multi-speed furnace blower motor

[daestrom]

So in addition to a reactor operator and sub captain and an engineer,
now you're a motor man. Fascinating.

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.

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?

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.

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.

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.

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.

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.

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.

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.

 

[[email protected]]

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

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

 

[daestrom]

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.

Because some folks disagree with you? Pity.

Really? So you think that air is a better thermal conductor than
iron, eh? Fascinating.

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.

...


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.

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.

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.

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].

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>.

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.

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.

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.

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=20071114104737AAHdl0M

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.

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.

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

 

[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.

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.

 

[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.

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.

 

[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

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.

 

[Don Kelly]

----------------------------

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.

---
In support:

Typically the maximum torque will be in the range of 75% or above
(depending on the R/X ratio of the machine) and this speed is independent
of voltage. If the motor has a high R/X ratio, then a wide speed range can
be obtained by voltage control. However, the available torque is
proportional to the square of the voltage so that, with a high R/X ratio a
good speed range can be obtained. What is gained- lower speed at a given
torque but also lower efficiency. If you have a low slip motor (e.g. 1740
rpm 60 Hz or 3.33% slip at rated load, then at or about 1700rpm, the motor
peak torque is near maximum. Assuming the peak is near 10% slip, then
V^2(s) (approx) at rated torque for a high Tmax/Tfl =2 does imply that the
peak torque is the same as full load torque at 81% voltage. The torque
required at 1700 rpm is roughly (1700/1740)^2 =95% of the full load torque
= OOPS A problem exists- motor torque is insufficient and a stall or an
attempt to operate on the "backside" occurs along with funny smells.
If the motor has a high R/X ratio with a peak torque at a low speed, one can
get away with effective speed control- effective but inefficient.
----------------

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.

------------------------------
A capacitor in series with an induction motor will do very little of use as
far as the motor is considered. It could have unwanted effects. Not a good
idea because the effective inductance of the motor changes with speed. How
much is beneficial- it depends. What is beneficial at one load may be
disaster at another load.
------>

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.

-----
Avoid spending on a good machine and you might find one but do you want it?
---

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.

---
---
The purpose of R/X approaching unity is for peak torque at starting.
Considering the single phase motor, a better approach (as used) is to
provide a capacitor start where the phase difference between start and run
windings (and physical position of the start winding) is as close to 90
degrees as possible. Then good starting torque is provided, and once the
start winding is switched out, the motor appears as a low R/X machine
holding near constant speed over a range of loads. Use of a run/start
capacitor setup allows a two level capacitance on the "start or secondary"
winding to simulate a true 2 phase machine at start and at some point near
full load slip.

The problem is not so much the heating but a generalization from a
particular example. In any application the speed torque characteristic of
the load and the speed torque characteristic of the motor must match at some
point where (a) the matching point is stable (frontside) and (b) the current
required is not excessive. There are devices available commercially which
purport energy savings through voltage control. This is true provided
certain conditions are met but the ads don't point out these conditions or
any disadvantages of a slower speed. I would like a slower speed (and could
do it by lowering voltage a bit ) on my furnace to reduce noise but that
would require a lower air flow and this would result in a higher temperature
at the heat exchanger so that I would then have a lower useful life of the
furnace. I don't relish the thought of a premature failure of my furnace in
mid winter so I haven't made modifications. I could get away with it as the
starting torque of a fan is low but...?