Experiment 2-8

Single-Phase Full-Controlled AC Voltage Controller

OBJECTIVE

1. To understand the differences between the single-phase full-controlled ac voltage controller and the single-phase semi-controlled ac voltage controller.

2. To understand the characteristics of single-phase full-controlled ac voltage controller.

3. To measure the voltage and current of single-phase full-controlled ac voltage controller.

4. To verify characteristics of single-phase full-controlled ac voltage controller.

DISCUSSION

Compared with the single-phase semi-controlled (or halt-wave) ac voltage controller, both the positive and negative cycles of output voltage waveform of the single-phase full controlled (or full-wave) ac voltage controller are symmetrical and variable. To achieve these function two SCRs (inverse parallel operation) are employed in the single-phase semi-controlled ac voltage controller.

Figure 2-8-1 shows the circuit and waveforms of single-phase semi-controlled (full- wave) ac voltage controller with purely resistive load. During the positive half cycle of Vi, Q2 is reverse-biased and tuned off, Q1 is triggered and conducts at. t=. , in the interval. ≤. t≤. , Vi is connected to the load through Q1, and the output voltage is variable; during the negative cycle of Vi,Q1 is reverse-biased and turned off, Q2 is triggered to conduct at . t=. +. , in the interval . +. ≤. t≤2. , Vi is connected to load through Q2, and the output voltage is also variable. Triggering angle. can be varied from 0° to 180°, the corresponding voltage waveform is shown in Figure 2-8-1 (b). The triggering pulses of thyristors Q1 and Q2 must be isolated from each other, usually two pulse transformers are used to couple the triggering signals to the gates; otherwise, short circuit may occur. Since the pulse width of gate triggering signal is usually 20μs approximately, the required pulse transformer is quite small in size.

Figure 2-8-1 Circuit and waveforms of single-phase semi-controlled ac voltage controller with purely resistive load

In the single-phase semi-controlled ac voltage controller with a purely resistive load, input current li and output current lo are equal. As can be seen from Figure 2-8-1 (b), The input current of single-phase semi-controlled ac voltage controller is alternating symmetrical and has no dc component.

If V_i(rms) is the rms input voltage, by varying the SCR triggering angle . from 0° to 180°, the rms output voltage v . _(rms) of single-phase semi-controlled ac voltage controller with purely resistive load can be varied from 0 to V_i(rms) . When . =0°, the output voltage waveform is identical to that of a single-phase full-wave diode rectifier, therefore V_0°(rms) = V_i(rms) . When . = 180°, there is no output waveform; that is, V_180°(rms)=0V. In the same way, the rms input current I . _(rms) can be varied from 0 to I_i(rms), where I_i(rms) = V_i(rms) /R. The power delivered to load P. will vary from 0 to P_i and P_i = V_i(rms) x I_i(rms).

If V . _(rms) is the rms value of ac output voltage of single-phase full-controlled ac voltage controller at firing angle . ,
is the peak value of ac input voltage, thus

(2-8-2)

If I . _(rms) is the rms value of ac current (this value is equal to the rms values of li and lo) of single-phase full-controlled ac voltage controller at firing angle . , I_{Q1(. )} and I_{Q2(. )} are the rms value of ac currents flowing through SCR Q1 and SRC Q2, respectively. Since positive and negative half cycles are symmetric, therefore I Q1(. ) are equal. Thus

(2-8-2)

Figure 2-8-2 shows he circuit and waveforms of single-phase full-controlled ac voltage controller with inductive to load. During the positive cycle of Vi, Q1 is fired to conduct at . t=. , Vi is applied to the load circuit through Q1 during the interval . ≤. t≤. . During the negative half cycle of Vi, due to the inductive load, Q1 continues to conduct until it reaches the angle . ; Q2 conducts at . t=. +. , in the . +. ≤. t≤2. interval, Vi is connected to load through Q2. During the next positive half cycle of Vi, Q2 continues to conduct until the angle 2. +. is reached.

Assume that the conduction angle of thyristor is. , impedance angle of load is ϕ, thus . =. -. ,
. As can be seen from the waveforms in Figure 2-8-2(b), the conduction angle . cannot be larger than 180° and the triggering angle . cannot be less than ϕ; Otherwise, only one of the two thyristors can conduct in a complete cycle. For example, if Q1 is now conducting, gating signal I_G2 cannot make Q2 conduct, therefore input current and load current are asymmertic and has dc component. There are two solutions: one is to extend the width of triggering pulse (continuous pulse) but a bigger pulse transformer is requited; the other is the use of high-frequency carrier gating signal (of pulse train). The second solution is more popular.

Figure 2-8-2 Circuit and waveforms of the single-phase full-controlled ac voltage controller with inductive load

EQUIPMENT REQUIRED

1. PE-5340-3A Isolating Transformer x1

2. PE-5310-5B Fuse set x1

3. PE-5310-5C Thyristor Set x1

4. PE-5310-3A R.M.S Meter x1

5. PE-5310-2B Differential Amplifier x1

6. PE-5310-3C Resistor Load Unit x1

7. PE-5310-3E Inductive Load Unit x1

8. PE-5310-2C Current Transducer x1

9. PE-5310-1A DC Power Supply x1

10. PE-5310-2A Reference Variable Generator x1

11. PE-5310-2D 3ϕ Phase Angle Controller x1

12. Digital Storage Oscilloscope (DSO) x1

13. Connecting Wires and Bridging Plugs

PROCEDURE

1. Put modules PE05310-1A, PE-5310-2A, PE-5310-2D, and PE-5310-5C in Experimental Frame. Place DSO, PE-5310-3C and PE-5340-3A modules on workbench. Complete the connections of Figure 2-8-3 using bridging plugs (curved lines) and connecting wires.

