multi-speed furnace blower motor

First off, I have no idea if this is an inappropriate forum for this question, so please redirect me if there is a better group for this.
I've got an old (1970's) gas-fired forced-air furnace in my basement. It has, as one might expect, a single-speed fan motor. The motor is a 1725rpm 1/4hp. During the winter we heat primarily with wood. We have a woodstove in the livingroom, and for the past few years have turned the furnace fan on during the day to circulate the heat to the rest of the house. We plug the cold-air intake during the winter, because otherwise we get outside-temp air mixed with the recirculated air which can have more of a cooling than heating effect when it's 0F outside. The house is >60 years old and has plenty of air leaks already, so please don't bother warning me about plugging the cold air intake... we've been doing it for years and nothing's broken down and everybody's still alive.
I'd like to switch my fan motor to a two-speed for a number of reasons. It will allow me to re-connect the cold air intake to the furnace, it will lower my electricity costs as I don't need the furnace fan on full speed just to keep heat ciruclating, etc. The problem is that the only motors I see available are 1725/1140rpm motors. 1140rpm is too fast for me, and will definitely still pull in too much cold air, plus I doubt I'll save that much $$ using it. I was thinking more like 500rpm.
So, first off I'd like to know if anyone knows of a manufacturer (and even better a retailer) who sells such a motor? I have been perusing the internet for an hour and while I see references to multi-speed FURNACES I don't see any mutli-speed motors for sale (other than the aforementioned 1725/1140 motors).
Secondly, how do the multi-speed newer furnaces which run at a low speed all or most of the time deal with the cold-air intake issue?
I don't want to invest a pile of money in this ancient furnace, which would preclude any type of microprocessor-controlled, sensor-driven, cfm-regulated, variable-speed ECM motor system... I just want a two-speed fan to circulate the air and not pull cold air into the house and which switches to high speed when the thermostat kicks the furnace on.
Thanks in advance for any replies.
Dave British Columbia, Canada
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You already have lots of good advice but your inability to find the motor you want is probably because you have only looked at 2 speed motors. It makes sense that a 1725 RPM (4 pole) motor would be unlikely to have anything except about 1140 RPM (6 poles) as its second speed. For lower speeds you could just get a 3 or 4 speed motor. There are a lot of variable speed motors used in HVAC systems but they often have rather complicated controls and high costs.
Don Young
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Don Young wrote:

Having rewound more than a few induction motors, I can tell you it is very easy to wind a 4-pole motor and reconnect it for 8 poles (~900 RPM) *if* the connections between poles are brought out to the terminal head (one connection commonly used is called 'consequent pole', but it comes in three variations, a fan application would use "variable torque / variable power" connection). To get 4 and 6 poles takes two separate windings that is harder and costlier.
Using a dimmer on anything but the smallest of induction motors is asking for trouble. Running induction motors on less than their rated voltage is a pretty sure-fire way to burn them out. Same with putting a cap in series with the line, it does the same thing and reduces the terminal voltage to the motor.
A 'throttle plate' would work well so long as the OP doesn't have to run the furnace with the throttle plate installed. If the furnace runs with too little air flow, the heat-exchanger safety switch will trip when the heat-exchanger hood overheats from a lack of air flow.
daestrom
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On Thu, 25 Sep 2008 16:55:07 -0400, "daestrom"

Shhhh. Please don't tell all the motors around my place that are running on dimmers and/or capacitors that! The might go on strike or something. This includes the 3/4 hp squirrel cage fan that constitutes my whole house fan. Or the 1 hp gear motor on my BBQ sauce mixer. Yep, another Lutron dimmer controlling it.
How's that old saying go? "Those who say it can't be done should stand aside so as not to obstruct those of us who are doing it."
There ARE some single phase motors that simply cannot be slowed with a phase angle controller but they aren't used in furnace blowers and so there is no need to muddy the water with those details.
John
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Neon John wrote:

And if you don't know what you're doing, just jury-rig the heck out of it and claim success.

Induction motors of any size can be burned out when run on lower than rated voltage. It's a simple fact. Ever have to replace a 'fridge compressor because of low line voltage? How about the 'acrid odor' from a burned out blower motor?
Squirrel cage induction motors only slow down on low voltage because they can't develop enough torque and the current draw goes up considerably. Now, universal motors on a dimmer, that's fine. Hook a dimmer up to the blower motor of your furnace and it will most likely burn up.
A 1 hp gear motor for a sauce mixer? LOL. if the speed varies widely with voltage, it probably isn't a squirrel cage motor in the first place.
Let's see if some EE's in alt-engineering-electrical think your idea is worthwhile. Don, Charles, what do you think of using a dimmer to slow down a furnace blower?
daestrom
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Neon John wrote:

And then there are those of us that spent more than a few years in motor re-wind shop, fixing burned out motors and re-winding them. I've had a few customers that burned out their motor and was surprised when I told them it wasn't worth rewinding but they should buy a new one and run it at the proper voltage.

