Speed control for capacitor start AC motors

Cap-start motors usually have a centrifugal switch that drops out the start capacitor at about 2/3 - 3/4 of full speed. If you try to reduce the fan speed below the centrifugal switch's cut-in speed, you will get the effects complained of.

I don't think that there is any easy answer here, short of changing the fan motor for one which is more amenable to speed control. A shaded-pole motor would be a good start - these can be speed controlled quite easily with an inexpensive electronic controller which is similar, but not the same as a light dimmer.

I doubt if a shaded pole motor is made that is big enough to do this job......I have only ever seen fractional horsepower motors with shaded pole....

A possible way to possibly fix this cheaply is to change the cap to a "Run" quality and to short out the motor centrifugal switch, assuming that the start winding is man enough for the job......never personally tried it though.....make sure that the motor does not overheat in any way.....remote temperature sensing is always good....

A new motor with a proper electronic control would be better....

Would it be possible to wire the start capacitor so that it would be disconnected from the circuit when switched to lower speed? A single pole double throw switch that would feed the capacitor or the variable speed device. My electronic knowledge is very limited. Just thought it might work. Good luck.

I, too, have such a fan, mounted in the attic, with louvers beneath it (in the hallway ceiling).

As mentioned, the start-cap (and start-winding too, as they are in series) are cut out of the circuit after the motor reaches 2/3 to 3/4 of its running speed. If the switch (centrifugal in years gone by, and, tho still in use, many motors are being fitted with electronic switches) is intentionally shorted, the motor will 'growl' violently, and either burn the start windings or blow the capacitor very quickly.

The speed of an a.c. induction motor is inherently determined by the number of poles wound on the stator and the frequency of the applied power. These motors do not lend themselves to the sort of speed control that you desire. It *might* be possible to rewind your existing unit for a "High" & "Lo" speed... but I doubt it. Most people do not realize how severe a duty that air-handling is for a motor. They are designed so as to be operating at full-tilt, so-to-speak. You won't find a fan motor that is only loaded at 2/3 or 3/4 of its load capacity. So, although there probably isn't room (in the slots in your motor) for an upgrade/re-design, there are certainly multi-speed units available that can be bought off the shelf to accomplish what you want to do.

Many air handlers for furnaces have either a 2 or 3 speed motor in them, and yet are wired so as to only be using 1 of them! You will probably want to stick with the same design that you already have though... which is probably a rubber-bushed cradle-mounted 48 or 56 Frame unit (find the nameplate).

Best wishes ~

A few motors may "growl" as you put it, but not all.....it is worth a short test to find out before buying a new motor.....no harm will be done for a few minutes test!! or switch off immediately if it "growls"!!

Certainly not meaning to sound argumentative, but the fact remains that the poster has a motor that was designed and wound for Split Phase/Capacitor-Start duty. The start windings will be a larger gauge wire than a comparable Permanent Split Capacitor motor ("Cap-Run"). To try and "fool" the motor by changing the cap and shorting the switch ... "just-to-see-what-happens" ... well; "OK"; I'll admit, if he has all afternoon to kill, and a grocery-bag full of various caps to try in various series/parallel arrays ... maybe he'll get lucky!

Then again, it's still going to fry prematurely, because PSC designs (whether driving an axial fan or a squirrel-cage fan) are, for all intents and purposes, always placed in the direct airflow, like this one. The one he has, being a cap-start design is (I'm willing to bet) mounted like this one. It will be drawing more current than it is supposed to draw, running slower (thus less cooling from its own internal fan), and ... again I'll place a wager ... it'll be growling quite unhappily. I fooled-around a bunch in my earlier days in an EASA rewind shop, and it is a rare motor indeed that will tolerate being 'hot-rodded' without a complete engineering redesign.

Now ... something we HAVEN'T addressed yet --- assuming that it IS, in fact, like the split phase one linked above ... how about a multi-stepped pulley, with adjuster?

Best wishes to all ~

Believe me, I really feel that a modern motor with the correct electronics for the speed control is the right way to go. It is also the way I would go......

But for someone who is more experimental, they can have some fun surely? Will you simply forbid the experiment?

Aw c'mon...! Never in a googol eons would I propose 'forbidding' the experiment, especially for someone who already : "attempted to use several different AC motor speed controls but this has only resulted in a 'on & off slamming' effect."

By the same token, I fully believe that in many cases (this just happens to be one of them) we should all put our BEST foot forward in an attempt to BEST answer the OP, by providing concrete enlightenment ... lessons already learned by those who've gone before us. He asked:

"Is there any practical and affordable way to control the speed of this existing motor?"

The final line of my previous post attempts that ... there are plenty of bone-yards full of kilotons of old lathes, drill presses, and multitudes of other machinery from which variable pulleys can be purchased for less than a good oil-filled run cap!

I'd rather see this guy get turned-onto an idea that has true potential, as opposed to possibly damaging his motor any further than he may have already done, that's all.

How much control is even remotely possible by attempting to make start windings function as (directional) run windings...? [And, this simple drop of a couple hundred rpm's would be at the cost of overheating, and subsequent premature failure.]

If Edison had had his way, there'd be no need for this discussion...

No orneriness present in these words ---

Simply, Best Wishes ~

I have done it on a 3 speed Aircon motor by using a Split phase capacitor.

I had to replace a Carrier 3 speed Air Con motor but could not get the correct motor from anywhere (800, 900 & 1000 rpm). Eventually I had to go with a larger RPM motor which was (1100, 1200 &1300 rpm), I had to cut the axle shaft to suit my much smaller unit & customized the motor mount to suit the larger motor. It was extremely easy to do and the finished customization fitted the aircon box like a glove.

But even the lowest speed of the new motor was much to high for a lounge room application so hence, I used a Spit phase capacitor to slow it down. It still provides the required voltage to start the fan & start the Aircon pump when needed but once it has done its job it goes off line (so to speak) & the motor runs on the continuous second phase which is resisted slightly. There is no loud Hum and it is no more noisy than the original motor & it does not run hotter than it should.

I chose a capacitor with a second phase that would only slow the motor by 15% which is enough to drop the RPM to around 900 on the lowest speed setting & also at this speed the motor still gets enough air blown across it & stays cool and quiet.

Thanks to all for your replies. I was pretty much thinking that I was S.O.L. with this older set-up. I guess it's time for a trip to the home improvement store for a more modern fan. Again.... THANKS!!!! :-)

Roger H Smith