ac-ac voltage controllers
DESCRIPTION
controllersTRANSCRIPT
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