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.
British Columbia, Canada
Reply to
Loading thread data ...
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
Reply to
Don Young
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.
Reply to
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.
Reply to
Neon John
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?
Reply to
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.
Reply to
One can use a dimmer on an induction motor provided that the motor is operating below its rated and the dimmer is rated for motor operation (more expensive than light dimmers). On a fan, as the speed drops, the torque required will generally drop faster than the speed so a small speed change will result in a greater drop in load torque (and air flow) so one might get away with it without burning out the motor. However, trying to decrease speed this way has its limits in that starting can be questionable and stalling is a definite possibility. There are devices on the market which do this and great claims are made -but the downside conditions are ignored. Neon John is lucky. Ignorance is bliss in that case.
The gear motor may be a "universal motor" or even a "brushless DC" motor. (1HP rating - pretty big compared to the average mixmaster).
As for series capacitors- Not a good idea and the effect on terminal voltage will be dependent on the operating point. How close to series resonance do you want to go?
I use a motor rated dimmer on a small fan motor for a fireplace- It allows me to set a minimum speed so that I couldn't stall the motor and it requires me to start the fan at full voltage. Would I do it with my furnace motor? No way!
Don Kelly remove the X to answer
Reply to
Don Kelly
You sound like some bully in the school yard that resorts to name-calling when someone disagrees with them. Real mature. Point is you won't take my word for it, so I simply asked that some other experts, with much more experience than you join in the conversation.
It's obvious that *you* are the one that is afraid of real experts, as you keep dropping the group from the discussion. Why is that John, afraid someone there might actually know more about the subject than you?? Not to worry, I added it back for you. I've posted there many times over the years and found some very knowledgable folks there (but of course it has a few crackpots as well). My experience with rotating machinery has been well received there, why don't you go read some and learn some.
Going to experts to solve a disagreement isn't 'running off like a coward', it's the smart and mature thing to do. Trying to make it look like the act of a coward is pretty childish.
If you read Don's reply you'll see that his position seems somewhere in between ours. He points out that some loads like fans have a rapid drop in their power requirements at slow speeds and so the load on the motor could drop fast enough at slow speeds to avoid damage to the motor. So for some things, Don thinks a suitably rated dimmer would be okay for variable speed control. But he also doesn't think it would be appropriate for a furnace blower.
As if your ten minute video would 'prove' anything. Just grandstanding in front of your admirers??
Temperature rise in the motor is not something you can measure with a thermocouple attached on the outside, or even on the winding. It's internal to the slot where the heat has the hardest time being conducted away from the source. Large motors often have temperature detectors embedded in the winding during manufacture. Motors that are 'internally protected' often have the protective device embedded as well. You going to take your motor and unwind it to put a detector in the proper place? Better not, you probably wouldn't know how to re-wind it again.
A mild case of overheating internally won't show up as a failure in an hour or two, or even a day. But it will shorten the life of the insulation in the slots and cause failure.
Ad hominem attack, the last resort of wannabes and losers. You don't know why I use a nym, but you see it as point to attack, so you go for it. You have no way to judge my integrity, but why not attack that too?
I haven't 'crawfished' or backtracked from you in previous arguments, yet you try to portray me as one who does. That's more a strawman tactic, but I expect about as much from you.
Ah, trying to foment some sort of 'mob mentality'. Why don't you wrap yourself in the flag and blame me for all that's wrong with the world while you're at it. A lot of petty dictators do that. It works, for a while.
Or you could just admit that you're just a tinkerer that plays around with stuff and "doesn't play well with others."
Reply to
My KitchenAid mixer's universal motor is rated at 355W. (Not 1 HP.)
It's probably a bad idea, as others have mentioned. However:
About three years ago the condenser fan motor on my refrigerator failed. After paying almost $200 to have it replaced, the fridge sounded like a jet taking off. The repair man's response was: "You're lucky I could find a replacement of any kind!" The unit was a thirty year old Chambers built-in, and my usual parts source could not help.
I put three 110V 60W lamps in parallel, and put them in the 120V line to the fan motor. This quieted things down considerably and the fan has been blowing at this slower speed for the past three plus years.
I did replace the lamp bulbs with a small 120V/multi-low-volt-output transformer connected in a "bucking" configuration to get the same voltage, (about 90V), at the motor.
This was a very small induction type motor, (about 35W ), and was marked "impedance protected". I assume this was to protect the motor in the case dust build-up stopped it completely, but this type of induction motor may be more amenable to lower voltage operation.
I would not recommend doing a similar thing with a furnace blower motor! Most of the ones I have seen, have an adjustable pulley arrangement to allow the fan to be run at a slower speed, while the motor runs at its rated voltage. Using a conventional "dimmer" produces a "chopped" AC waveform, which might contribute to motor heating.