2. This ac voltage controller operates from a single-phase 220V and the load circuit is a 200. resistor connected in series with a 200 mH inductor. On Reference Variable Generator module, set the Vc Range selector switch to 0~+10V position, and set the V control knob to its 0% position. On 3ϕ Phase Angle Controller module, select Single Pulse output (down position), set . _min = 0° and . _max=180°. The triggering angle can be varied from 0° to 180° by turning the V control knob of Reference Variable Generator.

Figure 2-8-3 Wiring diagram for single-phase full-controlled ac voltage controller experiment

3. Short out the load inductor by placing a connecting wire directly across the inductor terminals . This makes a purely resistive load circuit. Turn the V control knob of Reference Variable Generator to read triggering angle . =60° from the 7-segment display on 3ϕ Phase Angle Controller module. Set the V Range selector switches of Differential Amplifiers Ch. A and Ch.C to 500V. Use DSO to measure the ac input voltage (CH1) and the load voltage (CH2) waveforms of ac voltage controller as shown in Figure 2-8-4. Practice a variety of triggering angles . and observe the changes in load voltage waveform.

Figure 2-8-4 Measured input voltage (CH1) and load voltage (CH2) waveforms of dingle-phase full-controlled ac voltage controller with purely resistive load

4. Place the AC+DC/AC and RMS/AV selector switches of RMS meter in AC+DC and RMS positions, respectively. Adjust the V control knob of Reference Variable Generator to read triggering angle . =90° from the 7-segment display on 3ϕ Phase Angle Controller module. Measure the rms load current (equal to input current) value I. _(rms) = _____A. Using the RMS Meter, measure the rms value of SCR Q1 current I_Q1(.) = _____A., and the rms value of SCR Q2 current I_{Q2(. )}= _____A. Does the result meet Eq. (2-8-2)? _____

5. Remain the settings of RMS Meter in Step 4 uncharged. Set each of SCR triggering angles . indicated in Table 2-8-1, measure and record the corresponding rms voltage of the ac voltage controller.

Table 2-8-1 Measured V. Values of single-phase full-controlled ac voltage controller with purely resistive load.

6. Using recorded values of V ._(rms) in Table 2-8-1, calculate and record the ratio of

V ._(rms)/V_i(rms) for each value of . , where V_i(rms) is the V ._(rms) at . =0°.

Plot the V ._(rms)/V_i(rms) versus. curve in Figure 2-8-5.

Figure 2-8-5 V ._(rms) / V_i(rms) vs. . curve of single-phase full-controlled ac voltage controller with purely resistive load

7. When the triggering angle . =90°, use Eq. (2-8-1) to calculate V_90°(rms)=_____ V_i(rms) and V_90°(rms)/ V_i(rms) =_____. Are the calculated values equal to the recorded values in table 2-8-1? _____

8. Recover the load inductor by removing the connecting wire from inductor terminals. This modifies the purely resistive load to an inductive load. Repeat step 3 to measure the ac input voltage (CH1) and the load voltage (CH2) wave forms of ac voltage controller when . =60° and record the results in Figure 2-8-6.

Note: With inductive load, Single Pulse trigging is unsuitable when . <ϕ. Select Pulse Train output (up position) from 3ϕ Phase Angle Controller module.

CH1:

DIF AMP V Range:___

VOLTS/DIV:___

SEC/DIV: ___

CH2:

DIF AMP V Range:___

VOLTS/DIV:___

SEC/DIV:___

Figure 2-8-6 Measured input voltage (CH1) and load voltage (CH2) waveforms of

single-phase full-controlled ac voltage controller with inductive load .

9. Remain the load circuit and settings of triggering angles in Step 8 uncharged. Modify the connections of Figure 2-8-3 to measure the ac input voltage (CH1) and the load current (CH2) via Current Transducer module, and record the results in Figure 2-8-7.

CH1:

DIF AMP V Range:___

VOLTS/DIV:___

SEC/DIV: ___

CH2:

DIF AMP V Range:___

Current Transducer

I Range: ___

VOLTS/DIV:___

SEC/DIV:___

Figure 2-8-7 Measured input voltage (CH1) and load current (CH2) waveforms of

single-phase full-controlled ac voltage controller with inductive load .

COMPUTER SIMULATION

1. Run TINAPro. In Schematic Editor window, complete the simulation circuit as shown in Figure 2-8-8. Set input voltage source Vi as Sine wave, Amplitude to 311V, Frequency to 60Hz.

Figure 2-7-8 Simulation circuit of single-phase full-controlled ac voltage controller

2. In Signal Editor dialog, select General wave and create the triggering pulses V_G as reffering to Figure 2-4-9. Set Amplitude to 10V (A1=10V, A2=0V), Period to 16.67ms (T2=1ms, T6=15.67ms). set the triggering angle of SCR Q1 to 60° (TS=2.78ms), the triggering angle of SCR Q2 to 240° (TS=11.1ms). Once completed, click OK.

3. Modyfi the load to a purely resistive circuit by setting inductance L to 0H. Run Analysis/Transient. In Transient Analysis dialog, set Stop display to 33.3ms and check the Draw excitation box. Once completed, click OK. The TR result is shown in Figure 2-8-9. Is there agreement between the TR result and your measured result of Figure

2-8-4? _________

Figure 2-8-9 TR result of single-phase full-controlled ac voltage controller with purely resistive load

4. Modify the inductance L to 200mH to construct an inductive load . Repeat Step 3 to run Analysis/Transient and obtain the TR result as shown in Figure 2-8-10. Is there good agreement between the TR result and your measured results of Figures 2-8-6 and 2-8-7? ________

Figure 2-7-10 TR result of single-phase full-controlled ac voltage controller with inductive load

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