AC Voltage Converters (Phase Controlled Converters) - An Introduction

AC voltage Converters:

We have discussed already about various power electronics converter circuits in our previous posts....
We know that a power electronic
Circuit which converts DC into AC is known as Rectifier (or simply converters)
Circuit converting DC into DC with varying the voltage level is known as Choppers.
Circuit converting AC into AC with another frequency level (voltage remains same) is known as Cycloconverters.
Circuit converting AC to AC with another voltage level (frequency remains same) is known as AC voltage converters (AC Voltage Regulators) or Phase Controlled Converters.
In this post let us discuss about the AC voltage Converter circuits...

Definition:
The AC voltage converters are the SCR based Power electronics circuits which convert fixed Alternating voltage directly into variable Alternating voltage without a change in frequency.
Phase_Controlled_Controllers

AC voltage Converter Basics:

  • In domestic and industrial applications, most of the loads require variable AC supply. For example Induction motors, heating furnace, pumps, blowers, lights, fans etc.
  • Earlier days, to drive such loads devices like auto-transformer, magnetic amplifiers, saturable reactors, tap-changing transformers are used.
  • But now a days AC voltage controllers are used to provide the variable AC supply.

The AC voltage controller circuit consists of SCRs as switching device.(In some applications, TRIAC is used instead of antiparallel SCRs). Normally circuit takes mains supply as the input and provides variable AC to the load.

  • In ac voltage controllers, the SCRs are turned on by providing the gate signal.
  • They are turned OFF by natural commutation (as the input is AC main supply).
  • So complex commutation circuit, additional components are not required. Hence the ac voltage controller circuit is simple and easy to implement.
  • These SCRs control the RMS value of load voltage either ON-OFF control method or Phase control method.
    (1)  In ON-OFF control, the SCRs are turned ON for few complete cycles and turned off for complete cycles. Thus the power to the load is intermittent.

    (2)  In phase control method, the SCRs are triggered in each half cycle with some delay. The delay is based on the power requirement from the load side.

Some of the applications of AC Voltage Converters are:

  • Lighting and Fan control
  • Speed control of single phase and three phase ac drives
  • Domestic and industrial heating
  • Starting of Induction motors
  • Transformer tap changing

Advantages of AC Voltage Converters:

  • High efficiency. Since device losses are reduced, they have high efficiency.
  • Flexibility in control
  • Compact size
  • Less maintenance.
  • No extra commutation circuit, components required as they use line commutation.

Types of AC voltage converters:

The ac voltage controllers can be single phase or three phase. Each of them can be unidirectional or bidirectional. It is listed below:

  • Single Phase ac voltage controller
    (i) Single Phase Unidirectional (half wave controller)

    (ii) Single Phase Bidirectional (Full wave controller)
  • Three Phase ac voltage controllers
    (i) Three Phase Unidirectional (half wave controller)
    (ii) Three Phase Bidirectional (Full wave controller)

Disadvantages of AC Voltage Converters:

The major disadvantage in ac voltage controllers is the load current is not sinusoidal. So harmonics are introduced in load voltage and supply current waveforms. It will more at reduced output voltage levels.

Read More:

Power Electronics Interview Questions: Set-1 
Uninterruptible Power Supply (UPS) - Basics, Types 
Comparison of MOSFET with BJT 
Power Diode Basics 

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