multi-speed furnace blower motor

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?

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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

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Reply to
Neon John
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But, most importantly, did you then put the "circles and arrows on each one explaining what each one was"?

Reply to
Dave

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

Reply to
news

LOL. I think you missed the point...

Reply to
Dave

But no circles. On the other hand, it did take me about 18 minutes to read it all carefully...

Reply to
Morris Dovey

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

Reply to
Morris Dovey

Go easy on him, now. I took his quip as a joke.

John

-- John De Armond See my website for my current email address

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Reply to
Neon John

Even those of us in Group 'W' have standards to maintain...

ROFL

Reply to
Morris Dovey

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".......

Reply to
You

$ wget

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16:59:40--
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208.113.199.178 Connecting to
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|208.113.199.178|:80... failed: Connection refused.

Reply to
Steve Ackman

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.

Reply to
daestrom

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

Reply to
news

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

Reply to
daestrom

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.

Reply to
Cydrome Leader

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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Reply to
clare at snyder dot ontario dot canada

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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Reply to
clare at snyder dot ontario dot canada

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.

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it looks close to figure K.

Reply to
Cydrome Leader

I wanted everyone to get to see your entire blog-site! I found it fascinating. You covered many of the things I dabbled in while in high school! I even had a "neon", (argon of course), "cold lamp" for my 4X5 enlarger. I had to settle for an available phosphor. It worked OK, but variable contrast enlarging papers, (VariGam), didn't like the discontinuous spectrum.

BTY, what is your amateur call sign?

Reply to
VWWall

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

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Reply to
Neon John

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.

Reply to
Cydrome Leader

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