In this script, we will discuss the most important topic of   Power Electronics, we are already discussing 

Now we will discuss
  • The principle of the On-Off ac voltage controller
  • Advantages of disadvantages of On-Off ac voltage controller

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On-off Control: Thyristor switches join the load to the ac source for a few cycles of input voltage and then disjoin it for another few cycles. The on-off control is used only in limited applications.


Working Principle of AC Voltage Controller:

An ac voltage controller produces a variable ac voltage at a fixed or variable frequency from a fixed ac supply voltage. The input voltage to an ac voltage controller is the normal ac supply at 120v, 60 Hz, or 240 v, 50 Hz. The output should ideally be a pure sine wave at a fixed or a variable frequency, but the output of a practical voltage controller contains harmonics or ripples.

Figure (a): Single-phase full-wave control

In this figure, the gating circuits for thyristors T1 and T2 must be isolated. A single-phase full-wave controller with a resistive load is shown in the figure. When thyristor (T1) is forward biased for the positive half of the input supply voltage whereas thyristor (T2) is forward biased for the negative half cycle of the input AC supply. The method of phase control is used to achieve output voltage control.

The firing pulses to T1 and T2 are kept 180°. apart. the output voltage and gating signals for T1 and T2 are shown in Figure (b)

Figure (b): Output Waveforms

The input voltage to an ac voltage controller is the normal ac supply at 120v, 60 Hz, or 240 v, 50 Hz as shown in figure (b). The output should ideally be a pure sine wave at a fixed or a variable frequency, but the output of a practical voltage controller contains harmonics or ripples as shown in figure (c). Thus, the voltage may be controlled by having control of the firing angle. In this way, voltage control is achieved in AC voltage Controller.

Advantages of disadvantages of On-Off ac voltage controller

Advantages  of On-Off ac voltage controller
  1. better input power
  2. Less switching devices 
  3. high efficiency,
  4. simplicity,
  5. low cost 
  6. ability to control a large amount of power
  7. light dimming and industrial heating

 Disadvantages of On-Off ac voltage controller
  1. It is only used for low power resistive load.
  2. The on-off control is used only in limited applications
  3. It is only used for low-power resistive load