What cap value to use for window “box” fan?

Question

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.

Answer 1

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.

Answer 2

Thank you. 6.85 uF 250v cap now starts and runs the motor. Can’t yet tell how it runs with blade on, but it’s difficult to stall with fingers on high speed (of 3 speed settings). Current is 300 mA, 500 mA, 900 mA. Seems good. I’ll watch current with blade on to see how high it goes with load.

Cheers!

Answer 3

If you haven't already, search for the fan by make and model to see if the information is available somewhere. Maybe in a manual or from a parts replacement shop or what have you.

I don't consider what follows to be great advice. Hopefully someone will have a better answer for you. But if not, then you can consider following this advice.

Try several different values of start cap. Try to establish a range of values that work to start the motor spinning. Once you have that range, pick a solution in the middle of the range. This idea of establishing a range works for a lot of things when good analysis is not practical. If the highest cap is many x bigger than the smallest, consider using the geometric mean rather than the arithmetic mean.