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Model Forum / Radio Controlled / Air Models / November 2006Futaba S3003 servo current draw?
What is the current draw on the servo while running? Not just the idel current. Know it will varry with load but a nominal value will be a good start. Tried the Futaba site to no avail. Thanks for any help Gary > What is the current draw on the servo while running? Not just the idel > current. [quoted text clipped - 6 lines] > > Gary Dunno but 9gm HS55's peak at about 250mA, a bigger one might be up to an amp or so. Plug a meter in and try... > What is the current draw on the servo while running? Not just the idel > current. [quoted text clipped - 6 lines] > > Gary Figure on 125ma, but it can easily go as high as 3-4 amps with large control surfaces at high speeds... Cheers, Bill >> What is the current draw on the servo while running? Not just the idel >> current. [quoted text clipped - 14 lines] > > Bill To date the testing we have done tends to indicate STANDARD servos (42 to 50 ounce inch range) stall at about 250 to 270 ma. It is reasonable to assume that higher torque servos draw more power. >>> What is the current draw on the servo while running? Not just the idel >>> current. [quoted text clipped - 18 lines] > ounce inch range) stall at about 250 to 270 ma. It is reasonable to assume > that higher torque servos draw more power. All of this tells me that servo ratings should be taken with a large grain of salt. The S3003 is rated at 57 in-oz of torque, with .16sec / 60deg. That means that if it did a full circle, it'd take about 1 sec. For a one-inch arm, that'd be a distance of pi inches. And pulling 57 ounces, we'd get... 57 pi oz-in/sec Using my favorite units-conversion program, that's a power of about 7.6 Watts. At 6V, that should take about 1.3 amps, *assuming no losses*. So if the thing is stalling out at 250ma, it's clearly not managing to produce the rated torque. As for current going as high as 3-4 amps, that'd mean that the power losses were about a factor of 3, and that the servo would be putting out as much heat as a 15W bulb, more or less. Those (the ones for sewing machines, for instance) are about the same size as a servo, and they get pretty toasty; I have a hard time believing that a servo could get that warm without melting something inside. So: stall at 250ma => at most 1/4 of rated torque is being produced actually draw 4A => huge inefficiency and LOTS of heat If I had to bet one way or the other, I'd believe the 250ma estimate. Then again, if I were betting my plane, I might want to up that to 1.5 amperes just to be a little more on the safe side. -John >>>> What is the current draw on the servo while running? Not just the idel >>>> current. [quoted text clipped - 24 lines] > sec. For a one-inch arm, that'd be a distance of pi inches. And > pulling 57 ounces, we'd get... Ooops! Make that 2pi inches, so double everything: > 57 pi oz-in/sec > > Using my favorite units-conversion program, that's a power of > about 7.6 Watts. At 6V, that should take about 1.3 amps, *assuming no > losses*. Corrected version: 15W, so 2.6amps. > So if the thing is stalling out at 250ma, it's clearly not > managing to produce the rated torque. As for current going as high as > 3-4 amps, that'd mean that the power losses were about a factor of 3, > and that the servo would be putting out as much heat as a 15W bulb, Revised: power losses of 40%, 6W heat output. Not too extreme. > more or less. Those (the ones for sewing machines, for instance) are > about the same size as a servo, and they get pretty toasty; I have a [quoted text clipped - 3 lines] > So: stall at 250ma => at most 1/4 of rated torque is being produced > actually draw 4A => huge inefficiency and LOTS of heat Revised: stall at 250ma -> at most 1/8 of rated torque (!) draw 4A -> modest inefficiency, and some heat. > If I had to bet one way or the other, I'd believe the 250ma > estimate. Then again, if I were betting my plane, I might want to up > that to 1.5 amperes just to be a little more on the safe side. OK. With the new numbers, I'm thinking I'd design for 3A to be safe... --John (with egg on his face) >>>>> What is the current draw on the servo while running? Not just the idel >>>>> current. [quoted text clipped - 60 lines] > > --John (with egg on his face) but is the speed quoted at zero load or full load I suspect its either 57oz of torque or .16/60deg not both >>>>> What is the current draw on the servo while running? Not just the idel >>>>> current. [quoted text clipped - 60 lines] > > --John (with egg on his face) Interesting :) but wouldn't that only be correct if the motor was directly connected to the servo arm?  SignatureRick -- <estimates of current deleted> > Interesting :) but wouldn't that only be correct if the motor was > directly connected to the servo arm? Nope: a certain amount of power goes into the servo mechanism; as a result, it does a certain amount of work over time, and generates some heat. The "power in" has to equal the "power out" (which is why "power" is a thing that's worthwhile to define as a physical term!). If you have a 6-to-1 gear-down, then instead of 57in-oz of torque, the motor only has to produce 9.5 in-oz of torque...but it does so over a far greater distance (i.e., it has to make 6 times as many revolutions). The net power consumption is the same as if you had a tougher motor, turning slower. (Ignoring all frictional losses, etc.) --John > <estimates of current deleted> >> Interesting :) but wouldn't that only be correct if the motor was [quoted text clipped - 5 lines] > "power" is a thing that's worthwhile to define as a physical > term!). The point that was made earlier by someone else, which completely belies your analysis and which seems to have totally escaped your attention, is that the specifications of servo maximum torque and speed are made in such a way as to prohibit multiplying the two to get the servo output power. This means that the power figure you used to start with is completely specious and so working back to get a current draw is also meaningless. I have measured high(ish) torque Futaba 9450 servos stalled current draw at 6V to be around 1.1A, 3A might just possibly apply to some very high end servos but not to any "standard" analogue servo currently available. SignatureBoo >> <estimates of current deleted> >>> Interesting :) but wouldn't that only be correct if the motor was [quoted text clipped - 16 lines] > to be around 1.1A, 3A might just possibly apply to some very high