Reply to
They haven't used that architecture in home-sized air handles for oh, 30 years or so. Fan are now and have been for decades, direct drive, driven by a motor that is easily slowed any desired method of voltage reduction - phase angle control, variac, series reactance (capacitive or inductive) or resistance.
This has been a fascinating thread, watching all the "mights", "coulds", "possibilities" and other weasel words from people who've never tried it and therefore don't know what they're talking about. Usenet at its worst, I guess.
With opinions like that I'm not sure I'd advertise the fact. -- John De Armond See my website for my current email address
formatting link
Reply to
Neon John
Guess who wrote this: :-)
"Though I?m new to blogging, I?m an old hand on the net, being active back before there WAS an internet. While I?m interested in lively discussion about things I write about, I will not tolerate flame wars, drive-by flames or anonymous comment spamming. If you want such an environment then go somewhere else. We?re all adults so let?s act like adults. And by all means, have fun."
formatting link
Reply to
I started to write a fact-based rebuttal to this garbage but then I realized that I would be wasting my time. As Mom taught me, when one lays down with hogs, he comes up smelling like sh*t. The air in this thread is foul at the moment.
Really? So you think that air is a better thermal conductor than iron, eh? Fascinating.
But we're not talking about large motors are we? The notion of a thermal protection thermostat being embedded in the tiny slots of a furnace fan blower is hilarious. The "proper place" for the thermal sensor is strapped to the outside of the windings' end turns. Or to the end bell.
Crawfishing. Just what I expected. From this I take it that you're not willing to put your money where your mouth is. Again, what I expected.
The original poster can, of course, select whichever advice he desires. Mine, based on experience from someone with a long track record or hypothetical nonsense from an anonymous poster who hides behind a nym and has claimed in the past to be everything from a submarine driver to a nuclear reactor operator. .
Back to the original question. As I said in my original post, SCR control would not be my first choice in this particular application. There is little need to vary the fan speed once you select one that you like so a multi-tap motor or fixed series impedance (capacitive, inductive or even resistive - you are heating, after all) would do. OTOH, an SCR control will let you pick the speed you want.
On the outside change that your furnace has a blower motor that can't be controlled with an SCR, it will be immediately obvious. You won't have little to no control over the motor's speed. It'll either run at nearly full speed or stall. I've never seen that in an air handling fan motor, though I have occasionally seen it in condenser fan motors, but anything's possible, I suppose.
If you want to understand why this works (and why the writings of the likes of daestrom are pure BS), read this article
formatting link
The first respondent went to the effort of drawing a graph to aid in understanding why this works with fans while at the same time it usually doesn't with other loads. A fan's power vs speed profile just happens to be perfectly matched to voltage (actually variable slip) speed control of an induction motor.
In searching around for info on a related topic I found an HVAC distributor that sells SCR speed control modules for AC fans. Unfortunately I didn't grab the URL. It shouldn't take too much googling to find it again. I doubt that they'd be any cheaper than the Lutron commercial-grade dimmers that I like to use so the effort may be wasted.
If you'd like to take any further discussion off-list to avoid the hecklers, feel free to visit my website and click on the "mail John" button. I'll be glad to help you with the particulars of your project.
-- John De Armond See my website for my current email address
formatting link
Reply to
Neon John
My blog, my rules. They're there to keep people like you and your BS away. Thanks for the plug for my site, BTW. I appreciate all the Google ad clicks that I can get. Of course, you DID post the wrong link to the text you quoted. The correct link is
formatting link
Geez, if you can't even get THAT right....
I do have to wonder why you deviated so sharply from the thread's topic, which is methods of slowing fractional HP AC motors. Oh wait, I know. "If you can dazzle 'em with brilliance, baffle 'em with BS". Virg, the PROFESSIONAL baffler. You backed yourself into a corner and are trying to weasel out.
Anyway, just to toy around with ya a bit since you showed the poor judgment of jumping into a thread where you didn't belong, I'll offer you the same opportunity to put your professional money where your mouth is that I did daestrom.
Both you two claim that one can't slow a fractional HP permanent split phase capacitor run motor with reduced voltage without some ill-defined but awful effect damaging the motor. Even though neither of you have ever tried it, of course.
I offer you the same opportunity that I did to daestrom to make some cash if you're so sure of your position. That is, I bet him $1000 that an HVAC squirrel-cage air handling fan motor, can have its speed varied with reduced voltage, either by an SCR phase angle controller or variac, AND that the motor will operate within the manufacturer's safe thermal envelope. That is, that the winding temperature will stay below the insulation Class rating and the thermal protective device will not trip. I have three different kinds of fans here. If you're man enough to call this bet, I'll post pictures and you can pick the one to use for the test.
