multi-speed furnace blower motor

Neon John wrote:


<snip>
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 alt.engineering.electrical 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."
daestrom
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On Sat, 27 Sep 2008 08:50:03 -0400, "daestrom"
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. <chuckle>.
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
http://answers.yahoo.com/question/index?qid 071114104737AAHdl0M
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
-- 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 Serenity: That feeling of knowing that your secretary will never tell either of your wives.
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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.
wrote:

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wrote:

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.
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Neon John wrote:

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.
--
Benjamin D Miller, PE
www.bmillerengineering.com
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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.
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Cydrome Leader wrote:

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.
--
Benjamin D Miller, PE
www.bmillerengineering.com
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On Tue, 30 Sep 2008 17:51:14 +0000 (UTC), Cydrome Leader

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 http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN You can't turn [MS] shovelware into reliable software by patching it a whole lot. -Marcus Ranum
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I grabbed the first blower assembly I had laying around. It's the type used in high efficiency furnaces. It's completely generic and boring. The motor is about 4" in diameter and doesn't use a run cap. It just has two leads.
the motor is Fasco 7021-10399 type U21B rated 208 to 230V and 0.55A, it's also marked Class B.
Looking in from the bell I can clearly see a thermal protector wedged against the windings and in the slot. Only its leads extend past the laminated iron core.
If there's an easy way to remove the spring steel clips Fasco uses to hold the ends of motors on, let me know an I'll take the bell off and take a photo.
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On Thu, 2 Oct 2008 04:12:26 +0000 (UTC), Cydrome Leader

Perhaps the european standards (240 volt motor indicates not North American) are different than on this side of the briney. In MANY instances they are - and double the voltage would stress insulation more, and the rires in the slots could well be half the thickness they are over here on 120 volt motors.

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clare wrote:

It's a very US motor. 208 to 230 indicates it's for use one one leg of three phase service, or split phase 240, like in a house. Fast seems to like 115 vs 120 for rating their stuff.
http://www.fasco.com/pdf/Blowers_2008.pdf
it looks close to figure K.
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On Thu, 2 Oct 2008 14:55:22 +0000 (UTC), Cydrome Leader

Naw, he's just trying to save face by citing a toy motor. This thing works out to 110 watts (assuming 0.9pf) which is a little over a tenth of a HP. A toy. Similar to the one in my $12 Wal-mart special desk fan. That motor doesn't have slots per se. It has several (can't recall if 2 or 4) opposed stator bobbins with pole faces onto which simple coils are wound. The thermostat is "in the stator" in that it is taped in place within the thickness of the stator iron.
If there is any interest, I'll be happy to disassemble one of these motors (either my desk fan or one like his) and photograph it.
This is NOT, of course, what we refer to when talking about fractional HP squirrel cage motors. The most common term I've heard used (other than "toy") is sub-fractional. You can get a scale of the thing by looking at Figure P which uses what us old-timers call a "record player motor" because that is where they were commonly found. It's a single winding, shaded pole motor and yes, the thermal protection switch is taped to the single coil. It's usually a thermal fuse. Such a toy motor would never be used in an air handling unit's blower, of course.
Most, perhaps all fans on that page are used to supply combustion air to high efficiency gas furnaces and/or serve as induced draft fans to blow the flue gases out of the furnace.
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 I don't speak Stupid so do speak slowly.
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It's interesting you call a US made motor a toy, then talk about your chinese bullshit fan as some sort of reference unit of motors.
Your credibility has just dropped to zero.
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Neon John wrote:

Because some folks disagree with you? Pity.

Apparently you know little of the thermal calcs for motor windings. Of course iron conducts heat better on a watt/cm basis. But anyone familiar with the necessary calcs knows that the iron is cooled by air so is hotter than the air. And the slot insulation between the copper and the iron is another impediment to cooling, which the end turns of the winding don't have to deal with. Not to mention that the iron is quite a bit thicker than the layer of moving air surrounding the end turns.
What's fascinating is that someone that claims to have rewound some burned out motors can't seem to remember where the damage typically is in the windings. While end turns sometimes are burned, more often than not one or more slots are toasted. Once the insulation starts to fail in a slot, when it shorts some turns of the coil, it degrades pretty fast and heats that whole coil group up quite a bit.

