Experiment 2-7

Single-Phase Semi-Controlled AC Voltage Controller

OBJECTIVE

1. To understand the differences between ac voltage controller and rectifier.

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

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

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

DISCUSSION

Both controlled rectifiers and ac voltage controllers introduce thyristors between a constant-voltage ac source and the load circuit. The former varies the average value of dc voltage applied to the load by varying the triggering angle of thyristors; whereas the latter varies the rms value of ac voltage applied to the load by varying the triggering angle of thyristors. In addition, the former is suited for dc loads, whereas the latter is suited for ac loads.

Same as the single-phase controlled rectifiers, single-phase ac voltage controllers can be divided into semi-controlled and full-controllers. If a half cycle of output voltage can be varied by varying the triggering angle of thyristor, but another half cycle cannot be controlled, this controller is called the single-phase semi-controlled (or half-wave) ac voltage controller. If both the positive and negative half cycles of the output voltage are controllable, the controller is called the single-phase full-controlled (or full-wave) ac voltage controller.

The single-phase semi-controlled ac voltage controlled ac voltage consists of an SCR and a power diode connected inverse parallel. Figure 2-7-1 illustrates the circuit and waveforms of single-phase semi-controlled ac voltage controller with a purely resistive load. During the positive half cycle of Vi, Q1 is turned on at. t=. And Vi is connected to load through Q1 during the interval. ≤. t≤. , so the output voltage is controllable. During the negative half cycle of Vi, Vi is connected to load through D2 and thus the output voltage is uncontrollable. By varying the triggering angle. between 0° and 180°, the corresponding output voltage waveform is shown in Figure 2-7-1 (b).

Figure 2-7-1 Circuit and waveforms of single-phase semi-controlled ac voltage controller with purely resistive load

The input current li and output current lo of the single-phase semi-controlled ac voltage controller with purely resistive load are equal. As can be seen from Figure 2-7-1(b), although the single-phase semi-controlled ac voltage controller features low cost, but its input current is asymmetric (dc component introduced into input current) and transformer core may be saturated, so it is impractical.

If V_i(rms) is the rms input voltage, by varying the 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 V_i(rms) to 0.707 V_i(rms). Since the output waveform is identical to that of a single-phase full-wave diode rectifier when .=0°, so V_0°(rms) = V_i(rms). When . =180°, the output voltage waveform is identical to that of a single-phase half-wave diode rectifier, therefore V_180°(rms) = 0.707 V_i(rms). In the like manner, the rms value of ac current I. _(rms) will vary from I_i(rms) to 0.707 I_i(rms), where I_i(rms) = V_i(rms) /R; the power delivered to the load P. will vary from P_i to 0.5 P_i, where P_i = V_i(rms) x I_i(rms) .

If I. _(rms) is the rms value of ac current of the single-phase semi-controlled ac voltage controller at the triggering angle . . I_{Q1(. )} and I_{D2(. )} are the values of ac currents flowing through the SCR Q1 and power diode D2, respectively. Thus

2-7-2

Figure 2-7-2 shows the circuit and waveforms of the single-phase semi-controlled ac voltage controller with inductive load. During the positive half cycle of Vi, Q1 is triggered to conduct at. t=. , Vi is connected to load though Q1 in the interval. ≤. t≤. . During the negative half cycle of Vi, Q1 continues to conduct caused by the inductive load until the extinction angle. is reached. At this time, D2 is forward-biased and conducts until the positive half cycle of Vi appears and. t=2. +. .

(a)

Figure 2-7-2 Circuit and waveforms of single-phase semi-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-5310-5A Power Diode Set x1

4. PE-5310-5C Thyristor Set x1

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

6. PE-5310-2B Differential Amplifier x1

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

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

9. PE-5310-2C Current Transducer x1

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

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

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

13. Digital Storage Oscilloscope (DSO) x1

14. Connecting Wires and Bridging Plugs

PROCEDURE

1. Put modules PE-5310-1A, PE-5310-2A, PE-5310-2D, PE-5310-5A, and PE-5310-5C in Experimental Frame. Place DSO, PE-5310-3C and PE-5340-3A modules on workbench. Complete the connections shown in Figure 2-7-3 using bridging plugs (curved lines) and connecting wires.