I will vary the speed from 100% to 50% (at which point air flow has about ceased) using only reduced voltage. I will measure the rotational speed using a General Radio StroboTach. The motor voltage will be measured with a Beckman true-RMS voltmeter. I will measure the winding's temperature with an Extech thermocouple meter. The motor will be allowed to run at each speed point until the temperature stabilizes and remains stable within 5 deg F for 30 minutes (or whatever value you like, within reason.)
To address daestrom's ludicrous claim that the thermal protection device is in a different place than where I'd put my thermocouple, I'll define the location. I'll put the thermocouple between the thermal protection device and the winding that it is protecting. Or if you like, I'll un-lace the winding, spread the end turns, bury the thermocouple as close to the center as I can and re-lace the winding back around it. It makes no difference to me nor to the experiment.
If the motor doesn't have a thermal protection device (they are, after all, relatively new for HVAC blower motors) then I'll simply attach the thermocouple to the winding with a bit of putty and we'll use the insulation Class rating as the decision parameter.
I'll video the dynamic parts of the experiment, use time-lapse photography for the boring stuff such as during whatever time you'd like the motor to run at each speed and post it all to the net. You may send an observer to watch the experiment if you like. The Tellico mountains are gorgeous this time of year so I'm sure you can find someone who'd like to take a little junket.
So. Are you man enough to call my bet and submit your claims to an actual test or will you be like dastrom and slither back into the ooze of the anonymous internet after posting nonsense? If you actually have a spine, pick a reputable internet-based escrow service, we'll put our kilobucks on deposit, agree on the test protocol and may the best engineer win.
I'll warn you ahead of time that you WILL lose, as I've done this kind of testing in the past and have quite a few such fans running on SCR controllers (including in the electric motor shop where I moonlight) but hey, you're the one claiming that it won't work. I think I know what your response to my challenge will be but I'm willing to be surprised.
Even if you don't call my bet (your crawfishing being as certain as death and taxes), I think that I'll do the experiment anyway and post a web page, complete with yours and daestrom's "advice" just to show how unqualified you two are. It'll be something fun to point to whenever daestrom pokes his head up out of the swamp the next time. This'll be fun!
John -- John De Armond See my website for my current email address
formatting link
Reply to
Neon John
This is based on years of transformer testing and the hottest spot that will break down is definitely not the average iron temperature. daestrom knows what he is talking about on this one.
Reply to
Solar Flare
Don't bother, you won't have an audience.
It is in this case, since the MOVING air removes the heat. You obviously ignored (or don't understand) daestrom's point.
Obviously you are wrong again.
I've always read Daestrom's posts carefully, and have never found any substantial error. I rarely read your posts, you have an ego problem. Daestrom is correct.
Reply to
He is absolutely correct. The wires buried in the slot run hotter than the wires exposed to the air at each end of the stator. It is not just about thermal conductivity of the air and steel, but rather the total thermal resistance from the winding conductors to the surrounding ambient. This is well known to those who measure winding temperatures or run UL tests on motors.
Reply to
Ben Miller
It's really not.
plenty of small blower type motors have a thermal protector wedged into the slots for the windings.
tear some motors apart and take a look. The resettable ones are usually rectangular cross section metal tubes wrapped in a plastic or paper sleeve, with leads coming out one side.
Reply to
Cydrome Leader
To be fair to NJ, you could install a thermal protector on the ends of the windings, with the knowledge that it would see temperatures perhaps 10-15c cooler than the maxcimum, and with an appropriate setting. However, I have not seen them there very often. On the other hand, the earlier discussion was about measuring the winding temperature during operation, and this should be done in the slot to get the worst case. Another method often used is rise of resistance, which gives an avarage temperature. Again, you would need to apply a factor to get the maximum temp.
Reply to
Ben Miller
OK, I have an open mind. I've been into literally thousands of motors and have never seen a thermal switch inserted in the slot of a fractional HP motor. Frankly, I can't imagine a manufacturer wasting the slot space on one since it works just as well buried in or attached to the end windings. But I'm open to learning. Please show or point me to a fractional HP motor with a slot-embedded thermal overload switch.
Yep, we still have several hundred of the things, dead inventory, left over from when we used to rewind fractional HP motors. Today ChiCom-made motors are so cheap, especially compared to labor cost, that such motors are disposable.
None of those would fit in a stator slot even without any wire. They're almost always tied to a winding bundle, either with lacing ribbon or winding tape. Usually the ribbon.
Seriously, if you know of a fractional HP motor (NOT a tiny "record player" single coil shaded pole motor) with a thermal overload embedded in a stator slot, I'd certainly like to know about it. It will certainly be an oddity.
John -- John De Armond See my website for my current email address
formatting link
Reply to
Neon John

Site Timeline

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.