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DC motors

Discussion in 'Home Power and Microgeneration' started by mark Ransley, Oct 10, 2003.

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  1. mark Ransley

    mark Ransley Guest

    DC motors are 50 to 60% more efficient and are used in some furnaces
    and on the fan of the Artica refrigerator. So why arnt more products
    using them. Table and ceiling fans etc. There are brushless DC s made,
    is it reliability or cost. I think i would pay extra for a DC fan.
     
  2. Nick Pine

    Nick Pine Guest

    Grainger sells AC motors that are 96% efficient.
    Would a DC version be 146-156% efficient? :)

    Nick
     
  3. mark Ransley

    mark Ransley Guest

    Nick then why do dc motor furnaces take 1/3 to 50 less power to run.
    Because if i understand it they are more efficient. As a DC furnace
    increases AC seer by 1. You compared ac to ac, not dc. My AC motor
    furnace pulls 3a my new DC will pull 1a
     
  4. Ian Stirling

    Ian Stirling Guest

    You'r wrong.
    AC motors (nearly all of them) beat 90% efficiancy.
    DC motors are generally around the same, certainly not 30-50% better.
    Apples and oranges.
    The motor converts electricity to movement.
    If you change the amount of mechanical force you need by changing the
    appliance, then naturally the voltage will change.
    Some AC motors may be very inefficient, but it's not inherent, and
    is a design choice to use the inefficient ones.
     
  5. mark Ransley

    mark Ransley Guest

    Then how is Carrier which uses the GE DC ecm motor on the Infinity
    58mvp getting its claim of 66% less electrical usage. I guess I am
    missing it.
     
  6. Ian Stirling

    Ian Stirling Guest

    Either the AC motor is very inefficient (some blower motors, in some
    configurations can have very bad efficiency) or the DC motor is
    producing less mechanical energy.
    AC and DC motors usually have efficiancies well over 60%, even the ones
    in toys.

    Or it's a meaningless marketing number based on the start-up surges.
     
  7. mark Ransley

    mark Ransley Guest

    its not a marketing ploy , i have amp draw sheets, know people who
    have them, but true i dont know the whole picture , My unit pulls 3a at
    low speed, my new one will pull 1-1.5a at same cfm. The motors are
    expensive apx 600
     
  8. daestrom

    daestrom Guest

    Considering that integral horsepower AC motors are well into the 80%
    efficient range, and some close to 90%, what does it mean to say '50 to 60%
    more efficient'?? I dislike it when lay persons use words like '50% more
    efficient'. If the AC is 80%, does that mean you think DC is 130%?? That's
    silly. If a DC motor were 90% and and AC were 80% (which they are not),
    wouldn't it be more appropriate to say 90%/80% for 125%? Or maybe that DC
    is 10% more efficient (90 - 80)??

    DC motors of significant horsepower are usually not brushless. If they have
    brushes, there is a life/maintenance issue. Brushless ones simply use
    electronics to invert the DC supply to drive what is essentially an AC
    motor. Since the fundamental motor is the same, yet you have to add
    electronics, how can it be *more* efficient.

    Frankly, I think you're premise is bad. DC motors are *not* more efficient
    than AC except in poorly designed AC units.

    daestrom
     
  9. daestrom

    daestrom Guest

    Carrier, Trane, and others have come out with lines of super-efficient
    units. That is true, and the motors *are* DC. But it isn't a fundamental
    of motors being DC that is efficient.

    The super-efficient units have *variable-speed* motors. (easy to accomplish
    with DC motors, even 'brushless' DC) This allows the controls to vary the
    speed of fans and compressors to suit the A/C load. By running continuously
    at a lower speed, the unit can provide just enough cooling without operating
    over-rated. Some of the losses calculated in SEER ratings include starting
    and stopping of units. Building up initial pressure in freon to begin
    cooling process is avoided. It may draw less current, but it runs longer
    (i.e. hours at a time).

    Avoiding starts/stops saves energy and improves SEER, but it isn't 'magic'.