Certainly *my* furnace blower is in that category. But then a lot of them are internally protected so they'll just shut themselves down when they start to overheat. The user may wonder why it stops and then restarts after cooling.
daestrom
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On Fri, 26 Sep 2008 19:25:57 -0400, "daestrom"

So in addition to a reactor operator and sub captain and an engineer, now you're a motor man. Fascinating. Well, let's see how that holds up to scrutiny. I don't know WHO you are but I know WHAT you are - a lying bull-sh*tter. And a crawfish, of course. To address just one of your claims, that a fan motor's thermal protection has to be in a slot (or some variation thereof, I really couldn't keep track), tonight I did a little work.
See, I really DO moonlight in a motor shop, Jerry's Electric Motor Service in Cleveland, TN, as a matter of fact. I decided to pull a fan off the shelf and dissect it. in the process taking "8X10 Color Glossy Photographs" that would have made Arlo Guthrie proud. This text-only format doesn't lend itself very well to graphical presentation so without further ado, shall we shift to my blog?
http://www.johndearmond.com/2008/10/01/dissection-of-a-furnace-fan-motor /
For Part 2 of this little "Daestrom Disrobing", I'm going to reassemble that fan after instrumenting it with thermocouples, run it at various speeds and present the test results, again to demonstrate just how full of excrement you and your sycophants from alt.engineering(sic).electrical are.
Since none of you had the spine nor the balls to put your money where your mouths were, I'm going to do it just for fun. I'll put a thermocouple under that cute l'il thermal switch. I think I can slide a fine one a little ways into a slot - just to demonstrate the BS about slot temperatures in small motors. And I'll have a recording spot infrared pyrometer looking at the rotor just for added fun. Of course, it'll be instrumented for electrical parameters.
In fact, I think that I'll run several different brands of motors just to demonstrate that there is so little difference between 'em as not to matter. Finally, if I have time and still have the notion, I think that I'll run a motor that WON'T operate on a phase angle controller to show folks what that looks and sounds like.
Then it all goes up on my blog, complete with links from Google Groups to this and the original thread. This'll take a couple of days so don't go away.
The only question left to resolve is what you'll claim to be next. Or what nym you'll use now that daestrom has become so toxic and discredited. To modify an old saying, "A pile of BS by any other name smells just as rotten"
Bye for now,
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN What do you call 4 Blondes in an Abrams? Air Tank.
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But, most importantly, did you then put the "circles and arrows on each one explaining what each one was"?
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Had you followed the link, you would know that he used color-coded letters and arrows to create an explanation even the engineers could understand.
John
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LOL. I think you missed the point...
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snipped-for-privacy@picaxe.us wrote:

But no circles. On the other hand, it did take me about 18 minutes to read it all carefully... <g>
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Morris Dovey
DeSoto Solar
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snipped-for-privacy@picaxe.us wrote:

Spoken like a true usenet engineer - your parser was calibrated in the wrong unit system... ;-)
You failed to register the "Alice's Restaurant" reference. :-D
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Morris Dovey
DeSoto Solar
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wrote:

Never did care for Alice's Restaurant.
May have something to do with being born a tech geek. First noticed by my mother when I took my electric train engine apart (I was 6) - and it worked when I put it back together ;-)
John
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On Wed, 01 Oct 2008 12:48:41 -0400, snipped-for-privacy@picaxe.us wrote:

Go easy on him, now. I took his quip as a joke.
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN There are only 10 types of people in this world Those who understand binary and those who don't.
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Neon John wrote:

Even those of us in Group 'W' have standards to maintain...
ROFL
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Morris Dovey
DeSoto Solar
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wrote:

Not just "Circles and Arrows", but you also need the "Paragraph on the Back, Explaining what each Photograph was, and what the Circles and Arrows showed".......
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["Followup-To:" header set to alt.energy.homepower.]
03:57:03 -0400, Neon John, snipped-for-privacy@never.com wrote:

$ wget http://www.johndearmond.com/2008/10/01/dissection-of-a-furnace-fan-motor / --2008-10-01 16:59:40-- http://www.johndearmond.com/2008/10/01/dissection-of-a-furnace-fan-motor / Resolving www.johndearmond.com... 208.113.199.178 Connecting to www.johndearmond.com|208.113.199.178|:80... failed: Connection refused.
--
☯☯



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Neon John wrote:

Nope. It's really simple, started out as an electrican on submarines in nuclear power. Worked my way up to chief petty officer and along the way did a tour in R-3 which is Navy lingo for Repair-3 on a sub base. Can you guess what kind of repair facility R-3 is that they would have need for some Electrican's Mates? After I got out, I went to work in commercial nuclear power in operations and went to school to get a degree. Not at all that fascinating. Pity you aren't in operations yourself instead of just deconning a shutdown plant or you probably would have recognized the career path, it's not unique.

Hmm... you took apart one fan motor and now you know more about EE than anyone else on the planet. And from your example, you claim that all motors are built the same way. Nice try. I've probably cleaned out and rewound more motors than you've even had a chance to wire up the leads to.

Ah, when confronted with recognized experts, you crabwalk sideways and try to discredit the experts. Don for one has more experience in electrical machinery than you could ever hope for. At least this time you didn't run away like the schoolyard bully when an adult shows up by removing the newsgroup that disagrees with you. There may be hope for you yet.

Ah, again with the name-calling and narrow-mindedness. If someone doesn't accept your wager you assume the worst about them. It probably never occurs to you that I don't take wagers for another reason. So childish of you.

Nice try. Again with the schoolyard antics. If you bothered to look (but then you might learn something, heaven forbid), you'd find I've been posting with this 'nym' for quite a few years now. Unlike some, I don't use a different 'nym every week or two. Other than the few crackpots that think energy is free, you're about the only one that has had a long running feud with me. You're the relative newcomer to this group, unless you were lurking for the past four or five years yourself under a 'nym'.
Now, if you really knew very much about motors, you'd know that induction motor speed is a function of line frequency, number of poles and the amount of slip the rotor undergoes. And you'd know that the pull out torque point for most motors (even furnace blower motors) is above 75% of synchronous speed. That means voltage reduction at best can reduce the speed down to about 80% and can't get anywhere near the OP's desired 50% without shifting below the pull-out torque point. Operating on the 'backside' of the speed-torque curve is a really bad place to be.
Even the folks making portable box fans you can buy at the department store have recognized that the best way to get 50% speed out of an induction motor is to use a different combination of windings to get a different number of magnetic poles. For them cost is king and if switching a capacitor in series with such a fan motor were all that was needed (as you suggested in a post), that would certainly be cheaper than bringing four or five leads out from the motor to the switch. Guess those manufacturers would rather not burn out their motors. Or can you site a commercially available induction motor that shifts speed by adding a capacitor in series? I won't hold my breath on that one.
By the way, remember this statement of yours? "A centrifugal blower is a square law device. That is the flow is proportional to the square of the speed. 1140 RPM will probably drop the flow by half."
Go back and look it up. The flow is linear with speed, it is the head that follows the square of speed. Google for 'pump affinity laws' or 'fan curves explained' or some such if you don't believe me (which, of course you won't). The basic affinity laws apply for any centrifugal device where the density of the fluid doesn't appreciably change while going through the device. And power follows the cube of speed.
Bet you snip that little mistake out of your reply. Or you just pass it off as a minor typo. But I'm sure the one thing you *won't* do is admit you were wrong.
Repeating again so you just might get the point, lowering voltage on a squirrel cage induction motor doesn't drop the speed linearly with voltage applied. Not unless it is a very small one with a high resistance winding or specially designed with a very lower L/R ratio. And low L/R ratio motors are rare, I doubt you could even find one.
If you don't believe that, go study wound-rotor motors. These are often used in variable speed applications (and other special applications where very high inertia loads require special starting considerations). The principle of wound-rotor motors use the phenomenon of shifting the maximum torque point towards a lower speed point by increasing the R of the rotor circuit. But I doubt you'll find a single-phase motor of the wound-rotor design. I only mention it because it is a concrete example of how to lower the speed of an induction motor without shifting the winding connections.
daestrom
P.S. Did you know that the maximum torque point is defined by the rotor resistance being equal to the reactance? No, of course you didn't, that's just some detail that gets in the way of your view of the world. But I had to learn details like that over 30 years ago just to make rate in the Navy as an Electrican's Mate.
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----------------------------
wrote:

----------snip-------------
--
In support:

Typically the maximum torque will be in the range of 75% or above
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----------------------------
remove the X to answer

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