"klem kedidelhopper" wrote in message
...
We have a ceiling fan in our bedroom that we sometimes like to run at
very slow speed at night. The fan was purchased as new surplus many
years ago and never came with a speed control. So I have been using a
generic solid state speed controller. With this unit we are able to
take the unit down to complete "crawl" however there has always been a
slight rhythmic buzzing noise whose loudness and duty cycle are both
directly proportional to the motor speed. I know it's related to the
pulse type of control I'm using but short of just using a variac or a
rheostat for that matter, does anyone know of a method to achieve a
very slow speed without the noise?

Also this motor has no brushes and so even though its not labeled as
such I wonder if it could be a "Universal" type of motor that could
run on DC? It wouldn't take much to put a bridge and perhaps even a
filter in the electrical box ahead of the motor. I'm thinking that DC
operation could possibly be quieter. Thanks, Lenny

Klem,

First off, remove the light dimmer from the circuit. Dimmers are known to
cause hum/buzzing in ceiling fan induction motors. Ceiling fan induction
motors like sine waves. If the fan still buzzes on pure sine wave AC
current, the motor has loose/separated stator laminations or loose winding
coils on the stator; this is usually not repairable, although a good lacquer
bath could theoretically tighten it up.

The older spinner fans (especially ones that do not have a pull chain switch
to proivde the lower speeds) used a multiple tapped transformer to provide
the lower voltages. It was often too large to fit within a standard wall
switch box and was designed for surface mounting on the wall.

There are controls made specifically for ceiling fans that are incremental
(not continuously variable) that provide reduced voltages for the lower
speeds but keep the sine wave intact. Transformers are no longer used.

Using a capacitor inline with the power to provide the lower speeds is the
preferred method in fan manufacturing today. Today's fans and fan controls
use a dual stage capacitor - paired together in parallel for Medium and one
stage for Low Speed. For 120VAC 60Hz, the capacitor's rated voltage should
be at least 240VAC for the speed control capacitors. Speed capacitor values
range from 1.5µF to 12µF, with the most common falling between 3µ and 5µ.
There should be a running capacitor wired to one end of one motor winding,
probably mounted above the motor. The µF value of this capacitor is a good
starting point for low speed from speed control capacitors.

Many "universal" ceiling fan controls use 5µF + 5µF or 5µF + 6µF dual
capacitors. This is around 2.5µF for Medium and 5µF or 6µF for Low Speed.
Slowing the motor beyond the natural Low Speed range, to a "crawl" as it
were, with a large capacitor value will likely produce some harmonic noise
anyway, but should not buzz.

The numbering sequence for fan controls always starts at 1 for High Speed
because fans should always be started on High Speed then lowered to the
desired speed. (At a dead stop, the motor is an effective, coupled
inductor, add the speed capacitors and running capacitor, and it can create
voltages in the circuit higher than component ratings.)

Hope this helps
Scott
Dunedin, FL