AC VOLTAGE CONTROLLER (RMS VOLTAGE CONTROLLERS)

Transcription

1 AC VOLTAGE CONTROLLER (RMS VOLTAGE CONTROLLERS)

2 INTRODUCTION AC voltage controllers (AC line voltage controllers): are employed to vary the RMS value of the alternating voltage applied to a load circuit by introducing Thyristors between the load and a constant voltage AC source. The RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the Thyristors in the AC voltage controller circuits.

3 INTRODUCTION AC voltage controller is a type of thyristor power converter which is used to convert a fixed voltage, fixed frequency AC input supply to obtain a variable voltage AC output. The RMS value of the AC output voltage and the AC power flow to the load is controlled by varying (adjusting) the trigger angle α

4 APPLICATIONS OF AC VOLTAGE CONTROLLERS Lighting / Illumination control in AC power circuits 2 4 Induction heating Transformer tap changing (on load transformer tap changing) Industrial heating & Domestic heating Speed control of induction motors (single phase and poly phase AC induction motor control) 6 AC magnet controls

5 Two different types of thyristor control used in practice to control the AC power flow ON-OFF CONTROL PHASE CONTROL

6 ON-OFF CONTROL On-Off control technique: Thyristors are used as switches to connect the load circuit to the AC supply (source) for a few cycles of the input AC supply and then to disconnect it for few input cycles. The Thyristors thus act as a high speed contactor (or high speed AC switch).

7 ON-OFF CONTROL V s n m wt V o i o wt i g1 Gate pulse of T 1 wt i g2 Gate pulse of T 2 wt

8 PHASE CONTROL In phase control the Thyristors are used as switches to connect the load circuit to the input AC supply, for a part of every input cycle That is the AC supply voltage is chopped using Thyristors during a part of each input cycle

9 PHASE CONTROL

10 THE SINGLE-PHASE FULL WAVE AC VOLTAGE CONTROLLER WITH AN R LOAD Single phase full wave AC voltage controller (Bi-directional Controller) using SCRs

11 Single phase full wave ac voltage controller (Bi-directional Controller) using TRIAC

12 PRINCIPLE OPERATION--R LOAD Using 2 SCRs or single TRIAC (generally used in most of the AC control applications) Also referred as bi-directional controller. S1 is forward biased during the positive half cycle of the input supply voltage. S1 is triggered at a delay angle When the input voltage falls to zero the thyristor current falls to zero S1 naturally turns off S2 is forward biased during the negative cycle of input supply When S2 is triggered at a delay angle, output volt follows the negative half cycle of input. The input supply falls to zero and hence the load current also falls to zero S2 turn of naturally

13 RMS OUTPUT VOLTAGE POWER FACTOR OF THE LOAD RMS SCR CURRENT & AVERAGE SCR CURRENT

14 EXAMPLE 1: A Single Phase Full Wave Controller has an input voltage of 120 Vrms and a load resistance of 6 Ω. The firing angle of thyristor is 90. Find: a) RMS output voltage b) Power output c) Power factor d) Average and RMS thyristor current

15 OUTPUT VOLTAGE VS. DELAY ANGLE Normalized RMS load voltage vs. delay angle for a single-phase AC voltage controller with a resistive load.

16 HARMONIC AMPLITUDES VS. DELAY ANGLE Normalized Ampitude Normalized harmonic content vs. delay angle for a single-phase AC voltage controller with a resistive load.

17 EXAMPLE 2: The Single Phase AC voltage controller has a 120 Vrms 60 Hz source. The load resistance is 15 Ω. Determine: a) the delay angle required deliverer 500 W to the load. b) the RMS source current c) the RMS and average currents in the SCRs d) the power factor e) the total harmonic distortion (THD) of the source current.

18 T 1 THE SINGLE-PHASE AC VOLTAGE CONTROLLER WITH AN R-L LOAD Waveforms of Input supply voltage, Load Current, Load Voltage and Thyristor Voltage across

19 Input supply voltage & Thyristor current waveforms Gating Signals

20 PRINCIPLE OPERATION RL LOAD 1 The S1 is forward biased during the positive half cycle of input supply. Let us assume that is triggered at, by applying a suitable gate trigger pulse to S1 during the positive half cycle of input supply. 2 The output voltage across the load follows the input supply voltage when S1 is ON. 3 The load current flows through the S1 and through the load in the downward direction. This load current pulse flowing through can be considered as the positive current pulse. 4 Due to the inductance in the load, the load current flowing through S1 would not fall to zero at, when the input supply voltage starts to become negative.

21 PRINCIPLE cont.. 5 The S1 will continue to conduct the load current until all the inductive energy stored in the load inductor L is completely utilized and the load current through S1 falls to zero at, where is referred to as the Extinction angle, at which the load current falls to zero. 6 The extinction angle is measured from the point of the beginning of the positive half cycle of input supply to the point where the load current falls to zero. 7 The S1 conducts from. The conduction angle of is, which depends on the delay angle and the load impedance angle

22 EXAMPLE using RL- Load Controlling the speed of a single phase AC induction motor, the load which is the induction motor winding is an RL type of load. #R represents the motor winding resistance #L represents the motor winding inductance Regulate the RMS voltage across the terminals of AC motor (induction motor). #used to control the temperature of furnace by varying the RMS output voltage.

23 RMS OUTPUT VOLTAGE RMS OUTPUT CURRENT RMS SCR CURRENT & AVERAGE SCR CURRENT

24 EXAMPLE 3: A Single Phase voltage controller, the source is 120 Vrms at 60 Hz and the load is series RL combination with R= 20 Ω and L= 50mH. The delay angle 90 and extinction angle 220. Determine: a) an expression for load current b) the rms load current c) average and RMS SCR current d) power delivered to the load e) power factor