AC Voltage Controllers PDF

AC VOLTAGE CONTROLLERS Presented by - Sancoy Barua Assistant Professor, Department of EEE, CUET  Ref. T.B.: M.H. Rashid, Power Electronics: Circuits, Devices and Applications, 3rd Edition,...

AC VOLTAGE CONTROLLERS Presented by - Sancoy Barua Assistant Professor, Department of EEE, CUET  Ref. T.B.: M.H. Rashid, Power Electronics: Circuits, Devices and Applications, 3rd Edition, 2016-2017 Chapter 11 Articles 11.1,11.2,11.3,11.4 (Cont.) Derivations Examples SANCOY,EEE,CUET 2 2/6/2025  INTRODUCTION An ac voltage controller is a converter that controls the voltage, current, and average power delivered to an ac load from an ac source. Some applications: 1. Light dimmer circuits 2. Speed control of induction motors 3. Industrial heating 4. Electric welding 5. On load tap changing of transformer SANCOY,EEE,CUET 2/6/2025 3  AC VOLTAGE CONTROLLERS 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. RMS value of the alternating voltage is controlled by integral cycle control or controlling the triggering angle of the Thyristors. SANCOY,EEE,CUET 2/6/2025 4  WORKING PRINCIPLE OF AC VOLTAGE CONTROLLERS 1. Phase Control: The controller uses phase control techniques, which adjust the phase angle of the AC voltage. By delaying the triggering of the semiconductors (like thyristors or triacs), it controls the amount of energy delivered to the load. The longer the delay, the less power is supplied. 2. Firing Angle: The delay introduced in triggering the device is known as the firing angle. A smaller firing angle means more of the AC cycle is used, resulting in higher average voltage and power to the load. SANCOY,EEE,CUET 2/6/2025 5  WORKING PRINCIPLE OF AC VOLTAGE CONTROLLERS 3. Voltage Regulation: By varying the firing angle, the AC voltage controller can effectively regulate the output voltage. This leads to a controlled power supply to devices, allowing for variations in brightness for lights, speed for motors, and temperature for heaters. 4. Four-Wire Connection: The AC voltage controller is typically connected in a four-wire configuration, with two wires providing the input AC supply and two for the output to the load. 5. Feedback Mechanism: In advanced systems, feedback mechanisms may be utilized to monitor the load conditions, which allows the controller to adjust the firing angle dynamically to maintain the desired output. 6 CLASSIFICATION SANCOY,EEE,CUET 2/6/2025 7  TYPES OF CONTROLLING AC voltage controllers utilize several control mechanisms to regulate the output voltage and power supplied to the load. The main types of control mechanisms include: 1. Phase Control (Firing Angle Control) 2. Pulse Width Modulation (PWM) 3. On-Off Control (Integral Cycle Control) 4. Closed-Loop Control 5. Open-Loop Control SANCOY,EEE,CUET 2/6/2025 8  TYPES OF CONTROLLING 1. Phase Control (Firing Angle Control): This is the most common method used in AC voltage controllers. The controller adjusts the firing angle of the thyristors or triacs, effectively controlling the duration for which the load is energized during each AC cycle. By delaying the triggering point within the cycle, the controller modulates the power delivered to the load. Pros: Simple and effective for resistive loads, allows easy dimming and speed control. Cons: Generates higher harmonic distortion and may affect the performance of inductive loads. SANCOY,EEE,CUET 2/6/2025 9  TYPES OF CONTROLLING 2. Pulse Width Modulation (PWM):This method involves switching the output on and off rapidly instead of varying the firing angle. By changing the width of the pulses (duty cycle), the average voltage delivered to the load can be controlled. Pros: More efficient and minimizes harmonic generation compared to phase