Subject:
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Re: Using the Motor as a Tachometer
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 9 Aug 1999 07:28:59 GMT
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Viewed:
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929 times
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In article <FG671F.8BH@lugnet.com>, "Todd Lehman" <lehman@javanet.com> wrote:
> I just hooked an 1x2 LEGO incandescent lamp up to a geared-motor to see what
> would happen, and sure enough, the faster I turned the wheel, the brighter
> the lamp shone.
>
> One funny thing I noticed: I had to apply about twice as much torque to
> turn the wheel with the lamp attached to the motor as I did when the motor
> was hooked instead to a second motor. That is, it "felt" about twice as
> hard to turn the wheel at the same speed. Is this because the lamp is
> super-inefficient at converting electomagnetic energy to visible light,
> while a LEGO motor is super-efficient at converting electromagnetic energy
> back into kinetic energy?
I'm not a EE, but here's my take on the situation...
Turning the motor increases the electrical potential (voltage) between its
two contacts. If the motor isn't connected to anything, then its easy to
turn - relatively little energy is required to make one of the motor
contacts sit at a higher potential than the other. If you short the motor
contacts together, however, then you have a different situation. In order
to maintain a difference in potential, you need a sizeable current moving
through the wires. This requires a lot of work, thus its hard to turn the
motor. Thus how "hard" it is to turn the first motor has to due with how
much current needs to be moved in order to maintain a certain voltage.
At 9V, the incadescent lights will draw more current than the motor. My
rough measurements are about 30ma for the light, and less than 5ma for the
motor (free running). This doesn't directly relate to efficiency. Just
becasue one device draws more power than another, that doesn't necessarily
mean it is any less efficient. For example, a large switching power
supply will draw much more power than a small linear regulator, but the
switcher is almost certainly more efficient.
One reason the motor draws so little energy when running without a load is
that it hardly needs any energy to keep spinning. This doesn't
necessarily mean it is efficient at converting electrical to mechanical
energy. Its just efficient at not losing its present mechanical energy
(angular momentum of the flywheel) due to friction, etc.
To judge efficiency, we would need to quantify how much of the energe
going into the light is emitted as visible light. And for the motor we
would need to measure the amount of mechanical energy it produced.
Dave Baum
p.s. Probably a much longer answer than you were looking for, Todd.
--
reply to: dbaum at enteract dot com
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Message has 2 Replies:
 | | RE: Using the Motor as a Tachometer
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| (...) In fact, this is how the RCX motor controller internally sets the motor up for "brake" instead of "off"... Cheers, Ralph Hempel - P.Eng ---...--- Outside of a dog, a book is a man's best friend. Indside of a dog, it's too dark to read... (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: Using the Motor as a Tachometer
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| (...) Dave, this use of a motor as an input is fascinating! (How safe is it? Looking at some older threads, there seems to be a difference of opinions, or at least a non-consensus that it's safe...) I just hooked an 1x2 LEGO incandescent lamp up to (...) (25 years ago, 8-Aug-99, to lugnet.robotics)
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