- Slides: 24
AC VOLTAGE CONTROLLER (RMS VOLTAGE CONTROLLERS)
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.
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 ‘ ’
APPLICATIONS OF AC VOLTAGE CONTROLLERS 1 Lighting / Illumination control in AC power circuits 2 Transformer tap changing (on load transformer tap changing) 3 4 5 Induction heating Industrial heating & Domestic heating Speed control of induction motors (single phase and poly phase AC induction motor control) 6 AC magnet controls
Two different types of thyristor control used in practice to control the AC power flow ON-OFF CONTROL PHASE CONTRO L
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).
ON-OFF CONTROL www. themegallery. com
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
PHASE CONTROL www. themegallery. com
THE SINGLE-PHASE FULL WAVE AC VOLTAGE CONTROLLER WITH AN R LOAD Single phase full wave AC voltage controller (Bi-directional Controller) using SCRs
Single phase full wave ac voltage controller (Bi-directional Controller) using TRIAC www. themegallery. com
PRINCIPLE OPERATION--R LOAD q. Using 2 SCRs or single TRIAC (generally used in most of the AC control applications) q. Also referred as bidirectional controller. q. S 1 is forward biased during the positive half cycle of the input supply voltage. q. S 1 is triggered at a delay angle q. When the input voltage falls to zero the thyristor current falls to zero q. S 1 naturally turns off q. S 2 is forward biased during the negative cycle of input supply q. When S 2 is triggered at a delay angle, output volt follows the negative half cycle of input. q. The input supply falls to zero and hence the load current also falls to zero q. S 2 turn naturally of
RMS OUTPUT VOLTAGE POWER FACTOR OF THE LOAD RMS SCR CURRENT & AVERAGE SCR CURRENT www. themegallery. com
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 www. themegallery. com
OUTPUT VOLTAGE VS. DELAY ANGLE Normalized RMS load voltage vs. delay angle for a single-phase AC voltage controller with a resistive load.
HARMONIC AMPLITUDES VS. DELAY ANGLE Normalized Ampitude Normalized harmonic content vs. delay angle for a single-phase AC voltage controller with a resistive load.
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. www. themegallery. com
THE SINGLE-PHASE AC VOLTAGE CONTROLLER WITH AN R-L LOAD Waveforms of Input supply voltage, Load Current, Load Voltage and Thyristor Voltage across
Input supply voltage & Thyristor current waveforms Gating Signals www. themegallery. com
PRINCIPLE OPERATION—RL LOAD 1 The S 1 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 S 1 during the positive half cycle of input supply. The output voltage across the load follows the input supply voltage when S 1 is ON. 2 3 The load current flows through the S 1 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 S 1 would not fall to zero at , when the input supply voltage starts to become negative.
PRINCIPLE cont. . 5 The S 1 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 S 1 falls to zero at , where is referred to as the Extinction angle, at which the load current falls to zero. 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. 6 7 The S 1 conducts from. The conduction angle of is which depends on the delay angle and the load impedance angle ,
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. www. themegallery. com
RMS OUTPUT VOLTAGE RMS OUTPUT CURRENT RMS SCR CURRENT & AVERAGE SCR CURRENT www. themegallery. com
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= 50 m. H. 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 www. themegallery. com