Conference vmszoo::rc

    
    
      Bob,
    
           Its me again. YES I have seen similar measurments. I work
      in the NIO metrology lab and I hope I kbow what I'm doing. I have
      a trinity modified motor that draws almost 40 amps static with
      a 7.2v pack swinging an 7x6 prop. (too much prop I think) I also
      measured the RPMs at 10,500 when it was new. I do tend to go through
      a lot of brushes though. About every 10 flights of 4-5 minutes
      I start to lose power and the brushes start to get hot and then
      they start to cut out or sound real noisy. I think idealy 25amps
      static is what i want.

    I think you guys are confused.  I've flown a variety of electrics over
    the last few years.  I've never seen a Ferrite draw even as much as 20
    amps, except when stalled.  I check all my power system installations
    with a clamp on ammeter (which I periodically check for calibration)
    before I installa fuse.  I get a current reading with direct hookup on a
    bench, and then choose an appropriate fuse.

    As an example, an Astro 05, turning a 7-6 prop draws just about 18 amps
    at 7.2 volts, static, and will hold speed for about 3.5 minutes.
    Unloaded in the air, that's good for about a 5 minute full power run.

    There's no way you could be getting 240 watts out of a ferrite motor.
    125 watts out of a cobalt 05 is about the top of the range.  240 watts
    would fry the little sucker in an instant.

    To quote from an article by Keith Shaw "Designing Electric Scale
    Aircraft"...

    ***
    
    ...The following is a table relating static current drain to approximate
    motor run.

    Static Current  Motor Run (in minutes)
        10              8.5-11
        15              6-7.5
        20              4.5-5.5
        25              3.5-5.5
        30              3-3.5

    In addition, the maximum current capacity of the motor must be
    considered.  For the long motor runs that are common is sport scale, 15
    amps is about maximum for ferrite motors, while cobalt motors can go up
    to 20-30 amps depending on size.  For short motor runs (less than one
    minute) many cobalt motors can stand 40-60 amps!  I typically use a 20
    amp static load for my scale planes; this gives me 4.5 to 5.5 minutes of
    good performance, or 7-10 minutes of just cruising around the sky.

    ***

    from a separate source, the AstroFlight data on their family of Cobalts:

    Motor           Battery     Power Output
    Cobalt 020      4x800         50 watts
    Cobalt 035      5x800         90 watts
    Cobalt 05       7x900        125 watts
    Cobalt 05FAI    7x1200       200 watts
    Cobalt 15       12x900       200 watts
    Cobalt 25       14x1200      300 watts
    Cobalt 40       18x1200      450 watts
    Cobalt 60       28x1200     1200 watts

    ***
    
    I'de say you got either:

        A) Bad Meter
        B) Bad Shunt
        C) Need of a refresher course in electronics 8^)

    Seriously, try using your 25 amp meter directly, w/o a shunt, but give
    it a 20 amp fuse.  if the motor's acutally drawing the kind of current
    you seem to feel it is, the fuse will blow immediately, saving the meter
    from damage.  If it isn't, you'll know that the problem lies with the
    meter, or shunt.


    Randy

    I thought I knew and believed everything you said until I took the
    readings.  The meters were checked by running them in series, and the
    ones without the shunt go off scale.  At less than 25 amps the shunted
    meter indicates exactly 1/2 of the others, and at 4 to 16 amps, they 
    agree with my Simpson 269, which is a good quality meter.
    
    That's why I tried timing the discharge to see if I was in the
    ballpark.  Two minutes flat to discharge a 1200 mah seven cell pack
    works out to just about 35 amps. Both motor and batteries got hot, but
    not so hot you couln't hold them.  Some of the guys at this weekend's
    fun fly are running SAR cells so hot you don't want to touch them.
    
    Using the identical meter, my geared cobalt 15 swinging a 12" X 8"
    Master Airscrew folder, draws less than 20 amps from my 12 volt 
    trolling motor battery, and around 22 amps on its twelve cell pack.
    
    My logical mind says there is still something wrong with what I've
    observed, but whatever it is still escapes me.

re two notes by Randy  "I check .... with a clamp on ammeter"

    Ummmn, Randy.  Without entering the application discussion of these
    notes, I am puzzled by the reference to a clamp-on ammeter in a DC
    situation.  I had thought that clamp-on's measured AC current and were
    really little transformers with a flux path that could be opened.  I
    didn't think they were Hall effect (or similar) devices.  Although I
    own one as an attachment to my Simpson 262, I have never taken it
    apart.

    If the [otherwise] DC current were chopped, either intentionally or
    implicitly by the motor brushes, then the AC component could be
    significant but still generally different in value from the DC part.
    The AC reading would not be an accurate measure of the DC.

