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multi-speed furnace blower motor

Discussion in 'Home Power and Microgeneration' started by Dave, Sep 24, 2008.

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  1. Dave

    Dave Guest

    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
  2. Dave

    Dave Guest

    Thanks a lot for all of the information. I went to my local motor repair
    shop, and they tried to tell me that an 1140/1725 was all that was
    available. Maybe I'll try another shop.
    My fan is mounted inside the back of the furnace, and there is a filter just
    above the fan. I could very easily cut a piece of cardboard the same size
    as the filter and make a hole in it to only let a portion of air through.
    This will do what I want as far as restricting the air flow, but may present
    problems when the furnace turns on as I'd conceivably have much less
    efficient distribution of the heated air and more wasted heat up the chimney
    if I'm not getting my flow through the heat exchanger. In addition, as you
    note I may run into a motor overtemp situation.

    I like the SCR dimmer idea... does the dimmer dissipate the dropped power
    via heat? I am thinking that I could wire the dimmer in via a relay, so
    that when the furnace kicks on, the relay coil energizes and cuts the dimmer
    out of the circuit resulting in high speed. Do you see anything wrong with
    this approach?

    Why would anyone buy a new 2-speed motor if they could use a cheap dimmer
    and relay?
    Same concerns as above. It Would be pretty easy though. Is the idea that
    the capacitor introduces phase shift which makes the motor less efficient?
    My understanding of capacitors is that they pass AC with minimal power loss,
    otherwise they'd be hot all the time, which they are not.
    Just to clarify (you probably already figured this, but I'll state it just
    to be clear) my house has a duct system for heated air, and a separate
    system for cold air return. Each room in the house has two ducts, on
    opposite sides of the room in most cases. The cold air return duct system
    leads to the furnace and connects right above the filter and (below that)
    the fan and motor. There is a duct which runs from the outside of the house
    and connects to the cold air return ducting just prior to the furnace.

    When I run JUST THE FAN in the winter time, with the outside air duct open,
    it blows COLD AIR in my house, at least a whole lot colder than the ambient
    air inside the house. I don't know why this would be. The house is old,
    and the doors and windows probably do leak A BIT, but I'd think a fair
    amount of cold air needs to be being pulled in to feel as cold as it does.
  3. Dave

    Dave Guest

    Ahhh, yes. It can be challenging figuring out the right question to ask!
    Needing only two speeds, that's what I searched for. I think I will try the
    SCR idea for now as it's likely cheaper than any motor I could source. In
    addition I won't have to deal with mounting a motor with a different
    footprint than the one I'm replacing. I looked at various systems, ECM
    motors, etc. but really don't want to put hundreds of dollars into this old

    Thanks for the reply.

  4. Dave

    Dave Guest

    into a motor overtemp situation.
    Capacitive reactance is one of those terms I find tough to get a handle on.
    I repair electronic equipment as a hobby, and have (started to) read many
    books on electronics theory. Phase angle and capacitive reactance have
    never been clear though. You know the old first law of thermodynamics:
    energy is neither created nor destroyed, simply made to change forms.
    Resistors drop voltage, they get hot. (linear) voltage regulators drop
    voltage, they get hot. I can see where logic-based switching circuits, like
    PWM's, can vary voltage by switching, similar to an SCR so that less power
    is let through the circuit rather than having to dissipate excess. But
    capacitive reactance dropping voltage... where does the extra power go? I
    look at capacitors like voltage reservoirs, but I guess they are current
    limiters as well seen from a different angle.
    Yah, I used the term "cold air" instead of "fresh air".
    I've got wood-frame single-pane windows, for which I have built single-pane
    wood-frame storm windows. I seal the be-jesus out of them in the fall,
    using a product called "draft stop" which is a removable (doesn't exactly
    come off like they show in the commercial but...) silicone sealant around
    the outside of the inner windows and closed-cell foam around the edges of
    the storms. But I still get condensation between the storm and inner
    windows in the winter. It's been suggested to me that if I ran the fan 24/7
    on slow speed, with a fresh air intake connected, that I'd create a positive
    pressure inside the house which would tend to push dry air into the airspace
    between the windows rather than drawing cooler wet air in from outside. I
    have no idea if a) this is true or b) if this is attainable. Right now my
    fresh air intake is a 4" duct. Maybe if I damped this down (manual-style,
    stick a bat of insulation into it or something like that) it would help
    mitigate the seemingly excess amount of cold air drawn in to the return

    Again, many thanks for your time and expertise.

  5. If you look at capacitors similar to a lead acid battery it makes a bit
    more sense for AC.

    Say you connect a 400uF cap across 120 volts.

    It will charge up to a peak of around 180 or whatever. It just stored a

    Now the as the line voltage drops towards 0 and reverses, it's going to
    dump current back into the outlet, like a battery would. It's giving back
    what it took. This process of take some energy, then give it back keeps

    If you connect a current meter across a lead of this cap, you'd measure
    some amps of current flowing, but considering that it's not really using
    up much power (just it's internal losses which are really low for film cap
    of that size) no work is being done and the power draw is almost nothing.

    It obviously gets more complex when you deal with phase shifts or some
    inductance added into the mix, but a capacitor itself is more like a
    spring of sorts. It can store energy, but it has to dump it somewhere
    before it can store a new charge. resistors can dissipate energy, but
    store nothing.
  6. daestrom

    daestrom Guest

    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.

  7. Dave

    Dave Guest

    Okay, one more question regarding phase angle controllers and AC induction
    motors: Is the amount of strain which the use of an SCR puts on a motor
    proportional to the voltage drop? Because I'd want to use this dimmer to
    run the fan SLOW, probably a couple of hundred RPM's. Common sense (well,
    my common sense anyway) warns that whereas slowing it from 1725 to 1500 RPMs
    won't stress the motor significantly, maybe 400 RPMs would. Am I wrong?
  8. daestrom

    daestrom Guest

    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?

  9. daestrom

    daestrom Guest

    I'd say you are correct. But Neon seems to think you can do it (of course,
    he's jury-rigged a lot of things). Try it and when it burns out, send him
    the bill.

  10. daestrom

    daestrom Guest

    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

  11. Don Kelly

    Don Kelly Guest

    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
  12. daestrom

    daestrom Guest

    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

    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
    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."

  13. bw

    bw Guest

    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.
  14. Vaughn Simon

    Vaughn Simon Guest

    I live in south Florida, but was raised in Detroit. You are tickling some
    really old brain cells here... We had the wooden storm windows back then, and I
    don't remember fogging being a big problem. And yes, I think I do remember a
    little sheet metal gizmo in the bottom frame member. Of course, those windows
    weren't sealed in any effective way and provided little insulation effect. I
    think that their main function was to reduce drafts.

    Swapping the storm windows with the screens (that is also when we washed our
    windows) was a rite of season passage that I had long-forgotten. It was a
    process that took most of a weekend and was sometimes delayed too long.

  15. 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.
  16. Dave

    Dave Guest

    But, most importantly, did you then put the "circles and arrows on each one
    explaining what each one was"?
  17. Guest

    Had you followed the link, you would know that he used color-coded
    letters and arrows to create an explanation even the engineers could

  18. Dave

    Dave Guest

    LOL. I think you missed the point...
  19. You

    You Guest

    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".......
  20. Steve Ackman

    Steve Ackman Guest

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