Electronic – What cap value to use for window “box” fan

capacitorfan

Two years ago a 3-speed 19” (50 cm) 120vac window “box” fan stopped turning. I guessed the cap died so sourced a replacement and installed it. This didn’t fix the problem.

Now I’m getting back to diagnosing it. With the blade removed I now see that the motor hums and if I spin it, it will run fast (can’t say if this is the rated speed). This is when powering the motor’s high-speed leads, bypassing the 3-speed switch.

It’s not a mechanical issue—with power off the armature spins freely.

I don’t remember the original cap markings but this one is marked “PMS405J” and measures 4 uF. I’m usually good at looking up markings and matching replacement components but there’s always the possibility I couldn’t find the exact one and tried a different value.

Should I sub 2, 1.5, 1.2 uF caps and just see what works? How will I know when the value is correct? I presume torque and speed are max at one value and reduced at others?

I’ve watched countless videos of repairs of 120v fans and they used caps from 7 to 1.2 uF, so no joy looking for statistical help.

Suggestions welcome.

Thanks.

EDIT: I realize I’m going on the premise that the original cause IS a failed cap. Other ideas welcome.

Best Answer

I presume torque and speed are max at one value and reduced at others?

Increasing the capacitor value above the "correct" capacitor value, may result in increasing the torque and speed but the incremental speed increase will decline. Make sure that the current drawn does not exceed the rating marked on the fan. Lower capacitor values result is less speed and torque. Below some minimum capacitor value, the fan will not have enough torque to start and the speed may vary a lot.

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