control. Suitable for a variety of loads, including inductive and resistive. Cons: Requires more complex control circuitry. SANCOY,EEE,CUET 2/6/2025 10  TYPES OF CONTROLLING 3. On-Off Control (ICC): This basic method switches the load completely on or off, maintaining a constant voltage when on. It is typically used in heating applications where precise control is not as critical. Pros: Simple and inexpensive, suitable for low-cost applications. Cons: Less efficient, as it can cause voltage spikes and is not suitable for applications requiring variable control. SANCOY,EEE,CUET 2/6/2025 11  TYPES OF CONTROLLING 4. Closed-Loop Control: This involves using feedback from the output to adjust the control mechanism dynamically. It can employ sensors to monitor the voltage or current and continuously adjust the firing angle or PWM signal to maintain the desired output levels. Pros: Provides precise control and stability, compensating for variations in load and supply. Cons: More complex and costly due to the need for sensors and advanced control algorithms. SANCOY,EEE,CUET 2/6/2025 12  TYPES OF CONTROLLING 5. Open-Loop Control: In this system, the controller operates without feedback. It uses pre-defined settings based on expected load characteristics. Pros: Simple and cost-effective for applications where precise control is not necessary. Cons: Less adaptive to changing load conditions, which can lead to inefficiencies. SANCOY,EEE,CUET 2/6/2025 13 INTEGRAL-CYCLE OR ON-OFF CONTROL SANCOY,EEE,CUET 2/6/2025 14 INTEGRAL-CYCLE OR ON-OFF CONTROL SANCOY,EEE,CUET 2/6/2025 15 INTEGRAL-CYCLE OR ON-OFF CONTROL SANCOY,EEE,CUET 2/6/2025 16 INTEGRAL-CYCLE OR ON-OFF CONTROL SANCOY,EEE,CUET 2/6/2025 17 PERFORMANCE PARAMETERS OF AC VOLTAGE CONTROLLERS 1) RMS Output (Load) Voltage 2) Duty Cycle 3) RMS Load Current SANCOY,EEE,CUET 2/6/2025 18  PERFORMANCE PARAMETERS OF AC VOLTAGE CONTROLLERS 4) Output AC (Load) Power 5) Input Power Factor SANCOY,EEE,CUET 2/6/2025 19  PERFORMANCE PARAMETERS OF AC VOLTAGE CONTROLLERS 6) The Average Current of Thyristor, 𝑰𝑻(𝒂𝒗𝒈) 7) The RMS Current of Thyristor, 𝑰𝑻(𝑹𝑴𝑺) SANCOY,EEE,CUET 2/6/2025 20  To Derive An Expression For The RMS Value Of Output Voltage, For On-off Control Method 𝝎𝒕𝑶𝑵 𝟏 𝑽𝑶(𝑹𝑴𝑺) = න 𝒗𝑺 𝟐 𝒅 𝝎𝒕 𝝎𝑻𝑶 𝟎 SANCOY,EEE,CUET 2/6/2025 21 SANCOY,EEE,CUET 2/6/2025 22  To Derive An Expression For The Input Power Factor Displacement Real Power Power PF delivered to Load 𝑷 𝑽𝒐,𝑹𝑴𝑺. 𝑰𝒐,𝑹𝑴𝑺 cos ∅ 𝑷𝑭 = = 𝑺 𝑽𝒊,𝑹𝑴𝑺. 𝑰𝒊,𝑹𝑴𝑺 Apparent Power in the Circuit 𝑹𝒆𝒂𝒍 𝑷𝒐𝒘𝒆𝒓 𝑷𝑭𝒆𝒇𝒇 = 𝑨𝒑𝒑𝒂𝒓𝒆𝒏𝒕 𝑷𝒐𝒘𝒆𝒓 𝑰𝒏𝒄𝒍𝒖𝒅𝒊𝒏𝒈 𝑯𝒂𝒓𝒎𝒐𝒏𝒊𝒄𝒔 SANCOY,EEE,CUET 2/6/2025 23  To Derive An Expression For The Input Power Factor Key Points - ▪ The total PF (𝑷𝑭𝒆𝒇𝒇 ) of ICC method depends on Displacement (caused by Phase lag) & Distortion (caused by harmonics). ▪ Since ICC introduces harmonics, Apparent power increases due to non- sinusoidal current, reducing 𝑷𝑭𝒆𝒇𝒇. ▪ For resistive loads, 𝒄𝒐𝒔 ∅ is close to unity, but the total PF (𝑷𝑭𝒆𝒇𝒇 ) is reduced due to harmonics. ▪ For inductive loads, both displacement and 𝑷𝑭𝒆𝒇𝒇 are lower. ▪ The reduced PF is a common disadvantage of ICC, particularly in applications where harmonics & low PF are undesirable. 5) Input Power Factor SANCOY,EEE,CUET 2/6/2025 24  6) The Average Current of Thyristor, 𝑰𝑻(𝒂𝒗𝒈) Here, 𝑰𝑺 = 𝑰𝒎 sin 𝝎𝒕 7) The RMS Current of Thyristor, 𝑰𝑻(𝑹𝑴𝑺) 𝒌 𝝅 𝟐 𝑰𝑻(𝑹𝑴𝑺) = න 𝑰𝑺 𝒅 𝝎𝒕 𝟐𝝅 𝟎 Finally, 𝑰𝒎 𝒏 𝑰𝒎 𝑰𝑻(𝑹𝑴𝑺) = = 𝒌 𝟐 𝒎+𝒏 𝟐 SANCOY,EEE,CUET 2/6/2025 25 To Derive An Expression For The RMS value of Thyristor current, 𝑰𝑻(𝑹𝑴𝑺) SANCOY,EEE,CUET 2/6/2025 26 AC VOLTAGE CONTROLLERS SANCOY,EEE,CUET 2/6/2025 27 AC VOLTAGE CONTROLLERS SANCOY,EEE,CUET 2/6/2025 28 SANCOY,EEE,CUET 2/6/2025 29 PHASE ANGLE CONTROL OR, PHASE CONTROLLED SWITCHING ✓ In phase control the switching devices are used 