3-speed window fan runs only high speed

[iXod]

Bog standard Lasko, 125 vac, 19-inch "box" window fan worked for years. Now suddenly all positions on the 3-speed switch result in (normal) high speed.

I've replaced the original cap (unfortunately I've been working on this fan so long that I don't remember the value of the original cap) with a multi-value cap (4.5, 5, 6 uF). Wired each value in place of the original cap but no--or barely perceivable--effect (might have resulted in a slower start-up to get to speed).

All searches have shown only people repairing fans that are running slow on all switch positions (not fast like mine is), and are fixed by replacing the cap.

What am I missing? This can't be that hard...

I know that these fans are cheap and sometime can be had for free on Craigslist or Freecycle, but I'm taking this as a challenge. I will not be beat!

Thanks.

[TheMG]

Does the fan motor have a separate winding connection for each speed?

Could be a mechanical problem inside the speed selector switch would be my first guess.

If it's the type with with only a single run winding and a TRIAC based circuit to control the speed, then I would lean toward a shorted TRIAC or other problem with the circuit.

[Vincent]

I did mess with old multi-speed motors at some point. That cap might be a start capacitor. Its goal is to create a phase angle change in the start winding so that the rotor has a crude "rotating" magnetic field to get moving on. Changing that capacitor value modified the speed of that "rotation".

But many multi-speed motors are of the "shaded pole" type. The cap, if any, just corrects the power factor.

(I would've suggested to check the resistor assembly, but this is a household fan, not a car blower fan  )

The different speeds are probably achieved through a multi-tap stator. I'll assume the lower speeds use more of the windings and the highest speed uses the least? Less winding -> Less impedance -> More current -> More torque -> Higher speed? So if only the highest speed works, it's likely the motor is shorting out. A quick way to confirm this would be to measure its current at full speed.

[james_s]

My guess would be a fault in the switch itself. The fans I've worked on have been 20+ years old so it's possible things are different now but every one that I've seen has multiple windings and a multi-throw switch to select which one gets powered.

[cruff]

I bought a newer Lasko box fan a couple of years ago that had a faulty switch of such poor construction that it basically failed the second time I tried to used it. I replaced it with a four position rotary switch and knob from Digikey. The materials that went into the switch were of such low quality that I hesitate to apply "quality" to it at all

[TheMG]

The materials that went into the switch were of such low quality that I hesitate to apply "quality" to it at all

Yeah those things are cost-cut to the max. I have a Lasko I bought about 10 years ago. There's actually a plastic nub in the middle of the fan blade that's meant to prevent any other part of the blade from making contact with the front guard/grill which is made of plastic so thin it's flimsy and would never hold its shape well enough to maintain clearance from the fan blade.

The motor is also super weak, way undersized for the size of the blade (which makes more noise than it moves air). Had it in a window and the slightest wind pushing the opposite direction would completely stall the fan or even make it spin in reverse.

[wilfred]

If the fan starts and runs at full speed on all settings then it isn't the capacitor. I would say the most likely culprit is the switch being faulty and always switching the high speed contacts. I have had coils burned out where the fan just will not move or even twitch slightly, and when a capacitor is on the way out, they don't usually fail  completely, the fan is slow to get started and may not start on low speed. But you also need to observe how quickly a fan slows to a stop because that can suggest dried up lubrication. Which also makes starting harder.

I have found as others have that the very cheapest fans sold each Summer are becoming quite useless and seem to be designed with more attention to how small a box they can be squeezed into and light weight. Both to minimise shipping costs. Which is why half of them sprout from the nature strip at the end of Summer for collection in a city hard waste collection. They're not worth fixing in spite of the temptation too. Just buy a better one.

[james_s]

IMHO the best fans ever made are the Patton high velocity air circulators from the 80s-90s. They still make them although I'm sure they have been cost reduced by now like everything else. I have one from the 90s that is going strong, I put a few drops of oil in the motor bearings each year and it just keeps going. Moves air like no other fan I've had.

[floobydust]

Problem is the chinese special, they have conquered the small motor and appliance industry.
Huge hub yet small motor runs so hot but hey it's a fan. Low speed is still medium hurricane because that is how you make a motor using minimum copper. Big enclosure for big effect but with small blades.

I also have a Lasko box fan, it's so damn cheap. Last year the handle broke, this year I went to pick it up and my thumb poked through the thin grille. Grille is USD $12, handle $5.95 I have to laugh, as if they're repairable.
Seems fake: "Made in America, our 20-inch box fans from Lasko feature all the technological innovation you expect."

So I have no capacitors- only the three-tap switch and motor. I end up using it with a variac during summer to tone down the hurricane.
OP you might have a shorted switch or motor winding, with no run cap(s) I'd expect it to have little torque and not run at high speed.

[james_s]

I encountered a cheap box fan recently that was missing one of the screws that holds the grill on. I know it was missing out of the box because it was also missing the hole in the stamped frame for the screw to go into!