Electrical query -- series cap to run a fan very slowly?

[Nicholas Bodley]

Been wondering about this for several whiles.

I have a couple of 3-speed fans, one big box, one Honeywell Vornado
knockoff in trunnions like the Jodrell Bank dish; latter is very popular,
seems.

I'd like to try running them even more slowly than they do at their
slowest, and have thought of a putting an AC-rated cap of plenty of
voltage rating in series, but one thing makes me uneasy -- setting up an
inadvertent series-resonant circuit that would draw fault-level current.
I suspect that the actual inductance varies somewhat with speed (anybody
know?),and of course, I have some older switchmode power-harmonic
generators around, aka power supplies for my old but good 21-inch CRT
monitor and Blue Cone, the eMachines T6532 refurb.

I'd probably want an assortment of very good AC-rated caps, perhaps 1, 2,
and 4 [mu]F ,and if I could find one, an 8 mu. (Btw, no millifarads for
me when I mean microfarads. MF, megafarads, is still in the future (I
think! (A while back, a bus had 160 KF of ultracaps aboard. Loved
that.))).

Best,

--
Nicabod =+= Waltham, Mass.

 

[Wild_Bill]

I would be surprised if adding a capacitor in series to one of the AC
multi-speed fan motors would have any affect on the speed at any setting.
I'm fairly certain that the speeds in these type of fan motors are related
to the arrangement of the poles of the stator.

The only small induction motors that I'm aware of, where a capaitor value
change will affect the motor speed, would be the PSC permanent split
capacitor type motors.
The PSC types can also be controlled with fairly complex variable speed
controls.
See Oriental Motors technical data for some examples. The OM controllers
operate their PSC motors smoothly, quietly and efficiently (no excessive
overheating from wasted energy).

I'm not completely familiar with ceiling fan motors, but I believe at least
some of them are PSC type motors.
Some ceiling fan speed controllers are based on a triac delivering a chopped
current source, which is why some ceiling fans make so much noise when
they're operated at lower speeds.

Most AC/induction motors are intended to operate within certain speeds,
depending upon the design of the motors. Operating AC motors beyond their
design speed isn't a simple matter, and generally not a good idea, as
overheating is likely.
Controlling the speed of AC motors is quite a bit different than controlling
the speed of DC motors.
AC motors are completely dependent upon the supply voltage frequency, and
are intended to be powered with a conventional/normal sinewave voltage
source.

AC motor rated speeds are generally dependent upon how the stators are
wound, as 2, 4 or more poles.

The above comments are in regard to single phase motors. Three phase motors
are commonly controlled with VFD variable frequency drive controllers.

--
Cheers,
WB
..............


"Nicholas Bodley" <n_bod_ley@speakeasy.net> wrote in message
newsan.2009.07.20.00.06.48@speakeasy.net...

Been wondering about this for several whiles.

I have a couple of 3-speed fans, one big box, one Honeywell Vornado
knockoff in trunnions like the Jodrell Bank dish; latter is very popular,
seems.

I'd like to try running them even more slowly than they do at their
slowest, and have thought of a putting an AC-rated cap of plenty of
voltage rating in series, but one thing makes me uneasy -- setting up an
inadvertent series-resonant circuit that would draw fault-level current.
I suspect that the actual inductance varies somewhat with speed (anybody
know?),and of course, I have some older switchmode power-harmonic
generators around, aka power supplies for my old but good 21-inch CRT
monitor and Blue Cone, the eMachines T6532 refurb.

I'd probably want an assortment of very good AC-rated caps, perhaps 1, 2,
and 4 [mu]F ,and if I could find one, an 8 mu. (Btw, no millifarads for
me when I mean microfarads. MF, megafarads, is still in the future (I
think! (A while back, a bus had 160 KF of ultracaps aboard. Loved
that.))).

Best,

--
Nicabod =+= Waltham, Mass.

 

[Nicholas Bodley]

On Mon, 20 Jul 2009 13:17:17 -0400, Wild_Bill wrote:

I would be surprised if adding a capacitor in series to one of the AC
multi-speed fan motors would have any affect on the speed at any
setting.

