I have a single phase duct fan that that run by a capacitor and its running too fast. I don't need it at this speed.

Motor: Single Phase Motor 78w @ 230v : 2560 rpm

Running capactor: CBB61 2.5uF @ 500VAC : 50/60Hz

I tried running two capacitors in series and it's slow the motor down quite a bit but nowhere near as much, I would like to run at 1/4 the power. I know I could buy a speed controller but unfortunate can't afford one right now.

The question is; Can I run a different capacitor to determine the speed? Or what can I do on the cheap? Thanks.

  • \$\begingroup\$ Is this a shaded pole motor or induction motor with extra winding? Because you are refering at running capacitors that you find in single phase motor with extra winding. Maybe the easiest is to put a resistor in series. \$\endgroup\$ Commented Mar 27, 2016 at 16:55
  • 3
    \$\begingroup\$ I'm trying to imagine what a "duck fan" is. Do you mean "duct fan" by any chance? \$\endgroup\$
    – Dave Tweed
    Commented Mar 27, 2016 at 17:12
  • 2
    \$\begingroup\$ Quack! Duck Fan! \$\endgroup\$
    – lornix
    Commented Mar 27, 2016 at 23:57
  • \$\begingroup\$ @MarkoBuršič But it's coming from 220v source and I'd need a pretty big resistor to handle all that. I'm not sure, looked online hard to tell without opening it. \$\endgroup\$
    – Kivylius
    Commented Mar 28, 2016 at 11:01

1 Answer 1


Two capacitors in series is half the capacitance. Try three in series or one 0.5 uF capacitor. Reducing the capacitance reduces the available torque and allows the load to slow the motor down. At some point, the motor may not have enough torque to start reliably or may vary a lot in speed while running. At that point, you will know you are trying to reduce the speed more than the motor will tolerate.

  • \$\begingroup\$ Perfect; I order another capacitors. Will try it. Thanks. \$\endgroup\$
    – Kivylius
    Commented Mar 28, 2016 at 11:01
  • \$\begingroup\$ > Two capacitors in series is half the capacitance. Just to be clear, two caps of the same value will be half. Otherwise expect the series capacitance to be less than the lowest one in series. "the total value for capacitors in series equals the reciprocal of the sum of the reciprocals of the individual capacitances." electronics-tutorials.ws/capacitor/cap_7.html \$\endgroup\$ Commented Aug 1, 2022 at 7:27

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