Adding a Fourth, Lower Speed to Fan

My window-mounted, evaporative cooler has a three speed, permanent split capacitor (PSC) motor. Is it possible to reduce all three speeds via changing out the capacitor (without shortening the life of the motor)? The wiring diagram shows an 8 microfarad capacitor.
In the alternative, a guy at a review site for this cooler said he "added a fourth speed--1/3 of the lowest speed." Can anyone say how to do this?
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Not a good idea

Replace the 3 speed motor with a 4 speed motor, and the 3 speed switch with a 4 speed switch. Or you could just get a smaller cooler.
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As you have being told not good idea "more the capacitor keep motor to start in right direction. However since this is evaporative cooler yes you can reduce speed as fallow: set the speed to max range the cut the lead that supplying power to the motor and add variable control in serious that way you can control speed down to apx. 20% of it's rated speed. you can use one of dimmers that is variable from electrical supply stores. Caution dimmer most be made of use "triac" and not the SCR. scr's work great with resistive loads but not with inductive unless are two in parallel opposing each other which are use some time in heavy currents!

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Thank you. I will consider this. Meanwhile, here is a link to a schematic of the wiring: https://sites.google.com/site/hondalioness/pscmotormodification . See the top drawing.
I proposed a modification in the lower drawing to possibly achieve an extra slow speed. The starting winding (on the far right) changes to being in parallel with the run capacitor for the "extra slow" speed. Will this work?
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With respect to you, we are all reluctant here to tell you how to do make any electrical modification for your own safety . The best thing we can suggest is : Take the electrical schematic in to a Motor Repair Center in your locale (phone book yellow pages) for specific answers , or, you might want to consider simply blocking off a SMALL portion of the blower discharge on your Swamp Cooler using a piece of sheetmetal which will not cause any harm to a motor turning a forward curved blower wheel ...which will in turn decrease the air volume/noise exiting the unit. It will decrease the cooling capacity of the unit somewhat as well. .
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news:74470599-73c6-42c6-9288-

No, usually the winding with all of the taps (making it its own autotransformer) is the main (run) winding, and the capacitor is always in series with the start winding. The motor essentially undervolts itself when on a speed lower than high via the autotransformer. To slow it further you only need to drop the voltage to it even more, this can be done with a triac dimmer as mentioned or even better but more expensive, a variac, which is a variable autotransformer.
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IIBeBauck, I appreciate your tact, regarding safety.
Daniel, thank you for the further elaboration re triacs and variacs. I am continuing to consider these.
Steve, I did shop around a bit for a four-speed motor and think this has promise (may physically fit; is not too expensive; etc.), once the current motor dies.
Meanwhile, I am not fretting much about getting a lower speed. Since this window cooling setup for my house is so new, and by 4 PM, it's very hot where I am now, the fan is great at low or even medium speed, and I am tweaking things to cool the house. I am using different combinations of my ceiling fans, the furnace fan, and creating drafts with open windows. The furnace fan CFM is low. But its intake is at the level of my feet-to-knees in my one-story house and right in the area that the new window swamp cooler cools down magnificently. The furnace fan discharges through ceiling vents. Running the furnace fan helps some in cooling the whole house.
But --
Naturally my furnace thermostat does not have a "fan only" setting, nor the green wire yada from furnace control area to wall thermostat. I had to do a little temporary wiring with a temporary switch in the furnace control area to turn on the furnace fan only. (I am not wild about fish taping through the walls to get a third wire and then buying a new thermostat. Plus wear-and-tear on the furnace fan maybe is not a good idea. Wear and tear on the more accessible ceiling fans may be preferable.)
Some interesting info from the swamp cooler manufacturer: CFM at hi, medium and low speeds: 5500, 4400 (= 80% of hi), 3300 ( = 60% of hi). These don't make sense to me as far as the speed =~ 120 (frequency) / poles goes. I suppose it is an axial flow fan thing.
The manufacturer also told me that there is not an extra, unused winding in series with the main winding to add an extra fourth speed. So what this guy at the Home Depot review site did to add a single, fourth speed that is a third of the lowest speed still has me baffled.
Pardon the verbosity. It's for the interested motor and fan person helping those of us in DIY-land.
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Those are some pretty high cfm airflow values...should really blow the air around good . You must have alot of ductwork on the system .
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Yes but could it be that the static pressure of the old swamp cooler (with vane type blower) was quite a bit higher than that of the new swamp cooler (with three bladed axial blower)?

I installed this new window swamp cooler as a do-it-yourself thing. The only "design" I did was to note that the new swamp cooler is supposed to cool about 1600 square feet. Granted under certain conditions yada.
My old, roof mounted swamp cooler (vane-type blower) pressurized the house upon startup with a "va-voom." That is, if I did not have just about every window open an inch, then one could hear the house structure expanding a bit. The old cooler had to blow air through all the ductwork in the attic and then out ceiling vents. The old swamp cooler seemed to cool the house much more uniformly. The old swamp cooler had a 1/3 hp, 240 volt motor, at the roughly 2300 cfm rating at the low speed setting that I used.
Whereas the new swamp cooler, pushing air directly from the axial fan at a window opening, no ducting at all, does not pressurize the house. I could leave the windows shut and not have the same va-voom effect. Of course I do open the windows to get air circulating. But the new cooler's axial, three-bladed fan does not yield a very high pressurization, despite the higher CFM air flow rate.
Pardon my ignorance of HVAC, but is it simply that the old cooler's fan and motor setup puts out a lower CFM at a higher static pressure?
I am happy with the new window swamp cooler, because it is cooling the house pretty well, and I do not like going on the roof several times a season. I am tweaking things now with the new window cooler. But I am wondering.
I appreciate any insight you have on this. Mostly so I can learn something and maybe improve on my "tweaking" of the new cooler.
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IVe never looked closely at an Evaporative Cooler installation on a residence before since im in humid FLorida where they are not suitable. , but, dont the HVAC Contractors there install automatic relief vents (opening on pressure increase) so the air can be relieved thru these vents into the hot attic then to the outside ? Seems like this kind of setup would sufficiently prevent over- pressurization problems from bringing in 100% outside air thru the Unit , plus serve to drop the attic temp substantially.
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