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Low cost 3 - Phase electronic AC Variac

Low cost 3 - Phase AC Variac based on PWM USING Power MOSFET and 555 timer for pwm signals.

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Purpose of the project is to make a 3-Phase 440V digital Variac (dimmer) for 0.5 HP motor. The speed of motor is controlled by reducing voltage.


Simple circuit contains a 555 Timer in astable mode. The pulse width of oscillation will be controlled by pot. The output frequency will be around 10KHz. The output of 555 timer will be passed to gate driver. As the source of mosfet will be floating between 650V to -650 V it requires an isolated gate driver.

There is one mosfet per phase. Mosfet will be switched irrespective of AC half. During positive half the mosfet will be in forward mode and will switch input, thus modulating the voltage. While in reverse mode the reverse recovery diode will allow full wave to pass.

As in 3-Phase AC at least one phase will be in -Ve half. So when two mosfet in +Ve are modulating the 3rd mosfet will be sinking all the current that is passing through the motor. Thus inter-phase voltage is modulated.

Here is a block diagram for the project.

  • 3 × STW11NM80 800V 11A Power MOSFET
  • 3 × VOM1271T Photovoltaic MOSFET Driver with Integrated Fast Turn-Off
  • 3 × UF4007 1 Amp Glass Passivated Ultra Fast Recovery Rectifier 50 to 1000 Volts Features
  • 3 × ERZE14A681 680V 382J “ZNR” Transient/Surge Absorbents
  • 1 × SA555 555 Clock and Timer IC

View all 6 components

  • Circuit Diagram

    Dhawal Patel05/23/2015 at 12:21 0 comments

    I've completed circuit schematics today. Still I have to figure out a way to protect from short circuit and over current. If someone have any idea please share.

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Discussions

lancelot.good1 wrote 11/18/2016 at 07:32 point

sorry its for a primary 3phase welder

  Are you sure? yes | no

lancelot.good1 wrote 11/18/2016 at 07:32 point

sorry its for a primary 3phase welder

  Are you sure? yes | no

lancelot.good1 wrote 11/18/2016 at 07:31 point

help how can we make one 50 amps 415 volts 40kva to replace large mechanical variac

  Are you sure? yes | no

lancelot.good1 wrote 11/18/2016 at 07:31 point

help how can we make one 50 amps 415 volts 40kva to replace large mechanical variac

  Are you sure? yes | no

Afdhal Atiff Tan wrote 08/22/2016 at 02:17 point

How did you get away from the mosfet's body diode?

  Are you sure? yes | no

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