16) Set up the circuit of Figure 17.

  Figure 17: Controlled bridge rectifier/Inverter circuit.

(17) On the Thyristor Firing Unit, set the DC SOURCE control to MAX.

On the Tandem Rheostats, set the control knob to the centre position. On the Power Supply, make sure that the voltage control knob is set to the 0 position, then set the main power switch to 1 (ON). The Four-Pole Squirrel-Cage Induction Motor should began to rotate.

On the Power Supply, set the voltage control knob so that the voltage indicated by the Power Supply voltmeter id equal to 90 % of the nominal line-to-neutral voltage.

Adjust the Tandem Rheostate to obtain the voltage E₁ shown in Table 4 at the generator terminals of the motor-generator set.

LINE VOLTAGE	ACTIVE LOAD VOLTAGE E1 dc
V ac	V
120	80
220	160
240	160

Table 4: Active load voltage E₁.

Vary the firing angle and observe the effect on the waveforms and on the current delivered to the active load. How does the current vary as the firing angle is reduced to 0°? . . . . . . . . .

(18) On the Thyristor Firing Unit, adjust the FIRING ANGLE to 0°. Adjust the Tandem Rheostates to obtain the current I₁ shown in Table 5.

LINE VOLTAGE	CURRENT I1 dc
V ac	A
120	1.0
220	0.5
240	0.5

Table 5: Current I₁ delivered to load.

For each FIRING ANGLE in Table 6, adjust the Thyristor Firing Unit to the given firing angle, then adjust the Tandem Rheostates to obtain the current I₁ shown in Table 5. Observe the waveforms on the oscilloscope. Calculate the theoretical output voltage, record the measured voltage (E₁dc), and calculate the power delivered to the reversible dc power supply.

FIRING ANGLE	THEORETICAL VOLTAGE E0 = 0.9 Escos	MEASURED VOLTAGE E1 dc	POWER P = E1 × I1
degrees	V	V	W
0
15
30
45
60
75
90
105
120
135
150
165

Table 6: Data for bridge rectifier/inverter circuit.

(19) Turn the knob on the Tandem Rheostates to the centre position, so the field current of the DC Motor / Generator is zero. On the Power Supply, set the voltage control knob to 0 then set the main power switchand the 24-V ac power switch to 0 (OFF).

For what range of firing angle does the bridge operates as a rectifier?

For what range does it operate as a inverter? Explain. . . . . . . . . . . .

(20) In Figure 18, plot the voltage E₁ versus the firing angle. Then, in the same figure, plot the theoretical relationship E₀ = 0.9 E_s cos , where E_s is the line voltage. Compare the two curves.

Figure 18: Static transfer function for a single-phase bridge.

(21) Set the rocker switch on the Enclosure / Power Supply to the 0 position. Remove all leads and cables.

CONCLUSION

In this exercise, you observed that a single-phase thyristor bridge can operate both as a controlled rectifier and as inverter. You saw that a bridge made with two thyristors and two diodes has same characteristics as a four-thyristor bridge with a free-wheeling diode, but is more economical to build.

REVIEW QUESTIONS

1. In what direction is active power transferred by a bridge operating in the rectifier mode? . . . . . . . . . .
2. Under what conditions can a thyristor bridge operate in the inverter mode? . . . . . . . .
3. In what direction is active power tranferred by a bridge operating in the inverter mode? . . . . . . . . . . . .
4. Which bridge configuration using two thyristors and two diodes requires the addition of a free-wheeling diode? Explain why.    . . . . . . . . . . . . . . .
5. What is the role of the inductor in the bridge rectifier/inverter circuit?    . . . . . . . . . . . . . . . .

CAUTION

High voltages are present in the laboratory exercise! Do not make or modify any banana jack connection with the power on unless otherwise specified!

previous Rectifier and inverter modes

Rectifier and inverter modes

The function of the rectifying circuits seen so far is to convert ac current to dc current. The process is shown in Figure 6. Since the ideal rectifies does not consume power, the active power supplied by the source must be absorbed by the load. Note that the term load is used in its most general sense. A load can even be a voltage source, as in the case of a battery charger circuit.

An inverter is a circuit which performs the opposite function of a rectifier; it converts dc current to ac current. Figure 7 shows a general inverter system.

The active power supplied by the source is consumed by the load, as the ideal inverter does not consume power.
Inverters fall into two categories:

1. the self-commutated or autonomous inverter in which the frequency of the output ac current is proportional to the triggering rate of the thyristors;
2. the line-commutated or non-autonomous inverter in which the frequency of the output ac current is imposed by the ac network to which the inverter is connected.

Figure 6: A general rectifier system.

Figure 7: A general inverter system.

The frequency of self-commutated inverters is fixed. You will use the line-commutated inverter in this exercise.

The single-phase thyristor bridge can be used both as a rectifier and as an inverter. The inductor is large enough to ensure continuous conduction over a wide range of firing angles. The power supplied to the battery is P₀ = I × E₀, where E₀ is the average dc voltage at the output of the bridge.

Figure 8: Single-phase bridge with active dc load.

The average dc voltage E₀ decreases as the firing angle α is increased, as shown in Figure 9 and 10. However, current flows only as long as voltage E₀ is more positive than the battery voltage E_B. If α is large, say 75°, voltage E₀ is very small and current will only flow if voltage E_B is near zero.

Figure 9: Waveform for single-phase bridge in rectifier mode (α = 0°).

Figure 10: Waveforms for single-phase bridge in rectifier mode (α = 30°)

If the firing angle α is greater than 90°, E_D becomes negative. In this case, current will only flow if the polarity of the battery voltage is reversed, so that E_D is still higher than E_B. This is illustrated in Figure 11. The power supplied to the battery is still P₀ = I × E₀. However, since the voltage E₀ is negative, this power is negative, showing that power is actually transferred from the the battery to the ac source. The bridge is now operating as a line-commutated inverter, converting dc power to ac. 

Figure 11: Waveform for single-phase bridge in inverter mode (α = 150°)

Procedure summary

In the first part of the exercise, you will set up the equipment.

In the second part, you will observe the operation f a controlled bridge supplying a passive load, with both a resistive and an inductive load.

In the third part, you will set up two different bridge configurations having two thyristors and two diodes.

In the fourth part, you will operate a thyristor bridge in both the rectifier and the inverter modes. The reversible dc power supply circuit (see Familiarization with the Reversible DC Power Supply) is used to simulate a battery.

previous Bridge rectifier with two thyristors and two diodes

next Thyristor Single-Phase Bridge Rectifier/Inverter (PROCEDURE)