I need to speed control small ( 0.25 HP ) 3 phase 2800 rpm motor to achieve speed in the range of 6000-7000 rpm. I know basic stuffs. ( have created soft starter etc ... which is good enough for achieving slower speed than rated speed...) but How would I increase speed ? another problem is, I have 1 phase input.

I can work out the micro controller ( mostly AVR and ARM ) stuff but have no idea about power electronics

I have read to use pwm to create sine wave form which in turn can be smooth out by motor coil itself. I can generate the frequency and wave forms in microcontroller.

1) How do I create DC from 1 phase AC that can be good enough for 0.25 HP 3 phase motor. ( for at least 10 hour continuous running. ) 2) What do I need to convert my pwm waveform to actual ac using dc source. I know theory, but I am concerned about chip selection and detail circuitry.


1 Answer 1


So you want to rotate a motor at nearly three times its rated speed. Depending on how well the motor is built, it will fail immediately or later.

Then you build a 200W recitifier. This is the standard Diode Bridge, condensator setup.

And a 200W Pulse width sine generator. The tools for that are listed on Atmels AVR site. They have example code to do just that. You should be able to glean the code from some of the examples on


Wouldnt it be cheaper to get a faster motor in the first place?

  • \$\begingroup\$ what properties should I look in to motor if it has to run over a wide range ? i.e. from ~10 RPM to 7000 RPM \$\endgroup\$
    – iamgopal
    Oct 1, 2010 at 6:49
  • \$\begingroup\$ A gear reducer with a clutch. If you want any torque at the low-end, anyways. \$\endgroup\$ Oct 1, 2010 at 8:55
  • \$\begingroup\$ You may want to go for a brushless DC Motor, probably with sensors if you want to go for low RPM. You then could also use a RC driver. At least that is the way I would go if I needed a wide range drive. \$\endgroup\$
    – posipiet
    Oct 1, 2010 at 9:31
  • \$\begingroup\$ given my little knowledge in power ac electronics, and my power requirement in lower range, i think i will go with BLDC. thanks for the answer. \$\endgroup\$
    – iamgopal
    Oct 2, 2010 at 11:03

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