Variable Speed Boxes

[Deleted]

The vast majority of the ceiling fans I like use "variable speed boxes" to control speeds. If you are unfamiliar with these, they are a solid state speed control mounted to a small circuit card inside a cardbox box, small enough to fit in even a Tara/Fasco switch housing. They are also the same control that many companies use for industrial fans, mounted to the wall. Some examples of fans that use variable speed boxes:

K63 Casablancas, early K55 Casablancas
Homesteads
Key Largos
Taras
Early Fascos
Nutone Veranda Deluxes
Friction Drive Emersons
Ritzes
Mosses
Evergos
SMCs

and so on. Some of the variable speed boxes click to off, some simply go from high to low. But the best ones have what's called a "trim pot" which allows you to set how low exactly the lowest setting is.

For those of you who are familiar with all this, tell me if you've had this problem:

When I set the trim pot, I try to set it so it is at the very lowest setting it will go and yet the blades will still spin from a stopped position. The problem is, this is usually too fast for me for a low speed. If I turn the trim even a hair slower, the fan stops. HOWEVER if I leave the trim there, a hair past the slowest (and yet too fast) running speed, and then gradually turn up the variable speed dial, I am able to get a low speed that is very satisfactory, or even slower.

So, what is the problem? I dont WANT any of my fans to have a setting where, by turning the dial too far, you can have the motor still on but not moving. Other people operate my fans, so they must be idiot proof (if you've met my family you understand). I know most K55-type motors are impedance protected, but it still cant be good to leave it stalling.

So, I can either have it so the very lowest speed is still too fast, or so the very lowest speed is stopped (with the motor still getting juice) but turn it slightly higher and low is perfect.

For some reason, I never have this problem on fans that CAME from the factory with an aesthetic low speed, namedly K63 Casablancas and early Key Largos. They use the same motors and controls. ?

Any thoughts?

[Ben C.]

Question: A variable speed box, would this be like a fan dimmer, where it sends a pulse of electricity to the motor every x seconds to control the speed for does it limit the motor some other way?

[Deleted]

These are solid state controls. From what I understand they basically chop up the waveform of the AC, which is the same thing you are talking about. Brian Hicks would be able to give you a better answer.

[ulkesh54]

I thought they altered the frequency of the wave, as the slumber quiet does.
If they chop up the waveform, then they would be pretty noisy.

[Deleted]

I dont see how either can alter the frequency. 60 cycles is 60s cycles.

But I believe the reason that the variable speed boxes are noisy on some motors is because the waveform isnt clean.

I use to know this stuff, but I dont remember it now. Someone who knows what they are talking about had better open their mouth.

[ulkesh54]

spiffy said:

I dont see how either can alter the frequency. 60 cycles is 60s cycles.

But I believe the reason that the variable speed boxes are noisy on some motors is because the waveform isnt clean.

I use to know this stuff, but I dont remember it now. Someone who knows what they are talking about had better open their mouth.

Maybe I'm using the wrong terminology, but I know the slubmer quiet system keeps the wave form, but alters it somehow; not sure about variable speed boxes... do they work the same way?
If they chop the wave, then that would make the fan noisy...

[Deleted]

I think it just reduces it. Slumber quiet is a very simple system, much simpler than the variable speed boxes, but it uses larger better quality componants.

Here is a schematic (with explanation) of Slumber Quiet:

[ulkesh54]

slumber quiet is quiet since it doesn't chop the wave form. My guess is that the variable boxes do chop the wave and thus create some noise...
Cool diagram... The Fan Book has stuff on what slumber quiet does to the wave

[ulkesh54]

But, if some motors are quiet with a variable box, it must do something different... I've never had a fan with that kind of control

[Deleted]

Motors are quiet with the variable boxes when they are 18 pole, and the windings are in good shape and well varnished. Remember these are the exact same boxes that almost every company used, and some still do, for wall controls.

[Deleted]

Here's problem number two: Say I finally do get a satisfactory low speed. Then what happens is, the line voltage fluctuates slightly, and now the same low speed is either too fast or stalled out. I dont know WHY this only happens with certain fans (and not other fans on the same power source) but it does.

This is why I've taken to adding a capacitor low speed to many of my fans. I dont like that, though, and would prefer to keep them and operate them as original

[JW]

What does this "trim pot" look like? Is it a second knob, a notch on the control underneath the knob that limits its range, ??

I've also noticed a fluctuation on the lowest speed on my American Pride, but it always starts up from stopped.

[Deleted]

It's a tiny knob recessed in to the cardboard box that can only be turned with an eyeglass screwdriver.

You probably already posted somewhere, but remind me, about how slow is the lowest speed on your American Pride?

[Deleted]

Well, anyhow, here's my solution to this issue: I make something I call "Vari-Three". Vari-Low, which is used on Fascos and Nutones and others, allows you two speeds, high and low. However low is via the variable speed control dial, so you can set your low speed, to close to high or almost off. So it's off-high-variable speed.

Vari-Three, I use a three speed pullchain. off-high-variable speed-low, and the low is set by a capacitor. So the capacitor gives me my aesthetic/circulation low speed, high is for those rare times you use it, and my practical speed is set by the variable speed knob.

I prefer to have my fans operating as they would originally when they were new, but this isnt a bad solution.