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 switching devices during a part of each input cycle. ✓ The controller keeps the switch open (off) for a certain period within a supply voltage cycle (in general, less than half cycle), and closes the switch (on) for the remaining period of the cycle. SANCOY,EEE,CUET 2/6/2025 30  SINGLE PHASE HALF WAVE CONTROLLER OR UNIDIRECTIONAL CONTROLLER (ASYMMETRIC PHASE) ✓ Power flow to the load is controlled by delaying the firing angle, 𝜶 of 𝑇1. ✓ Due to the presence of 𝑫𝟏 , the control range is limited & the effective 𝑽𝑶(RMS) can only be varied between 70.7% and 100%. ✓ Suitable only for low-power resistive loads, such as heating & lighting. ✓ Since the power flow is controlled during the +ve half-cycle of input voltage, this type of controller is also known as ‘Unidirectional Controller’. SANCOY,EEE,CUET 2/6/2025 31  EQUATION OF OUTPUT VOLTAGE AND OUTPUT CURRENT ✓ If input AC Supply Voltage across the Transformer Secondary Winding is 𝒗𝑺 = 𝑽𝒎 sin 𝝎𝒕 = 𝟐𝑽𝑺 sin 𝝎𝒕 and delay angle of 𝑻𝟏 is ω𝒕 = 𝜶 Output Voltage: Output Current: SANCOY,EEE,CUET 2/6/2025 32 TO DERIVE AN EXPRESSION FOR RMS OUTPUT VOLTAGE Average Value (DC Value) of Output Voltage SANCOY,EEE,CUET 2/6/2025 33 Disadvantages of Single Phase Half Wave AC Voltage Controller ✓ The output load voltage 𝑽𝟎 has a DC component because the two halves of the output voltage waveform are not symmetrical with respect to ‘0’ level. The input supply current waveform also has a DC component (average value) which can result in the problem of core saturation of the input supply transformer. ✓ The half wave ac voltage controller using a single thyristor and a single diode provides control on the thyristor only in one half cycle of the input supply. Hence ac power flow to the load can be controlled only in one half cycle. SANCOY,EEE,CUET 2/6/2025 34 Disadvantages of Single Phase Half Wave AC Voltage Controller ✓ Half wave ac voltage controller gives limited range of RMS output voltage control. Because the RMS value of ac output voltage can be varied from a maximum 100% at a trigger angle, 𝜶 𝟎𝟎 to a minimum of 70.7% at 𝟏𝟖𝟎𝟎. This controller is suitable for low power resistive loads, such as heating and lighting. The drawbacks of single phase half wave ac voltage controller can be over come by using a single phase full wave ac voltage controller. SANCOY,EEE,CUET 2/6/2025 35 PLOT OF 𝑽𝟎(𝑹𝑴𝑺) VERSUS TRIGGER ANGLE 𝜶 FOR A SINGLE PHASE HALF-WAVE AC VOLTAGE CONTROLLER (UNIDIRECTIONAL CONTROLLER) Fig.: Control characteristics of single phase half-wave phase controlled ac voltage controller SANCOY,EEE,CUET 2/6/2025 36  Example 11.2 SANCOY,EEE,CUET 2/6/2025 37  Example 11.2 SANCOY,EEE,CUET 2/6/2025 38  Example 11.2 (c) SANCOY,EEE,CUET 2/6/2025 39  Single Phase Full Wave AC Voltage Controller (Bidirectional Controller) With Resistive Load SANCOY,EEE,CUET Fig: Single phase full wave ac voltage 2/6/2025 40 controller with Resistive load  Single Phase Full Wave AC Voltage Controller (Bidirectional Controller) With Resistive Load Fig: Single phase full wave ac voltage controller with Resistive load SANCOY,EEE,CUET 2/6/2025 41  Single Phase Full Wave AC Voltage Controller with Resistive Load Input voltage, Output RMS voltage, SANCOY,EEE,CUET 2/6/2025 42  Single Phase Full Wave AC Voltage Controller with Resistive Load RMS current in the load and the source, Power factor of the load, SANCOY,EEE,CUET 2/6/2025 43  Example 11.3 SANCOY,EEE,CUET 2/6/2025 44  Example 11.3 Or, Average current of load 𝟏 𝝅 𝑽𝒅𝒄 = න 𝑽 sin 𝝎𝒕 (𝒅𝝎𝒕) 𝟐𝝅 𝜶 𝒎 𝟐𝑽𝑺 𝑽𝒅𝒄 = cos 𝜶 + 𝟏 𝟐𝝅 SANCOY,EEE,CUET 2/6/2025 45  Example 11.3 (d) RMS value of load current, 𝑽𝑶 𝟖𝟒. 𝟖𝟓 𝑰𝑹 = = = 𝟖. 𝟒𝟗 𝑨 𝑹 𝟏𝟎 SANCOY,EEE,CUET 2/6/2025 46  THANK YOU SANCOY,EEE,CUET 2/6/2025 47