Author Topic: Basic DC Motor Question from newbie  (Read 3335 times)

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Offline JohnBotTopic starter

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Basic DC Motor Question from newbie
« on: December 24, 2009, 12:40:03 PM »
Hi All,

New to the board and hope you don't mind a few basic questions.

I'm just beginning to experiment with motor and robots. Here's my first basic question:

When I read motor specs that say 3-12 volts and 45 mA no load current (160 mA max) -does that mean that all I need to operate this motor at an efficient point is 12 volts and 45 to 160mA?
Maybe it sounds straight forward, but I'm sort of confused. 12 volts and less than half an amp should be no problem to generate. But it doesn't seem like a lot. The motor I'm considering buying (see below), claims that it can lift 527 oz-in. That's 32.9 pounds! Is it correct that this small, 12 volt motor can lift 32 pounds on a one inch pulley?

Motor: http://www.servocity.com/html/4_rpm_gear_motor.html

Also, if you have a motor that's much bigger, and requires 24 volts and maybe 5 amps or something, where do you get batteries for that? You must have to use a car battery, I suppose?

Anyway, thanks in advance for the help. I can't say enough how I've learned more from reading some archive posts that hours of pouring over books.

In case you're wondering, I'm planning on creating a test project... an arm on an axle that can throw a tennis ball. The arm will be powered with high caliber rubber bands. Then the motor will kick on, turn the axle back, and move the arm back to the 'ready' position. I'm hoping to use a one-way bearing to make this work.
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Offline Razor Concepts

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Re: Basic DC Motor Question from newbie
« Reply #1 on: December 24, 2009, 12:48:44 PM »
The 40 and 160ma are for no load - the motor isn't connected to anything. As soon as you add a load, the amp draw will increase significantly.
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Offline JohnBotTopic starter

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Re: Basic DC Motor Question from newbie
« Reply #2 on: December 24, 2009, 01:02:42 PM »
ah...ok. Anyway to predict how much current it will need?
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Offline waltr

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Re: Basic DC Motor Question from newbie
« Reply #3 on: December 24, 2009, 02:15:38 PM »
A motor will draw the max current when stalled (shaft not turning). The specs on the web page you link to does not give the stall current on the DC resistance so one can only guess at the stall current. I'll go with at least several Amps.

That is a gear motor that reduces the output shaft speed by 900 from the internal motor speed. This in effect multiplies the torque at the expense of shaft speed so yea, it'll lift the weight mentioned.

For motors that draw a lot of current many types of batteries can be used. An important battery spec is its capacity expressed in Amp-Hours or mA-Hr. This will tell you how long the battery will deliver current at the rated voltage with a specified load (current draw). So as an example with a 24V, 5A motor and a battery rated at 20A-Hr (2 medium sized sealed lead acid batteries wired in series): 20Ahr divided by 5A = 4 hours. A 1000mA-Hr battery would last 12 minutes.

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