Figure 2-7-3 Wiring diagram for single-phase semi-controlled ac voltage controller experiment

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

3. Short out the load inductor by placing a connecting wire directly across the inductor terminals. This constructs a purely resistive load. Adjust the V control knob of Reference Variable Generator to read the triggering angle. =90° 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. Using DSO, measure the ac input voltage (CH1) and the load voltage (CH2) waveforms of the ac voltage controller as shown in Figure 2-7-4. Practice a variety of triggering angles and observe the changes in load voltage waveform.

Figure 2-7-4 Measured input voltage (CH1) and load (CH2) waveforms of single-phase semi- 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. Measure the rms load current (equal to the input current) at triggering angle .=90°, I. _(rms) = _______ A. Using the RMS Meter, measure the rms value of SCR Q1 current I_{Q1(. )} = ________ A, and the rms value of power diode D2 current I_{D2(. )} = _______ A. Do the results agree with Eq. (2-7-1)? ________

5. Remain the RMS Meter settings in Step 4 uncharged. Measure the rms output voltage V. (rms) when triggering angle. =0°, 30°, 60°, 90°, 120°,150° and 180° and record the results in Table 2-7-1.

Table 2-7-1 Measured V. (rms) values of single-phase semi-controlled ac voltage controller with purely resistive load

6. Using the recorded values of V._(rms) in Table 2-7-1, calculate and record the corresponding ratio of V._(rms)/ V_i(rms) for each . value, where V_i(rms) is the V._(rms)

when . =0°. Plot the curve of V._(rms)/V_i(rms) versus . in Figure 2-7-5.

Figure 2-7-5 V._(rms)/ V_i(rms) vs. curve of single-phase semi-controlled ac voltage controller with purely resistive load.

7. 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) waveforms of the ac voltage controller, and record the measured waveforms in Figure 2-7-6.

Note: For an inductive load, in the case of . <ϕ
, Single Pulse triggering is unsuitable, set Pulse selector switch of 3ϕ Phase Angle Controller to Pulse Train output (up position).

CH1:

DIF AMP V Range:___

VOLTS/DIV:___

SEC/DIV: ___

CH2:

DIF AMP V Range:___

VOLTS/DIV:___

SEC/DIV:___

Figure 2-7-6 Measured input voltage (CH1) and load voltage (CH2) waveforms of

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

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

Figure 2-7-7 Measured input voltage (CH1) and the load current (CH2) waveforms of single-phase semi-controlled ac voltage controller with inductive load

COMPUTER SIMULATION

1. Run TINAPro. In Schematic Editor window, complete the simulation circuit shown in Figure 2-7-8. Set Vi to Sine wave, Amplitude to 311V, Frequency to 60Hz.

Figure 2-7-8 Simulation circuit of single-phase semi-controlled ac voltage controller with purely resistive load

2. In Signal Editor dialog, select General wave and create the triggering pulses V_G as follows: Amplitude to 10V (A1=10V, A2=0V), Period to 16.67ms (T2=1ms, T6=15.67ms), the triggering angle of SCR Q1 to 90° (TS=4.167ms). Once completed, click OK.

3. Set inductance L to 0H to form a purely resistive load. 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 2-7-9. Is there good agreement between the TR result and measured result of Figure 2-7-4? _________

Figure 2-7-4 TR result of single-phase semi-controlled ac voltage controller with purely resistive load

4. Modify inductor L to 200mH to construct an inductive load circuit. Repeat Step 3 for simulation and obtain the TR result as shown in Figure 2-7-10. Does the TR result agree with the results of Figures 2-7-6 and 2-7-7? ________

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

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