AC-Voltage Controller & Cyclo-Converter Study notes For Electrical Engineering

By Yash Bansal|Updated : October 25th, 2020

In this article, you will find the study notes on AC-Voltage Controller & Cyclo-Converter which will cover the topics such as AC Voltage Controller, Single Phase Half Wave Single Phase Full Wave AC Voltage Controller, ON-OFF Controller, Three Phase Half Wave & Full Wave AC Voltage Controller, Cyclo-Converter, Single Phase to Single Phase Cyclo-Converter & 3-Phase to Single Phase Cyclo-converter.

AC-Voltage Controller

In AC Voltage Controller RMS value of applied A.C input voltage is varied by introducing Thyristors between the load and a constant voltage ac source,& this RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the Thyristors in the ac voltage controller circuits.  



The RMS value of the ac output voltage and the ac power flow to the load is controlled by varying (adjusting) the trigger angle ‘α’.

There are two different types of thyristor control used in practice to control the ac power flow

• On-Off control

• Phase control

Application of AC Voltage Controller

• For Lighting control in ac power circuits.

• They are used in Induction heating,Industrial heating & Domestic heating.

• AC Voltage COntroller is also used in Tap Changing Transformer (on load transformer tap changing).

• AC Voltage Controller is also used Machines to control their speed.

Types of AC Voltage Controller

The ac voltage controllers are classified into two types based on the type of input ac supply applied to the circuit.

• Single Phase AC Controllers.

• Three Phase AC Controllers.

They are further classified as 

• Uni-directional or half wave ac controller.

• Bi-directional or full wave ac controller. In brief different types of ac voltage controllers are

• Single phase half wave ac voltage controller (uni-directional controller).

• Single phase full wave ac voltage controller (bi-directional controller).

• Three phase half wave ac voltage controller (uni-directional controller).

• Three phase full wave ac voltage controller (bi-directional controller).

Single Phase Half wave AC controller

In SIngle Phase HAlf wave AC voltage controller thyristor is connected in Anti-parallel with one diode.load is resistive, by changing the value of firing angle α of thyristor S we can controlled the load voltage.


  • In positive half cycle of AC supply SCR T is turn ON at ωt=α. during this load voltage is positive now SCR will be turn OFF due to natural commutation at ωt=π. load current and source current are positive.
  • In negative half cycle of AC supply Diode D1 will turn ON at ωt=π. During these cycle load voltage is negative. and load current and source current are negative.
  • By controlling the firing angle α of the SCR we can control AC load voltage.



Average value of output voltage is given by,

= √2 Es/2π (cosα-1)

If α is varied from 0 to π, output voltage varies from Es to Es/ √2 and average value of output voltage varies from 0 to -√2 Es / π .

Single Phase Full Wave AC Voltage Controller


In this configuration for control by phase angle delay, the thyristor gate trigger delay angle is α, where 0 ≤ α ≤ π,The fundamental of the output angular frequency is the same as the input angular frequency, ω = 2πfs. The thyristor current, shown in figure is defined by the equation


Ldi/dt + Ri = √2 V Sin wt ;  if α≤wt≤β

                    = 0  otherwise

The above differential equation has two solutions, depending on the delay angle α relative to the load natural power factor angle, φ = tan-1wL/R. Because of symmetry, the mean supply and load, voltages and currents, are zero.


Case 1: α>φ When the delay angle exceeds the load power factor angle the load current always reaches zero before π+φ, thus the differential equation boundary conditions are zero. The solution for i is




Case 2: α≤φ (continuous gate pulses)

When α φ ≤ , a pure sinusoidal load current flows, and substitution of α= φ then 


  • If a short duration gate trigger pulse is used and α < φ, then the current will be unidirectional load current & device to be turned on is reverse-biased by the conducting device.
  • Hence if the gate pulse ceases before the previous half-cycle load current has fallen to zero, only one device conducts. It is therefore usual to employ a continuous gate pulse, or stream of pulses, from α until π, then for α < φ a sine wave output current results. 

Principle of ON-OFF Controller



  • The thyristor switches T1 and T2 are turned on by applying appropriate gate trigger pulses to connect the input ac supply to the load for ‘n’ number of input cycles during the time interval tON.
  • The 3 thyristor switches T1 and T2 are turned off by blocking the gate trigger pulses for ‘m’ number of input cycles during the time interval tOFF  . The ac controller ON time tON  usually consists of an integral number of input cycles.


  • In this type of type of control is used in applications which have high mechanical inertia and high thermal time constant (Industrial heating and speed control of ac motors). Due to zero voltage and zero current switching of Thyristors, the harmonics generated by switching actions are reduced. For a sine wave input supply voltage,

     Vs = VmSin(wt) = √ VR Sin(wt)

    VR =RMS value of input ac supply

If the input ac supply is connected to load for ‘n’ number of input cycles and disconnected for ‘m’ number of input cycles, then

tON = nxT & tOFF = mxT

Where T =1/f  input cycle time (time period) and

tON = Controller ON Time

tOFF = Controller OFF Time

To = Output time period = tON+tOFF = (nT+mT)


where Vi(RMS) = RMS input supply voltage = VR


tON = An integral number of input cycles; Hence

tON = T,2T,3T,4T....& WtON = 2π,4π,6π,8π,...