There would be some value that would enable the motor to start, but not
run fast. The cap's reactance would let through enough current to start
the motor

I'm fairly certain that the speeds in these type of fan motors
are related to the arrangement of the poles of the stator.

Respectfully: sorry, not true. Multispeed AC motors that have different
numbers of poles for each speed are quite costly, and not too common,
pretty sure. Some pro audio decks with direct-drive capstans (such as the
Ampex AG-440) did have two sets of windings; they were hysteresis synch.
and cost $100s around 1975. (They were capacitor-run, sep. cap. for each
speed, iirc.)

That very-popular Honeywell tabletop fan, spiral front grille, sort of
hemispherical from the side, and a support yoke, has a two-pole shaded-
pale motor, with *six* leads coming out of the winding. I was mystified.
Two of the leads were black. Turns out that the black leads were for a
thermal fuse, and the winding had two taps.

I have removed the blades from older multispeed fans, and found that the
motors run at full speed (minimal load, just the bearings) regardless of
the speed setting.

As well, I have opened up a few, and the low-cost ones have one field
coil, and two shaded poles.

The only small induction motors that I'm aware of, where a capacitor
value change will affect the motor speed, would be the PSC permanent
split capacitor type motors.
The PSC types can also be controlled with fairly complex variable speed
controls.

Interesting! Reminds me of AC servo motors I encountered in military
equipment.

["PSC" means Permanently-connected (i.e., not just for starting)
Capacitors, Split phase. ("Permanent Split Capacitor") Two windings, one
in series with the phase-shifting cap., are almost equivalent to a two-
phase motor. They tend to be more costly, run relatively cool, are
efficient, and seem to last a long time. In my experience, they are quite
nice.

See Oriental Motors technical data for some examples. The OM controllers
operate their PSC motors smoothly, quietly and efficiently (no excessive
overheating from wasted energy).

I spent maybe 10 or 15 minutes searching the US site, and had no luck.
"Variable speed AC" gave me scads of brushless DC specifics, and a
sidebar for what looked like variable-frequency inverters. In all, about
1,000 hits! As soon as I added terms to the search string, it found
nothing. Quite-bad search engine, or too-concise database to compare
against. Unfortunate, because I was really curious to see what you are
referring to. (A model number might work well.)

I'm not completely familiar with ceiling fan motors, but I believe at
least some of them are PSC type motors.

I'd think so; also multipole, as well, most likely. In the Navy,
mid-1950s, our cooling blowers were typically PSC; they were excellent.

Some ceiling fan speed controllers are based on a triac delivering a
chopped current source, which is why some ceiling fans make so much
noise when they're operated at lower speeds.

I'm partly deaf, and only recently had an opportunity to experience
variable-speed AC ceiling fans.

Most AC/induction motors are intended to operate within certain speeds,
depending upon the design of the motors. Operating AC motors beyond
their design speed isn't a simple matter, and generally not a good idea,
as overheating is likely.

Indeed, not simple! You need a higher frequency than they ware designed
for.

Controlling the speed of AC motors is quite a bit different than
controlling the speed of DC motors.

Indeed so!

AC motors are completely dependent upon the supply voltage frequency,
and are intended to be powered with a conventional/normal sinewave
voltage source.

(One exception: Slo-Syn (TM, Superior Electric, or was) and similar
types, which are basically (two-phase?) steppers. They probably work fine
with square pulses, if inductive kick is taken care of.

AC motor rated speeds are generally dependent upon how the stators are
wound, as 2, 4 or more poles.

Surely so. However, all affordable multi-speed fan motors are carefully
designed, I dare say, and very likely for one specific type of blade.
Tapped stators cause lots of slip at the slower speeds.

The above comments are in regard to single phase motors. Three phase
motors are commonly controlled with VFD variable frequency drive
controllers.

Indeed so. As I understand it, vector drives work right down to "stop",
although really-low speeds such as maybe 5 rpm might not be practical.
Apparently, a vector drive creates a rotating field that's slightly
faster than the motor's slip with that load. Afaik, motors rated for such
service need much better cooling than typical.

--
Cheers,
WB

And to you! Thanks for your thoughts.

--
Nicabod =+= Waltham, Mass.