    Simple AC motors are not variable speed (at least not the style used in
    A/C). The unit develops more cooling than is really needed when running,
    and to avoid freezing you out, a thermostat turns the unit on and off. But
    this wastes some energy.

    daestrom
     
  10. In the simplest form, DC motors tend to find a balancing point, where the load
    on the motor and power usage are balanced. When the load increases, the speed
    of the shaft decreases and the torque increases. When the load lightens, the
    speed increases and the torque decreases. This type of self adjustment tends
    to make the motor run at the ideal point in the power curve IF the motor is
    matched properly to the load.

    In contrast, the common early AC motors tended to want to run at a fixed speed
    determined by the frequency of the current and the windings, and were not as
    efficiently responsive to changes in load as DC motors.

    DC motors, OTOH, could "run away" under low-load conditions, which made small
    AC motors a safer choice for home appliances and tools. Most tinkerers have
    played with washing machine motors and seen how they can stall out under load,
    when a comparable DC motor would hunker down, pull more amps, and torque the
    load into submission.

    Edison himself was a proponent of DC, and most early heavy usage motors were
    DC. Tesla's AC motors were not as capable as the DC motors of the time, but
    advances in switching technology and design have drastically reduced the gap
    between the two types of motors.

    http://www.trainweb.org/railwaytechnical/drives.html

    IME, the whole "efficiency" idea doesn't distill down to a single percentage
    rating. Different conditions may give one motor an advantage over another, and
    any fixed rating only reflects a single set of conditions. If efficiency was
    as cut and dried as some people think, there would not be the wide variety of
    motor types that the marketplace supports.
     
  11. mark Ransley

    mark Ransley Guest

    Daestrom, That makes more sence its because it is variable speed it
    can at low speed consume less.
     
  12. There is efficiency, and there is efficiency.
    An AC motor, lightly loaded, has an AWFUL power factor.
    Then there is the matter of efficiency at what speed. A DC motor can
    be run either constant speed or constant load (torque) As a blower, at
    constant speed, it takes a bit more power to start than to run, but
    the controller gives it a soft start, so it draws considerably less
    than an AC induction motor - either split phase or cap start. It can
    be easilly controlled to run at the required speed, where for an AC
    motor to run its most efficiently in an application it would have to
    be custom designed to that particular application.
    A 1/3 HP fan motor on a belted squirrel cage fan will never reach 90%
    electrical efficiency. Direct drive multispeed AC motors will do a bit
    better, but still are not terribly efficient., and the low speed
    winding on a two speed fan motor is much less efficient than the full
    power winding.
    To look at it another way, an induction motor is first of all a
    transformer with a broken core. It MUST have an air gap to allow the
    armature to turn within the stator. The transformer portion of the
    motor is not anywhere near 100% efficient. Even a real transformer is
    not. Now that you have gotten an electric current flowing in both the
    stator and rotor, there is resistance and eddie current losses in both
    the fixed and rotating windings and cores.

    I have just replaced my old Anthes furnace with a new Tempstar. When
    the fan kicked onto high with the old 2 speed AC motor my UPS would
    occaisionally beep and you could see some lights dim (barely
    perceptible, but there).
    With the DC motor you can barely tell when it ramps up to high
    speed.and the low speed most definitely uses less power.
    With nothing else in the house on, the meter used to turn at a very
    noticeable speed - and now it just crawls.
     
  13. daestrom

    daestrom Guest

    Such speed variation is known as 'speed regulation' and is usually
    calculated given in percent of no-load speed. Most shunt wound motors have
    a speed regulation less than 10% (5-7 is not unusual). Differentially-wound
    compound motors can be designed such that the speed regulation is actually 0
    or even slightly negative.

    Only series wound motors have the kind of poor speed regulation you seem to
    be thinking of. These motors can overspeed if unloaded and develop
    tremendous torque when stalled. This type of motor sees use in applications
    where high-torque but intermittent service are the norm. The classic
    example is the starter motor on cars/trucks.

    Even the earliest AC motors have the same behavior. They only develop as
    much horsepower as required by the load to keep it running at the speed of
    the motor. They do not draw rated kW from the line when only supplying a
    fraction of their rated horsepower at the shaft.
    Even modern induction motors run at nearly a fixed speed. The amount of
    torque they develop can vary from zero to full rating in as little as 3-5%
    speed variation. The amount of power they draw from the supply varies
    directly with the load on the shaft. Your statement 'not as efficiently
    responsive to changes in load' makes absolutely no sense at all. The
    response of even early AC motors is directly affected by shaft load.

