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Quick question on motor selection. — Parallax Forums

Quick question on motor selection.

morris4019morris4019 Posts: 145
edited 2009-07-05 17:18 in Robotics
So after blowing up one motor, i'm forced to choose through the box of motors that i have available for replacement.

The standard motors that come with the GM9 (Gear Motor from solarbotics), the resistance measurment taken accross the tabs was 8.5 ohms.

8v/8.5 = 941mA

The question i have is should i take these resistance measurements without the capacitors connected to the leads and case?

1amp is alright with me, but, looking through my other motors (and all my motors have bigger stranded coper wire for windings), and the measurements are comming out at about 1.5 to 2ohms. Well...

8v/2 = 4amps !!! ON ONE MOTOR

Is this the correct formula? Even if i run them at 5v thats still 2.5amps.

Would i be better of buying a few of the standard replacement motors that came with the GM9s? Or is there a way i can up the resistance on these motors?

-Michael Morris


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Comments

  • morris4019morris4019 Posts: 145
    edited 2009-07-04 22:57
    So i've found one 2 that closely match the standard motors, resistance is about 8.5 - 9 ohms. Which should be ok. But the problem is that if i run them on 8v, which i would like to, the regulators are spec'd to 1.5amp output.

    -Michael Morris

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  • TimmooreTimmoore Posts: 1,031
    edited 2009-07-04 23:11
    The current draw of a motor isn't purely based on its resistance. A motor is a generator as well. The reason you often get a non-load current and a stall current is this.
    1. non-load, the motor is generating a voltage that is close to the supplied voltage but in reverse, so the voltage across the motor is the difference of the supplied voltage and the generated voltage. This is the voltage to divide by the motor resistance so the non-load current is small. It increases has more load is taken from the motors (the motors slow down so the generated voltage is lower)
    2. Stall, the motor is stopped so it is not generating any voltage, so the current is the supplied voltage/motor resistance.
    The above is a simplification but explains the basics of why the stall current is much higher than teh non-load current and why the motor current increases as the motor does more work.
    Also the start currents drawn by the motors are very high - higher than voltage/resistance expects because the motors are inductors and they take current to generate their field. You often find that the start current is more than the stall current for very short periods of time, similar for reversing (in fact for reversing, the supplied voltage is in the same direction as the generated voltage so you have 2x voltage across the motors, so 2x stall current, which is why regulators for motors are normally a bad idea)

    Also pololu.com sell replacement motors for the solarbotics gearmotors. http://www.pololu.com/catalog/product/604
    and upgraded motors http://www.pololu.com/catalog/product/604
    they also sell http://www.pololu.com/catalog/product/24 which is a beter version of the l293d
  • W9GFOW9GFO Posts: 4,010
    edited 2009-07-04 23:27
    You could put a motor in series with a multimeter set to measure amps to really know what it draws. Even my el cheapo Harbor Freight meter can measure amps, up to 10 amps I think.

    Rich H
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-07-04 23:37
    And, if the motor's not too hefty, load it and watch the amps - H

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  • PrettybirdPrettybird Posts: 269
    edited 2009-07-05 10:36
    Watts are power no matter what. No load and stall current is often used on larger motors. Also something to keep in mind when using DC motors in servo situations (not meaning servos in model applications) Frequency is also a factor for changing speeds. Measuring amps with a meter is the best bet to make things simple.
  • kwinnkwinn Posts: 8,697
    edited 2009-07-05 17:06
    Michael, here is the info on the GM9 motor from the solarbotics web site.

    At 5 V, the gearbox and motor provide 43 oz-in of torque and 66 RPM, which is faster than a standard RC servo. The free-running current is 74 mA, and the stall current is 670 mA. This motor also comes with a built-in safety clutch.

    The 2 motors you have are probably very similar to that so they should be powered from a 5 or 6V supply to reduce the risk of cooking the motors. Even at 5V it is possible to cook the motors if they are stalled for a long period of time.

    If you use a 5-6V battery pack and connect the motor/L298 directly to the battery pack NO REGULATOR IS NEEDED FOR THE MOTOR CIRCUIT. The micro and other electronics will need regulators.
  • morris4019morris4019 Posts: 145
    edited 2009-07-05 17:18
    Ok, a redesign is needed then to incorporate another battery. That is that. Thank you all.

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