I brought up 'large motors' because the placement of temperature detectors in them illustrates my point. The hottest part of the winding is in the slots, not the end turns. That's why all the professionals that design motors with internal temperature sensors put them in the slots, not as you suggested in your challenge, "...with the winding temperature as indicated by an attached thermocouple "
If the end turns are just below the insulation class's temperature limit, you can pretty much be assured that the insulation inside the slots is above the limit.
Suggesting that the winding temperature can be measured from the end bell is ludicrous. The 'end bell' is the metal part of the housing that holds the bearing for the shaft and attaches to the outer casing. On many motor designs, the stator iron and casing of the motor aren't even the same piece of metal. About the only thing you can detect from a sensor on the end bell is if the *bearing* is overheating.

Nope. Simple truth. Insulation failure is one of those things that follows what is sometimes referred to as "Arrhenius equation". Simply put, the rate of insulation breakdown rises exponentially with temperature. So insulation that would normally last for 20 years can have it's life shorted to less than five years or even one by a mild bit of overheating. Just because a motor doesn't 'burst into flames' for the first month of operation at higher temperatures doesn't mean you aren't materially shortening the life of the motor.
But you probably just write off a burn-out after a few months as, "Oh, well it was an old motor anyway." It couldn't *possibly* be because you operated the thing at reduced voltage. After all, you're an 'expert' [sic].

Jealous of nuclear operators? My, my, what you 'radiation sponges' will say just to make yourself sound important.
BTW, you might quite twisting my statements for a moment and realize I never claimed to be a 'submarine driver' (whatever that's supposed to be). I served in the submarine fleet for many years as an Electrician's Mate on their nuclear power plants. We don't 'drive' the sub, we keep the lights on and the motors going roundy-roundy from back in the engineering spaces, not up in the control room where the helmsman stands watch.
Of course all submariners have to qualify submarines to get their Dolphins and that includes knowing how to rig every space and how to operate a lot of equipment outside one's normal watchstation.

Adding capacitive or resistive element in series with the motor will have it operating at lower than rated voltage. Pretty much the same problem with overheating and shortening the life. Although at least it avoids the harmonics a phase-controlled SCR would add. Those harmonics add to the motor overheating problem somewhat so at least you avoid that.

What a smuck! Talk about 'crabwalking'. First you say to use phase-controlled SCR to control the speed of the furnace blower, then you site an article that explains how you have to shift winding connections to change the speed of a ceiling fan. Your own citation explains that
"Some motors, such as shaded pole motors, have a torque capability curve that allows the motor speed to be controlled by reducing the applied voltage"
But I'm pretty darn sure that the OP's furnace blower motor is not a shaded pole motor. Rather, it most probably is a capacitive start / induction run motor or capacitive start/ capacitive run type.
But you'll note that even in that article, and the accompanying graph, you can't get down to 50% speed or so like the OP asked about without going to extremes.
And then you 'crabwalk' some more by admitting that there's a chance it "can't be controlled with an SCR..." Then you hedge your bets a bit with "I've never seen that...though I have occasionally seen it in [other motors]". Sounds like a lot of crabwalking and back-peddling to me.
I think all can see who the 'crabwalker' is here.

Too bad that the speed of induction motors isn't proportional to voltage. Or did you miss that point? Cut voltage in half, torque developed is about half, but the speed hasn't dropped nearly that much at all. Depends on the design curve of torque-speed. Capacitive start/run motors have the peak much farther to the right and reducing voltage doesn't pull it left nearly as much as your citation's graph for a shaded pole motor shows.
Only motors with very low L/R ratios will have that much drop in speed with voltage. Or you completely pass the pull-out torque point and start operating on the 'back side' of the speed-torque curve. And that's a bad place to operate a motor if you want it to last.
daestrom
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On Wed, 1 Oct 2008 23:38:46 -0400, "daestrom"

What you say to this point may be true - BUT Normal practice on furnace blower motors (the subject of this discussion) is to put the device inside the case, between the wire end connections and the metal case (current practice) Not that many years back the protection device was on the bell-end housing - almost exclusively. Perhaps not as effective as it could have been - or why would they have changed it. Perhaps the way it is done now is not the theoretical best way - but it IS the way it is done, and it is RELATIVELY effective.

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daestrom wrote:

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.
--
Virg Wall, P.E.

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Why!!
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 http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN The profligate use of energy is the sign of a healthy, expanding civilization. Conservation is a leap backward toward the caves.
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Neon John wrote:

Guess who wrote this: :-)
"Though Im new to blogging, Im an old hand on the net, being active back before there WAS an internet. While Im 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. Were all adults so lets act like adults. And by all means, have fun."
See: http://www.johndearmond.com
--
Virg Wall, P.E, K6EVE

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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
http://www.johndearmond.com/about /
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 http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN Remember, amateurs made the Ark, professionals made the Titanic.
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Me thinks medication would be more helpful, in this case.
wrote:

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