    AC *or* DC motors are very in-efficient at light load. By definition, they
    are both 0% efficient at no-load. All motors have a particular load level
    where they are most efficient. But change the load from that point and the
    efficiency of AC or DC motors drops off. No inherent advantage (efficiency
    wise) of either design.
    Only series-wound DC motors 'run away'. And they only do it when unloaded.
    There are several types of DC motor windings, you seemed focused on only one
    type. The series motor is used for 'traction motors' in locomotives because
    it can develop high torques for starting.

    Traction requirements are much different than other industrial settings.
    With many industrial/residental loads, the torque required by the load
    varies with speed squared. In pumping applications (and blowers), the
    torque required at half-speed is one quarter that needed at full speed. So
    the power required is one-eighth. Pumping power is proportional to speed
    cubed. If traction were like that, it would be as if the train got
    shorter/lighter as it slowed down.

    Traction requirements, on the other hand, have a couple of different
    components. First, rolling resistance varies with the speed of the train
    and the sharpness of any curve. So the torque required to overcome that
    varies with speed. But a *major* torque requirement is hauling a train up a
    grade. The tractive effort needed to pull a long train up even a modest 1%
    grade is sometimes 10 times the level-grade rolling resistance. This
    tractive effort shows up as a torque requirement on the motor that is nearly
    constant regardless of speed.

    While it is true that an *overloaded* AC motor will stall, and an
    *overloaded* DC motor will just slow down and burn up, that has little to do
    with practical applications. Most people don't deliberately overload the
    equipment.
    Edison was a proponent of DC at many levels and waged a vicsous campaign to
    discredit Westinghouse and Tesla. Fortunately, Westinghouse won that round.

    Early polyphase AC motors were just as capable as DC motors, they just
    developed a bit later. DC was first, but lost a lot of ground due to
    maintenance, size and power transmission issues.
    Ah ha!! Just as I suspected, you're thinking of DC series wound motors.
    They are an excellent 'fit' for that application, but not many others. In
    the case of A/C (the OP's question), series motors would *not* be the best
    fit. A shunt, or compound motor with solid-state armature voltage control
    would be better. But I believe they are actually variable-frequency driven
    AC motors with no brushes (can't find much information about them).

    So your comment of 'switching technology' is probably coming from the
    background of AC motor drives. Newer locomotives have AC generators on the
    diesel, supplying AC traction motors through a large bank of solid-state
    switching. This varies the voltage and frequency applied to the motors to
    suit the operating condition. The AC motor system is much less maintenance
    than the older DC traction motors (which appeals to RR management).
    Absolutely. Some applications require a constant power all the time. These
    are the simplest to design for. A motor that is most efficient at that
    speed and load is the best choice. But other applications present varying
    load conditions. A pump whose flow is varied by some process needs varying
    pumping power at different times. A variable speed drive might be the best
    fit, but capital cost vs. the energy cost to run a fixed speed pump with
    throttle valve have to be considered.

    A locomotive requires high torque for prolonged periods of time while
    starting and running at varying speeds. Each application presents unique
    factors that make motor choice/design a compromise of several factors.

    daestrom
     
  14. You are absolutely correct that I was focusing on the one type of motor, since
    there is has been a _lot_ written about the early advantages of DC motors,
    especially in railfan and traction magazines and books. My initial thought was
    that this might have been how the OP got derailed into dismissing AC, and I
    just continued that line of reasoning to the present day. Thanks for expanding
    the differences in motors in your post, and doing so in a reader friendly
    fashion. Posts like that are what make usenet good reading.
     