end servos > but not to any "standard" analogue servo currently available. The answer about "power in = power out" was to a question about gearing, and independent of the torque-and-speed-simultaneously issue. As for the torque and speed ratings being independent, it's pretty safe to say that their product puts an UPPER BOUND on the power draw; since I was only trying to see whether the "5 or 6 amps" (or whatever was claimed) figure was reasonable, using this is a good first shot. If the product had come out more than 6 amps, I'd have learned nothing. Since it comes out considerably less, I can dismiss the 6A figure (or figure that the heat losses would have to be huge). Actually measuring is clearly a superior method for finding the true numbers; I just happened to be at work and trying to use the information at hand to do a back-of-the-envelope sanity check on a figure that sounded odd to me. --John > The answer about "power in = power out" was to a question about > gearing, and independent of the torque-and-speed-simultaneously issue. Oh, well I should have read that more carefully then. > Actually measuring is clearly a superior method for finding the true > numbers; I just happened to be at work and trying to use the information > at hand to do a back-of-the-envelope sanity check on a figure that sounded > odd to me. Fair enough :-) SignatureBoo OK...OK.....OK I measured two 3003's (used) with virtually identicle results. Idle current = 26ma Unloaded transit current = 105ma Stall current = 430ma This was with a 4.8 volt nicad pack at 5.1 volts. I have no 6.0 volt packs with which to test. I think I read somewhere in this thread that the original question poser was thinking of using these servos with 12V. That should be interesting, at least for a few minutes......... > OK...OK.....OK > [quoted text clipped - 6 lines] > This was with a 4.8 volt nicad pack at 5.1 volts. I have no 6.0 volt packs > with which to test. I just tried a 3001 with a 6v pack (note the resistor I was using wasn't real accurate, so I'd allow 10% error, and the 6v pack might have been a little flat). I used a digital scope to calculate an average draw - it's more accurate than a normal meter as it can interpret the current-draw waveform in a more meaningful manner. Idle: 14mA Very lightly loaded transit: 60mA Stall: 500mA >>>> What is the current draw on the servo while running? Not just the idel >>>> current. [quoted text clipped - 32 lines] > losses*. So if the thing is stalling out at 250ma, it's clearly not > managing to produce the rated torque. I had to think hard to see where your reasening went wrong. But its at this point. You assume that at max torque the servo still manage to do 60 degrees in .16 seconds. But it does not. At stall torque it either stalls (no power in the movement) or it moves very slowly (again very little power in the movement). The current then only depends on the current the motor draws to generate the torque. And that is in a DC motor totally dependent on the resistance of the coils of the motor. Peter. | Peter J. de Vrijer e-mail: pdevrijer_nospam_@home.nl | zendamateurs: pa5dv_nospam_@amsat.org | | And perhaps the horse will learn to sing! SNIP > | Peter J. de Vrijer e-mail: pdevrijer_nospam_@home.nl > | zendamateurs: pa5dv_nospam_@amsat.org > | And perhaps the horse will learn to sing! Is this akin to pigs flying? > Figure on 125ma, but it can easily go as high as 3-4 amps with large control > surfaces at high speeds...# That's complete bollocks - you'll never draw an amp out of a 3003, more like 1/4 to 1/2 an amp absolute tops. You only get just over an amp from something like a 9450, let alone a piddly little analogue device like the 3003. SignatureBoo Thanks for the help. Got it (motor only) working on 12 volts. Only using the motor with limit switches for an open /closed servo. Cant get a reading with our meter as it is digital and to slow ro read before the servo hits the stop. Gary > > What is the current draw on the servo while running? Not just the idel > > current. [quoted text clipped - 13 lines] > > Bill I'm surprised you haven't let the smoke out of that servo... It's designed to operate on 6VDC max... It may work a while as long as you're not using the internal circuitry.. But I'll bet stall current will cause a meltdown in short order... Bill > Thanks for the help. Got it (motor only) working on 12 volts. Only using > the motor with limit switches for an open /closed servo. [quoted text clipped - 22 lines] > > > > Bill | I'm surprised you haven't let the smoke out of that servo... It's designed | to operate on 6VDC max... It may work a while as long as you're not using | the internal circuitry.. But I'll bet stall current will cause a meltdown | in short order... Well, we do often take 6V speed 400 can motors, and run them at 12 volts -- and do so for many minutes at a time. The motors don't last forever, but they don't usually burn up in a few minutes either. As for stall current, stalling your servo for several minutes at 6 volts would probably cause a meltdown too -- just a slower one. Three watts (0.5 amps x 6 volts) may not sound like much, but when it's all being generated in that tiny motor with next to no cooling ... it won't last long. (The 0.5 amp figure is a rounded off approximation of the 3003 stall current somebody just measured.) What I've always wondered about is *why* digital servos will create more torque. Higher power consumption, especially while idle makes sense, as the servo will `whine' as it tries to find the very center, but higher torque doesn't make much sense. Seems to me, if you take a given servo mechanism, with a given motor and a given geartrain, the maximum torque produced should be the same no matter what sort of electronics drive that motor. Or do digital servos typically just come with bigger motors? (so it's not the `digital' part that gives more torque -- it's the bigger motor that comes with them?) SignatureDoug McLaren, dougmc@frenzied.us Time is the fire in which we burn. > Thanks for the help. Got it (motor only) working on 12 volts. Only using > the motor with limit switches for an open /closed servo. > > Cant get a reading with our meter as it is digital and to slow ro read > before the servo hits the stop. Grab hold and don't let it turn for a quick reading. Stall current will be a good indication of maximum current draw. Startup may be higher, but it is so short, as to not impact the real world very much. SignatureJim in NC |
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