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Physics/What about the brush noise (pulse signal) of a DC motor ?

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Question
Hello Steve

I have a ferrite coil and would like to send a pulse signal through it, the science magazine suggests a DC motor which works between 6-12 volts. It is said that the brush noise from the DC motor is providing a pulse signal to the coil.
Hmmm, what kind of pulse would this be, I am curious, if I use a simple toy motor with an electrical potential of, lets say 9
volts ?

It this a kind of full wave rectification wave, with a disturbed (not harmonically) wave character ?
Hmmm, what would happen, when I use a signal generator with different frequencies and send the current to the coil, after a full wave rectification through a diode bridge ?
Here I can play with the DC frequency, am I right ? The pulses that I send would not be disturbed anymore, which other positive effects can you mention ?
Is here a limitation on the sent frequencies, for example can I send rectified AC currents in kHZ or MHz values, why ?

In which situations is it better to use a motor described above, to pulse the DC current input and in which situations should we rectify the current output of a signal generator over a diode bridge ? Why ?

Are both effects the same ?

Many thanks for your help,
Birol

Answer
Hello Birol,

There are several variations on the basic DC motor that Wikipedia describes in this article.
http://en.wikipedia.org/wiki/DC_motor
I'll assume that your magazine was referring to a motor like the one in Wikipedia's animation. The commutator has just 2 halves and accomplishes reversal of the current in the rotor every 1/2 revolution. The battery current to the motor would flow in only one direction with brief interruptions of zero current while the brush is loosing connection to one commutator half and making connection to the other commutator half. The current would flow continually at a reasonable constant amperage for an entire half revolution, go to zero briefly, and then resume flow for the next half revolution.

Hmmm, what kind of pulse would this be?
Where would you put this ferrite coil? A ferrite coil is basically an inductor. If you put it in series with the motor, so the motor current flows through the ferrite coil, the coil would try to maintain the current during the interruption. Where would you try to harvest a pulse?

It this a kind of full wave rectification wave, with a disturbed (not harmonically) wave character ?
No, it would be a pulse train: onnnnnnnnn,off,onnnnnnnnn,off,onnnnnnnnn and so forth.

Hmmm, what would happen, ...?
I need to understand the connections that you would make. I would need to see the circuit schematic before I could predict what the circuit would do.

Sorry, I really don't understand enough about what you're doing to try to answer any further.

Steve

Physics

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Steve Johnson

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I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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