I am controlling the speed of a single phase, 120vac, centrifugal blower with a particle photon microcontroller, optocoupler, triac, and snubber circuit. I'm currently using phase control, but intend to try integral cycle/burst fire control next.

I am able to control the speed, and am measuring the output with a hot wire anemometer. Full on, the fan is blowing 28 cfm. I can barely reduce this down to 27, and then there is this large jump at the next microcontroller output level down to 12 cfm, from which I can step it accurately down 1 cfm at a time to 0. This meets my purposes, as I need around 27, 10, 5, 3, and 1 cfm at different times in my project.

However, the output level resulting in 12 cfm will, if left running for 5 minutes, eventually speed the fan up to the higher step of 27 cfm.

What is happening? Why is there this big step from 12 to 27, and why if slowed down to the lower step would the fan eventually ramp up to the higher step when run long enough?

I'm currently running a temperature test to make sure I'm not overheating the fan motor with speed control, but I don't have full results in yet.

My second question is about phase control vs. integral cycle control. I've seen competing information on both being better for motor speed control. Any advice on which will give better results?

  • \$\begingroup\$ Measure the voltage across the blower during the different conditions. \$\endgroup\$
    – winny
    Commented Jan 27, 2017 at 17:40
  • \$\begingroup\$ sounds like a sensor measure problem out of range when fan at full speed. Post schema for better answers. \$\endgroup\$ Commented Jan 27, 2017 at 18:09

1 Answer 1


There are many different types of single phase motors, the majority of which cannot be used in variable speed applications, no matter how you accomplish it. Of the few that can have their speed varied, only one type, Shaded Pole motors, can do so with simple voltage reduction without consequences.

  • \$\begingroup\$ I have a shaded pole motor. Why would this step occur with a shaded pole motor? \$\endgroup\$
    – Topher
    Commented Jan 27, 2017 at 22:54
  • 1
    \$\begingroup\$ With voltage control, you are not really directly controlling speed, you are reducing torque, which at a given load RESULTS in a speed drop. But with a SP motor on a fan, if the air density changes, the load drops on the motor, so it speeds up given the same voltage. So I'd guess that after a few minutes, your air temperature rises and the fan doesn't have to work as hard, so the speed increases. \$\endgroup\$
    – JRaef
    Commented Jan 28, 2017 at 5:14
  • 1
    \$\begingroup\$ Thanks @J. Raefield! I've gathered temperatures, and what you're saying matches up. Once the fan motor gets to ~110F, the motor begins to consistently ramp up to a higher speed. \$\endgroup\$
    – Topher
    Commented Jan 31, 2017 at 15:09
  • \$\begingroup\$ @JRaef Stumbled upon this old answer. Your comment about the torque vs. air density was very insightful, I think it deserves to be edited into the actual answer! \$\endgroup\$
    – pipe
    Commented 2 hours ago

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