AC-AC Voltage Controllers

AC Voltage Controller• A type of thyristor power converter which is used to

convert a fixed voltage, fixed frequency ac input supply to obtain a variable voltage ac output.

• The RMS value of ac output voltage & the ac power flow to the load is controlled by varying the trigger angle, .

AC Voltage Controller• 2 types AC controllers: - on-off ~ connect & disconnect for several

cycles at a time- Phase Control ~ switching takes place during

every cycle of the source, in effect removing some of the source waveform before it reaches the load.

AC Voltage Controller• 2 types of AC voltage controllers :- Single Phase AC Controllers- Three Phase AC Controllers

• Single Phase AC Controllers operates with single AC power supply voltage of 230V RMS @ 50 Hz in our country.

• Three phase AC controllers operate with 3-phase ac supply from 400V @ 50Hz supply frequency.

• Applications: lighting/illumination control in power circuits, induction heating, industrial & domestic heating, transformer tap changing, etc.

Single Phase AC Controller with Resistive Load

• S1 conducts if a gate signal is applied during the positive half-cycle of the source until the current reaches zero.

• A gate signal is applied to S2 during the negative half-cycle of the source, providing a path for negative load current.

• If the gate signal for S2 is half period later than S1, analysis for the negative half-cycle is identical with the positive half, except for algebraic sign for the voltage & current.

Basic Operation

Single Phase AC Controller with Resistive Load

Basic Operation : Circuit & Waveforms

Recap: SCR in Controlled Half Wave Rectifier

• 2 conditions that SCR can conduct:- The SCR must be forward biased - A current must be applied to the gate of the SCR

• Unlike diode, the SCR will not begin to conduct as soon as the source becomes +ve. Conduction delayed until a gate current is applied basis of SCR as a means control.

• Once the SCR is conducting, the gate can be removed & the SCR remains until the current goes to zero.

Single Phase AC Controller with Resistive Load

• source voltage

• Output voltage

• The RMS load voltage is determined by taking advantage of positive & negative symmetry of the voltage form

𝑉 𝑆 (ω𝑡 )=𝑉𝑚sin ω𝑡

2 mo,rms m

sin 2V1V V sin t d t 122

Single Phase AC Controller with Resistive Load• Output voltage vs delay angle

• Normalized output voltage =

* Note that for α = 0, the load voltage is a sinusoid that has the same rms value as the source.

Single Phase AC Controller with Resistive Load• The RMS current in the load and the source is

• Power factor of the load is

* Note that pf = 1 for α = 0, which is the same as for an uncontrolled resistve load, and the power factor for α 0 is less than 1.

Single Phase AC Controller with Resistive Load

• The average SCR current is

• Since each SCR carries one half of the line current, the RMS current in each SCR is

Single Phase AC Controller with Resistive Load

• Since the source & load current is nonsinusoidal, harmonic distortion is a consideration.

• Only odd harmonics exist in the line current because the waveform has half-wave symmetry.

• Figure shows normalized harmonic content vs α.

• Base current is source voltage divided by resistance, which is the current for

α = 0

Example 5.1

Single Phase AC Controller with RL Load

Single Phase AC Controller with RL Load

The extinction angle, β is the angle at which the current returns to zero at ωt = β

Single Phase AC Controller with RL Load

Single Phase AC Controller with RL Load

connecting /disconnecting the load from AC source by gate control of the SCRs

Single Phase AC Controller with RL Load