  15. daestrom

    daestrom Guest

    Well, from one rail-fan to another, you're welcome. I too am very
    interested in trains and locomotives. I was pleasantly surprised once to
    find myself working on some large 16-cylinder EMD diesel-generators for
    standby power. Looking through the manuals, they kept commenting on the
    differences between stationary-power and 'road-service'. After a while it
    dawned on me, 'these are the same diesel engines used in railroad
    locomotives!!!'.

    daestrom
     
  16. Steve Thomas

    Steve Thomas Guest

    Power factor in AC motors has been mentioned already but there a couple
    of points that should be emphasized. The most basic is that in an AC
    circuit, power does not equal voltage times current unless the effective
    reactance of the circuit is zero, which of course is what power factor is
    all about. The thing is that while the power rating of a reactive device may
    be accurate for the power consumed in the device, the reactive current leads
    to increased power being wasted heating up the power distribution system.
    This difference is glaring in the case of most electronically ballasted
    compact flourescent lights. A typical example of a cheap one that I just
    bought has a current rating of 225 ma at 120 V but the power rating is 13 W.
    This device is rated to draw twice as much current as a purely resistive
    load consuming the same amount of power. Wave form and other issues make
    these ratings insufficient to calculate the exact average increased power
    dissipated in the wiring, but it could be twice as much a well.
     
  17. Mike Glover

    Mike Glover Guest

    I'm with you so far: 225ma x 120V = 27W, power factor = 13/27 = .48
    Now you've lost me. How does the power factor affect losses in the wire?
    And are those losses really enough to be significant?
     
  18. Guest


    This is not inherently so, at least not at those kind of numbers and
    AC motors are much simpler, and thus more reliable. I am not going to
    go into a electricity/motors 101 here tonight, but the reasoning
    behind these claims revolve around speed control issues, DC motors
    are simply easier/cheaper to apply speed control methods to, and the
    newer ones do so efficiently, energy wise. The ad's are still a bit of
    a red herring though, as the advertised savings are comparing a DC
    motor under an averaged variable load demand to a full speed AC motor
    under a full load at max demand. AC speed controls are expensive, the
    newer efficient DC motors are expensive. Overall you may see more DC
    motors in appliances in the future, unless that is AC speed controls
    get cheaper. Right now in industry, at least my view in hospital
    maintenance, we use AC motors with VFD's for the big items like 400
    ton chillers / 50hp AHU motors. The newest smaller items like washer
    sterilizers, assay equipment and such have DC motors with variable
    exciter voltage controllers. Note we have had a LOT of bad luck with
    these new DC motors as they seem extremely susceptible to surges and
    moisture. And they are expensive, in one recent event we found the OEM
    price for a German manufactured washer/sterlizer with an American made
    1/3 hp DC motor to be over $1700! I did save some by buying the same
    basic motor from Grainger ($600), minus the integral speed controls,
    and added an external DC benchtop power supply and SSR from Jameco
    ($100). This did give us the handy ability to alter the speed without
    removing the motor though. As for the general small appliance loads
    you mentioned, the savings take too long to payback, drive up the
    initial cost, and be negated or even worst untill the reliability
    issues are resolved. Unless that is you can get a marketing advantage
    out of it, or driving up the initial and/or long term cost is a
    desirable thing, like it seems to be with hospital equipment :). Hope
    this answered you question.

    Matt


    --
    "It's not what folks don't know that gets 'em in the most trouble,
    it's the things they do know that ain't so" Will Rodgers

    "Any sufficiently advanced technology appears as magic" Arthur C.
    Clarke
     
  19. Steve Thomas

    Steve Thomas Guest

    Reactive current is real current.
    To the extent that there is resistance in the wiring feeding a reactive
    load, there is power dissipated in the wiring in direct relationship to the
    total current. These power losses are real but the lost energy can't be
    calculated from the characteristics of the load device alone. The source
    resistance and reactance of the AC power supply at the point where it feeds
    the load would have to be known. If the source reactance is insignificant
    then whatever the resistance in the wiring, the CF lamp in this example
    creates twice as much heat in the wiring as an equivalent purely resistive
    load. Most high power reactive loads are inductive, so power companies
    switch banks of capacitors in and out of the grid to reduce the power factor
    and thus the circulating reactive current.
    Significance is a subjective measure.
    In this example, the energy lost in heating up the house wiring is twice
    what it would be if the lamp had a power factor of one. The total loss is a
    fraction of a watt in an installation of one lamp only. But if everything
    had such a poor power factor, and it was all one type of reactance, then the
    wiring could only deliver half of the power that would be possible if the
    loads were purely resistive.
     
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