    Alton who hopes his electricity understanding is better than his
    reference to female forms (in another note topic)

    
    
       Hmmmmm,  Maybe i am a bit confused, only I made the measurements
    many times. I used an tektronix am503 current probe. I will admit
    that I am the one who calibrated this probe.(don't say anything
    if I'm wrong) I surely am not an electric motor wizard but what
    I see is what I believe.  The motor I use is not ferrite, it's a
    wet magnet? . Not that I know the difference. Cobalt, from what
    I had understood enables you to heat it up with a reduced chance
    of demagnetizing right?. I beleive that the I/draw would mainly
    be determined by the resistance of the windings (lower the R the
    higher the I) Although there may be some amount of inductive feedback
    I don't think that adds to much. So if the motor can be stalled,
    with no fuse in place, can't it draw a max of what the minimum
    resistance of the winding (the one being fried at this point) and
    what the batt pack can supply?. If I measured the resistance stalled
    at .1 ohms and supply 7.2V I get 72 AMPS!!. of course the pack and
    the wiring and the motor won't last long. 
    
      So am I crazy? and would y'all send me your meters for calibration?
    
    kevin p (as in metrologeee)

    RE .39
    
>    I thought I knew and believed everything you said until I took the
>    readings.  The meters were checked by running them in series, and the
>    ones without the shunt go off scale.  At less than 25 amps the shunted
>    meter indicates exactly 1/2 of the others, and at 4 to 16 amps, they 
>    agree with my Simpson 269, which is a good quality meter.

    Sounds reasonable.
    
>    That's why I tried timing the discharge to see if I was in the
>    ballpark.  Two minutes flat to discharge a 1200 mah seven cell pack
>    works out to just about 35 amps. Both motor and batteries got hot, but
>    not so hot you couln't hold them.  Some of the guys at this weekend's
>    fun fly are running SAR cells so hot you don't want to touch them.

    I think you mean SCRs, and yeah, you can run 'em awful hot.  Still, 35
    Amps is a LOT.   If that's the power you're drawing, they would have
    gotten too hot to handle.  I've run 20 amp discharges, and the pack and
    motor (Astro 05) are more than uncomfortably warm after the run.  The
    shortest runs I've ever seen, or read about in electrics is about 1.5
    minutes, for pylon races run with with something like an FAI wind Cobalt
    05 that's way overpropped.  These are drawing 35+ Amps, and are at the
    limits for what they batteries can deliver, and the motors can handle.

    The capacity of an SCR cell is significantly impacted by discharge rate.
    At 1C, a 1200 MaH cell delivers slighty more than 100% of it's rated
    capacity.  At 4C, that figure is reduced to 75%, and at 10C (which is
    only 12 Amps, remember) the cell capacity is reduced to only about 60%
    (these figures from a Sanyo Applications guide).

    What are you considering "Discharged"?  You should stop timing when the
    pack gets to 1.0 Volts per cell (6 Volts on a 6 cell pack).
    
    What size prop are you spinning?  My Astro 15 will turn a 10-6 and draw
    25 Amps, static.  This prop is too large for the motor, and it gets
    *REAL* hot if I let it run (I don't actually use this prop, it was part
    of my experiments at propping the motor).
        
>    Using the identical meter, my geared cobalt 15 swinging a 12" X 8"
>    Master Airscrew folder, draws less than 20 amps from my 12 volt 
>    trolling motor battery, and around 22 amps on its twelve cell pack.

    This sounds right.
    
>    My logical mind says there is still something wrong with what I've
>    observed, but whatever it is still escapes me.

    I agree.


    Re .40
    
>    Ummmn, Randy.  Without entering the application discussion of these
>    notes, I am puzzled by the reference to a clamp-on ammeter in a DC
>    situation.  I had thought that clamp-on's measured AC current and were
>    really little transformers with a flux path that could be opened.  I
>    didn't think they were Hall effect (or similar) devices.  Although I
>    own one as an attachment to my Simpson 262, I have never taken it
>    apart.

    Well, it's got a switch for DC, and a set of arrows on the sensor
    showing which way current should flow for positive readings, and it
    reads signed values.  Being as it clearly does what I want, I can't say
    I ever stopped to consider how it worked (I know, where's my inquisitive
    mind?), but you're right, you sure couldn't measure DC with an inductive
    couple.  Perhaps it is Hall Effect.  Have to take a look at it tonight.
    

    Re .41
        
>    I see is what I believe.  The motor I use is not ferrite, it's a
>    wet magnet? . Not that I know the difference. Cobalt, from what
>    I had understood enables you to heat it up with a reduced chance
>    of demagnetizing right?. I beleive that the I/draw would mainly
>    be determined by the resistance of the windings (lower the R the
>    higher the I) Although there may be some amount of inductive feedback
>    I don't think that adds to much. So if the motor can be stalled,
>    with no fuse in place, can't it draw a max of what the minimum
>    resistance of the winding (the one being fried at this point) and
>    what the batt pack can supply?. If I measured the resistance stalled
>    at .1 ohms and supply 7.2V I get 72 AMPS!!. of course the pack and
>    the wiring and the motor won't last long. 