Where T is the input supply time period (T = input cycle time period). Thus we note that sin2wtON =0




Input Power Factor











The Average Current of Thyristor ITAVg



where Im=Vm/ RL

Half-controlled three-phase AC Voltage Controller

The half-controlled three-phase regulator requires only a single trigger pulse per thyristor and the return path is via a diode. Compared with the fully controlled regulator, the half-controlled regulator is simpler and does not give rise to dc components but does produce more line harmonics. resistive symmetrical load, line-to-neutral voltage waveforms for four different phase delay angles, α. Three distinctive conduction periods exist.

  • 0<α<π/3: Before turn-on, one diode and one thyristor conduct in the other two phases. After turnon two thyristors and one diode conduct, and the three-phase ac supply is impressed across the load.


  • π/3 ≤ α ≤2π/3: Only one thyristor conducts at one instant and the return current is shared at different intervals by one (⅓π ≤ α ≤ ½π) or two (½π ≤ α ≤ ⅔π) diodes. 


  • 2π/3 ≤ α ≤ 7π/6: Current flows in only one thyristor and one diode and at 7π/6 zero power is delivered to the load. 





Fully-controlled three-phase ac regulator with delta load


A delta connected load can be considered to be three independent single phase ac regulators, where the total power (for a balanced load) is three times that of one regulator, that is



  • 0 ≤ α ≤ π/3


  •  π/3≤ α ≤ 2π/3


  • 2π/3 ≤ α ≤ π



The CycloConverter has been traditionally used only in very high power drives, usually above one megawatt, where no other type of drive can be used. Examples are cement tube mill drives above 5 MW, the 13 MW German-Dutch wind tunnel fan drive, reversible rolling mill drives and ship propulsion drives.

Basic Principle

  • Principle of a cyclo-converter is explained with reference to an equivalent circuit shown in given figure below, Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals.
  • The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters. Normally, the ripple content in the output voltage is neglected.


  • The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal (ac) voltage at its output terminals.
  • It is possible for the mean power to flow either ‘to’ or ‘from’ the output terminals, and the cyclo-converter is inherently capable of operation with loads of any phase angle − inductive or capacitive.
  • Because of the uni-directional current carrying property of the individual converters, it is inherent that the positive half-cycle of load current must always be carried by the positive converter, and the negative half-cycle by the negative converter, regardless of the phase of the current with respect to the voltage.
  • This means that each two-quadrant converter operates both in its rectifying (converting) and in its inverting region during the period of its associated half-cycle of current.


Single-phase to Single-phase Cyclo-converter


  • In Single Phase to Single Phase Cycloconverter two fullwave fully controlled bridge converter circuits, using four thyristors for each bridge, are connected in opposite direction (back to back), with both bridges being fed from ac supply (50 Hz).
  • Bridge-1(P–positive) supplies load current in the positive half of the output cycle, while bridge-2(N–negative) supplies load current in the negative half.
  • These two bridges should not conduct together as this will produce short-circuit at the input. In this case, two thyristors come in series with each voltage source. When the load current is positive, the firing pulses to the thyristors of bridge 2 are inhibited, while the thyristors of bridge 1 are triggered by giving pulses at their gates at that time.
  • Similarly, when the load current is negative, the thyristors of bridge 2 are triggered by giving pulses at their gates, while the firing pulses to the thyristors of bridge 1 are inhibited at that time. This is the circulating-current free mode of operation.


Inductive (R-L) Load:

For this load, the load current may be continuous or discontinuous depending on the firing angle and load power factor. The load voltage and current waveforms are shown for continuous and discontinuous load current.


  • . In this case, the output frequency is assumed as ( f2= 12.5 Hz), the input frequency being same as (f1= 50 Hz), i.e.,f1=4.f2. So, four positive half cycles, or two full cycles of the input to the full-wave bridge converter.

Three-Phase to Single-Phase (3φ -1φ ) Cycloconverter

  • In 3-phase to single phase,there are two kinds of three-phase to single-phase (3φ-1φ) cycloconverters: 3φ-1φ half-wave cycloconverter and 3φ-1φ bridge cycloconverter.
  • Like the 1φ-1φ case, the 3φ-1φ cycloconverter applies rectified voltage to the load. Both positive and negative converters can generate voltages at either polarity, but the positive converter can only supply positive current and the negative converter can only supply negative current.
  • Thus, the cycloconverter can operate in four quadrants: (+v, +i) and (-v, -i) rectification modes and (+v, -i) and (-v, +i) inversion modes. The modulation of the output voltage and the fundamental output voltage are shown in Figure below.
  • here note that α is sinusoidally modulated over the cycle to generate a harmonically optimum output voltage. 



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