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  1. 1. LECTURE 28 AC Voltage Controllers Dr. Rostamkolai ECE 452 Power Electronics 1
  2. 2. Introduction  The power flow into a load can be controlled by varying the rms value of the load voltage  This can be accomplished by thyristors, and this type of power circuit is known as ac voltage controllers 2
  3. 3.  The most application of ac voltage controllers are:  Industrial heating  On-load transformer tap changing  Light controls  Speed control of induction motors  AC magnet controls 3
  4. 4.  For power transfer, two types of control are normally used:  On-off Control  Phase angle control  In on-off control, thyristor switches connect the load to the ac source for a few cycles of the input voltage and then disconnected for a few cycles  In phase control, thyristor switches connect the load to the ac source for a portion of each cycle 4
  5. 5.  The ac voltage controllers can be classified into two types:  Single-Phase Controllers  Three-Phase Controllers  Each type can be subdivided into:  Unidirectional or Half-Wave Control  Bidirectional or Full-Wave Control 5
  6. 6.  Since the input voltage is ac, thyristors are line commutated  Typically phase control thyristors which are cheaper are used  For applications up to 400 Hz, TRIACs are used 6
  7. 7. Performance Parameters  An ac voltage controller produces a variable ac voltage at a fixed or variable frequency  Input source is a fixed voltage and frequency ac supply  120 or 240 V  50 or 60 Hz  The output should ideally be a pure sine-wave 7
  8. 8. 8
  9. 9.  From the input side, the performance parameters are similar to those of diode rectifiers  Input power, Pi  Rms input current, Is  Total harmonic distortion of the input current, THDi  Crest factor of the input current, CFi  Harmonic factor of the input current, HFi  Form factor of the input current, FFi  Input transformer utilization factor, TUFi  Ripple factor of the input current, RFi 9
  10. 10.  From the output side, the performance parameters are similar to those of inverters  Output power, Po  Rms output current, Io  Output frequency, fo  Total harmonic distortion of the output voltage, THDv  Crest factor of the output voltage, CFv  Harmonic factor of the output voltage, HFv  Form factor of the output voltage, FFv  Ripple factor of the output voltage, RFv 10
  11. 11. Principle of On-Off Control  The principle of on-off control can be explained with the following single-phase full-wave controller 11
  12. 12. 12
  13. 13.  This type of control is applied in applications which have high mechanical inertia and high thermal time constant  Typical examples are industrial heating and speed control of large motors  If the input voltage is connected to load for n cycles and is disconnected for m cycles, the output load voltage is found from: 13
  14. 14.  Note that k is called the duty cycle, and the power factor and output voltage vary with the square root of k k V n m n V V t d t V m n n V s s rms o s rms o               2 / 1 2 0 2 2 ) ( sin 2 ) ( 2     14
  15. 15. Principle of Phase Control  The principle of phase control can be explained with the following circuit 15
  16. 16.  Due to the presence of diode D1, the control range is limited  The rms output voltage can only be varied between 70.7 to 100%  The output voltage and input current are asymmetrical and contain a dc component 16
  17. 17.  This circuit is a single-phase half-wave controller and is suitable only for low power resistive loads, such as heating and lighting  Since the power flow is controlled during the positive half-cycle of input voltage, this type of controller is also known as unidirectional controller 17
  18. 18.  The rms value of the output voltage is found from:  The average value of the output voltage is: 2 / 1 2 / 1 2 2 2 2 2 )] 2 2 sin 2 ( 2 1 [ )]} ( sin 2 ) ( sin 2 [ 2 1 {                     s o s s o V V t d t V t d t V V ) 1 (cos 2 2 )] ( sin 2 ) ( sin 2 [ 2 1 2                  s dc s s dc V V t d t V t d t V V 18
  19. 19. Single-Phase Full-Wave Controllers with Resistive Loads  The problem of dc input current can be prevented by using bidirectional or full-wave controller 19
  20. 20. 20
  21. 21.  The firing pulse of T1 and T2 are 180 degrees apart  The rms value of the output voltage is:  By varying α from 0 to π, Vo can be varied from Vs to 0 2 / 1 2 / 1 2 2 2 2 sin ( 1 ) ( sin 2 2 2                           s o s o V V t d t V V 21

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