    I don't know the difference, either. Cobalt magnets have higher
    magnetization for a given weight and take heat better without
    demagnetizing, as compared to Ferrites.

    As for the basics of electricity, yes, I agree.  Dead short the motor
    and it pulls the maximum current.  The batteries have an internal
    resistance, too, though, and they'll have a significant effect on the
    maximum current that can be delivered.  SCR cells typically have 5 to 10
    MilliOhms internal resistance.  In any case, I'de strongly dissuade you
    from actually trying this experiment.  Snap! Crackle! Pop! And I don't
    mean Rice Krispies, either.
    

    This discussion has gotten away from the Electricub, but seems worth
    continuing, so I'll leave it up to Alton to decide whether it should be
    moved elsewhere.
    
>>>    I think you mean SCRs, and yeah, you can run 'em awful hot...
    
    No, these people were using SARs, some of which were rated up to 1200
    mah.  The reason that I mentioned it, was that so many people seemed 
    to be using them. With the excessive current drains for those size 
    cells, I would expect very poor efficiency, but it sure makes for a 
    small pack.  My own packs are all SCRs.
    
>>>    The capacity of an SCR cell is significantly impacted by discharge rate.

    I was aware that internal resistance losses climbed with output current, 
    but had never seen any actual tables.  At 60% of rated capacity, a 1200
    mah cell would only last about 2 minutes at 22 amps.  My 15, drawing 22 
    amps would only run 1.5 minutes on 900 mah cells.  That seems low. 
    I'll have to time it to be sure.  BTW, actual flights at partial 
    throttle, with the Amptique, exceed ten minutes.
    
>>>    What are you considering "Discharged"?  
    
    Significant drop in RPM, indicating the pack is over the knee.  This
    was just a rough test to confirm or deny the meter readings.
    
>>>    What size prop are you spinning?  
    
    I believe its an 8"X4" Tornado, which shouldn't be a major overload.
    
    RE: .40
>>>    I beleive that the I/draw would mainly be determined by the 
>>>    resistance of the windings (lower the R the higher the I) Although 
>>>    there may be some amount of inductive feedback I don't think that 
>>>    adds to much. 
    
    In a stalled motor, there is no inductive feedback, so resistance does
    determine the current flow.  On the other hand, inductive feedback is
    the primary factor which determines maximum RPM.  With no load, one of 
    my motors draws only about two amps.  
    
    To increase power, you have to increase flux density with either more 
    current or more windings.  The difference between an Astro FAI 05 and 
    a 15 is a slightly larger iron case on the 15, and 4 turns versus 11.  
    They are both 200 watt motors but the 15 uses more voltage and less
    current.  All of these motors have an ideal RPM for peak efficiency, 
    where resistance losses are minimal.  That's why a geared motor is 
    more efficient than a direct drive loaded down with a big prop.

>    This discussion has gotten away from the Electricub, but seems worth
>    continuing, so I'll leave it up to Alton to decide whether it should be
>    moved elsewhere.

    I agree.
    
>    No, these people were using SARs, some of which were rated up to 1200
>    mah.  The reason that I mentioned it, was that so many people seemed 
>    to be using them. With the excessive current drains for those size 
>    cells, I would expect very poor efficiency, but it sure makes for a 
>    small pack.  My own packs are all SCRs.

    You must mean that the packs are "A" Cell size.  I didn't know anyone
    made SAR cells.  Are you sure they weren't the 2/3C or 3/4C that you
    typically find in 600 MaH or 800 MaH size packs (note that this size
    cell is available in capacities as high as 1250 MaH)?
    
>    I was aware that internal resistance losses climbed with output current, 
>    but had never seen any actual tables.  At 60% of rated capacity, a 1200
>    mah cell would only last about 2 minutes at 22 amps.  My 15, drawing 22 
>    amps would only run 1.5 minutes on 900 mah cells.  That seems low. 
>    I'll have to time it to be sure.  BTW, actual flights at partial 
>    throttle, with the Amptique, exceed ten minutes.

    The data I quoted was directly from a table published in a Sanyo
    Applications Guide.  I have it here at my desk, so the numbers weren't
    from memory, or anything.
    
>    Significant drop in RPM, indicating the pack is over the knee.  This
>    was just a rough test to confirm or deny the meter readings.

    That's not a valid measurment.  As current draw increases, the "knee"
    spreads out.  At low current drain, a typical SCR type cell maintains
    95%+ of it's rated voltage for better than 90% of the discharge period.
    As draw goes up, the "knee" straightens out a bit, and that figure drops
    to nearer 80%.  If you're just listening to the prop spin, you may be
    shortchanging your measurment.  Do what the manufacturers do, and
    measure the pack voltage, waiting until it reaches 1V/Cell.
    
>    I believe its an 8"X4" Tornado, which shouldn't be a major overload.

    No, not at all.