I'm trying to design a board to control an AC motor with PWM, forward and backward.

Since this is my first time designing an AC circuit, I thought I would get input from the community to make sure I'm not going to fry something.

The DIRECTION net will switch the relay to control direction of the motor. The SPEED_PWM net will be from a 5v microcontroller, which is using an AND logic gate to combine with the ON net to turn the triac on and off using the MOC3021.

The AD_IO net will cause an interrupt in the microcontroller to control the pwm.

The board will also provide 12v out through J3 pin 7.

Is this a decent design? I've tried to keep the DC components on the left side of the board, and the AC components on the right. Is there a simpler design that doesn't rely on the microcontroller interrupt to trigger the triac at the correct times, while still allowing a PWM input?

Thanks for taking the time to help a beginner!

Schematic Board Layout

BOM for all parts listed:


  • 1
    \$\begingroup\$ There are different types of AC motors. What type are you trying to control? \$\endgroup\$
    – Aaron
    Oct 10, 2022 at 18:27
  • \$\begingroup\$ A single phase 0.5hp motor like this one: amazon.com/dp/B07DQBD57C. \$\endgroup\$ Oct 10, 2022 at 21:00
  • 2
    \$\begingroup\$ You’ve violated many clearance requirements between the mains and control circuitry. This is a safety issue. With the schematic, convention is to have positive pointing up and gnd/0V pointing down. Unused inputs on cmos devices should be tied high or low - don’t leave them floating. You don’t do pwm with a triac - you want phase control. \$\endgroup\$
    – Kartman
    Oct 10, 2022 at 21:56

1 Answer 1


That is a PSC (permanent split capacitor) motor, possibly with a start capacitor as well. A phase modulated triac design does not work well for speed control of induction motors. A VFD (variable frequency drive) would work better, but most of them are three phase output, and won't work well (or at all) for a single phase motor. You can probably purchase a 1/2 HP three phase motor and an appropriate VFD for not much more than the motor shown on your Amazon link, and it will work over a wide range of speeds, with better efficiency and smoothness.

1/2 HP three phase motor $85: https://www.amazon.com/Mophorn-Electric-Standard-Flange-Mounted/dp/B07V32G8M4/ref=sr_1_3?crid=30CMAB46ED9DT&keywords=three+phase+motor+1%2F2+HP&qid=1665441299&s=industrial&sprefix=three+phase+motor+1%2F2+hp%2Cindustrial%2C81&sr=1-3&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

1/2 HP single to three phase VFD $60: https://www.amazon.com/Frequency-XCFDP-Converter-220V-Single-Phase/dp/B09L82VHTF/ref=sr_1_17?crid=CXXZMP8TOUCO&keywords=three+phase+VFD+1%2F2+HP&qid=1665441509&s=industrial&sprefix=three+phase+vfd+1%2F2+hp%2Cindustrial%2C99&sr=1-17&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

If you have only 110 VAC available, here is a 3 HP drive that will output 220 VAC with 110 VAC input, for $106: https://www.amazon.com/Liccx-Variable-Frequency-Converter-Single-Phase/dp/B09QQ72KZ3/ref=sr_1_25?crid=VY1HAHX0B5CU&keywords=three+phase+VFD+110V+1%2F2+HP&qid=1665442736&s=industrial&sprefix=three+phase+vfd+110v+1%2F2+hp%2Cindustrial%2C92&sr=1-25&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0


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