Insane amps drawn by motor?

Discussion in 'General Electronics Chat' started by strantor, Sep 25, 2011.

  1. strantor

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    Looking at this motor, I see the phase-phase winding resistance is .013Ω. Does that mean that the instant I apply 48V, before there is any BEMF it will draw 3,692A?

    How do I get around that? a shunt resistor? I planned on software PWM current limiting, but from what I see here, it looks like my MOSFETs will blow up in the first PWM period.
     
  2. Kermit2

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    Feb 5, 2010
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    It is probably a decimal position error. The closest realistic value would be 10 times that.

    For example:
    30 ft of 16 gauge magnet wire gives approx. .12 to .13 ohms.
    30 ft of 6 gauge is about .012 to .014 ohms.

    I seriously doubt they used anything near 6 gauge wire in the motor.

    The change of a decimal position also fits much more closely to the spec given of about 200 amps for one minute overload capacity on the motor.
     
  3. shortbus

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    Also you might look at the torque rating of these motors. The data sheet says "Peak Stall Torque if 38 Nm", converting that to Lb/FT = 28 ! Not much torque for the amp input.

    The motors you linked to in the other thread had similar ratings. When you look closely at all the data for these motors they don't make much sense, at least not to me. Maybe I'm looking at things wrong, been known to happen.
     
  4. strantor

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    I think it's right actually, other sellers are saying the same thing:
    Keep in mind it's supposed to be a 18hp motor that only weights 22kg. It probably comes down to the fact that it's 8 pole, so 4 high gauge wire coils in parallel, with no brush resistance.
    here's what the inside looks like:
    [​IMG]
    [​IMG]
     
  5. strantor

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    While I am probably axeing my brushless controller, this question is still important to me because I'm back to making a brushed motor controller and the brushed motor I'm considering has a resistance of .025ohms. I've already found this motor on plenty of websites so I am sure about the .025ohm rating.

    this document seems to suggest that software PWM current limiting can still work, but I am still skeptical. they are not talking about motors with nearly zero resistance.
     
  6. shortbus

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    These Guys are playing fast and loose with their HP and Torque ratings on all these motors. The one just linked to says -

    Peak power: 34.3 hp 7.22KW (HP = KW/746) 7220/746 = 9.68 HP

    Torque = 20.5 Nm thats 15.12 lb/ft @ 3480RPM

    HP= (rpm * ft/lb) /5252 = (3480 * 15.12) / 5252 = 52617.6 / 5252 = 10.02 HP

    So where are they getting their numbers? Formulas here; http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
     
  7. Adjuster

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    Dec 26, 2010
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    Perhaps the inductance of the winding has a role in constraining the rate of current rise at start-up. Don't you start with a narrow PWM pulse?
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    As Adjuster says ...

    It's not instantaneous, but the time is dictated by the time constant L/R. The inductance of 0.1 mH and the resistance of 0.013 Ohms gives you about 1 ms for the current to rise to about 10 percent of the 3692 Amps. That's still pretty fast, but it gives you some leeway for electronic current limiting in the brushless design.
     
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  9. strantor

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    Considering they would draw way more amps than they're rated for without a current limiting controller, their HP is really only limited by how far you are willing to push them (heat wise) past their rating. it's also common practice in racing community to exceed the voltage rating and the current limit at the same time. that's probaby where he's getting the 'out-to-lunch' figure of 35hp.
    here's the specs from a reputable retailer (not ebay):
    so 72V X 200A = 14.4KW, or 19.3HP for 10 min. But if you wanted to go for example 300A @96V for 10seconds, you could get 38HP out of it (maybe)
     
  10. strantor

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  11. shortbus

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    I see that they list your name on Ebay, in the bids page as a***n. Have you been bidding on some Kant-twist clamps lately?
     
  12. strantor

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    haha no not me.
     
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