AC Voltage Controller Ckts' RMS Voltage Controllers

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AC Voltage Controller Ckts.(RMS Voltage
Controllers)
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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Applications

• Lighting / Illumination control in ac power
  circuits.
• Induction heating.
• Industrial heating Domestic heating.
• Transformer tap changing (on load transformer tap
  changing).

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• Speed control of induction motors (single phase
  and poly phase ac induction motor control).
• AC magnet controls.

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Types of AC Voltage Controllers

• Single phase half wave ac voltage controller
  (unidirectional controller).
• Single phase full wave ac voltage controller
  (bidirectional controller).
• Three phase half wave ac voltage controller
  (unidirectional controller).
• Three phase full wave ac voltage controller
  (bidirectional controller).

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AC VoltageControl Techniques

• On-Off control.
• Phase control.

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Principle of ON-OFFControl Technique

• Single phase full wave AC voltage controller
  circuit

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To derive an expression for the RMS value of
output voltage (for ON-OFF control method)
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Performance parameters of AC voltage controllers

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RMS Output (Load) Voltage
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Duty Cycle
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RMS Load Current
Output AC (Load) Power
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Input Power Factor
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The Average Current of Thyristor
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RMS Thyristor Current
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Principle of ACPhase Control

• Single phase Half-wave AC phase controller
  (Unidirectional Controller)

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Equations

• Input AC Supply Voltage

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Output Load Voltage
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Output Load Current
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To Derive An Expression For RMS Output Voltage
VO(RMS)
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Control Characteristics
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Average Value of Output Voltage
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Disadvantage

• Output load voltage has a DC component as the two
  halves are not symmetrical with respect to 0
  level.
• Limited range of RMS output voltage control from
  100 of VS to 70.7 of VS, when we vary the
  trigger angle from zero to 180 degrees.

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Single Phase Full Wave AC Voltage Controller
using 2-SCRs
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Single Phase Full Wave AC Voltage Controller
using Triac
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Equations
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To Derive an Expression forthe RMS Value of
Output Voltage
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Control Characteristic of Single Phase FW AC
Voltage Controller With Resistive Load
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Need for Isolation

• Pulse Transformer

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Single Phase FW AC Voltage Controller With
Common Cathode
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Single Phase FW AC Voltage Controller Using a
Single Thyristor
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Single Phase FW AC Voltage Controller With
RL-Load
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Input Supply Voltage Thyristor Current
Waveforms
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Gating Signals
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AC Supply Voltage Load Current Waveforms
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Output (Load) Voltage Waveform
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Thyristor Voltage across T1
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To derive an expression for output load current

• When thyristor T1 is triggered at ?t ?, the
  load current flows through the load R
  inductor L from ?t ? to ?, when T1 conducts

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To Calculate Extinction Angle ?

• The extinction angle ? , which is the value
  of ?t at which the load current iO falls to
  zero and T1 is turned off can be estimated
  by using the condition that iO 0, at ?t ?
• By using the above expression for the output load
  current, we can write

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• The extinction angle ? can be determined from
  this transcendental equation by using the
  iterative method of solution (trial and error
  method). After ? is calculated, we can determine
  the conduction angle of thyristor T1 as ?(? -
  ?).
• ? is the extinction angle which depends upon the
  load inductance value. Conduction angle ?
  increases as ? is decreased for a known value of
  ?.

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• For ? lt ? radians, i.e., for (? - ?) lt ?
  radians, for (? ?) gt ? the load current
  wf appears as a Discontinuous current wf as shown
  in the fig. The o/p load current remains at zero
  during ?t ? to (? ?). Hence Discontinuous
  load current operation occurs for ? lt (? ?).
• When the trigger angle ? is decreased and made
  equal to the load impedance angle ? i.e., when ?
  ?, we obtain from the expression for sin (? -
  ?).

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• Each Thyristor conducts for 1800 (? radians).
• T1 conducts from ?t ? to (? ?) and provides
  a positive load current.
• T2 conducts from (? ?) to (2? ?) and
  provides a negative load current.
• Hence we obtain a continuous load current
  and the output voltage waveform appears as
  a continuous sine wave identical to the input
  supply voltage waveform for trigger angle ?
  ? ? and the control on the output is lost.

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Output Voltage CurrentWaveforms for a Single
Phase Full wave AC Voltage Controller with RL
load, for ? ? ?
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• We observe that for trigger angle ? ? ?, the load
  current tends to flow continuously and we have
  continuous load current operation.
• We obtain output voltage waveform which is a
  continuous sinusoidal waveform identical to the
  input supply voltage waveform.
• We loose the control on the output voltage for ?
  ? ?, as the output voltage becomes equal to the
  input supply voltage and thus we obtain

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To Derive an Expression for RMS Output Voltage of
a Single Phase Full-Wave AC Voltage Controller
With RL Load
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Problems in AC Voltage Controllers
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• A single phase full wave ac voltage controller
  supplies an RL load. The input supply voltage is
  230V, RMS at 50Hz. The load has L
  10mH, R 10?. The delay angle of thyristors
  T1 and T2 are equal, where ?1?2?/3 radians .
  Determine
• Conduction angle of the thyristor
• RMS output voltage
• The input power factor
• Comment on the type of operation.

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• A single phase full wave controller has an input
  voltage of 120 V (RMS) and a load resistance of 6
  ohm. The firing angle of thyristor is . Find
• RMS output voltage
• Power output
• Input power factor
• Average and RMS thyristor current.

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• Find the RMS and average current flowing through
  the heater shown in figure. The delay angle of
  both the SCRs is 450.

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• A single phase full wave ac voltage controller is
  employed for controlling the power flow from 220
  V, 50 Hz source into a load circuit consisting of
  R 4? ?L 6?. Calculate the following
• Control range of firing angle
• Maximum value of RMS load current
• Maximum power and power factor
• Maximum value of average and RMS thyristor
  current.

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