Re: Signal in a phase on a DC motor

MooseFET wrote:

On Jun 4, 7:24 am, redbelly <redbell...@xxxxxxxxx> wrote:
On Jun 3, 7:39 pm, Mark2o5...@xxxxxxxxx wrote:

Hello! How are you?

The other day I read this:

"There are three terminals inside most small DC motors, and it acts a
lot like three-phase alternating current [...]. A nice side effect of
this is that the position of the motor can be detected by taking one
of the phases straight into the microprocessor [RA4 in a PIC16C84]."

I would like to know how is it possible. How is the signal on the
phases? Is it a square wave?

Thank you!

First, those 3 terminals are (usually? always?) rotating with the
rotor, so I'm not sure how you could pick a signal off of them.

I guess you could describe things this way, but it would not be a
simple square wave, and would be at whatever frequency the rotor is
turning at.

For a "delta" configuration of 3 rotor coils, (AND if I am thinking
about this correctly), the signal across any two terminals would be:

At +V for 1/6 cycle
At +V/2 for 1/6 cycle
At -V/2 for 1/6 cycle
At -V for 1/6 cycle
At -V/2 for 1/6 cycle
At +V/2 for 1/6 cycle

And then repeat the cycle.

The V/2 values won't be seen in real life. When the brush breaks
contact with the section, there will be a largish spike. After the
spike dies down, there will be some mixture of the V/2 value with a
sine wave. The three windings aren't perfectly coupled to each other
so the nonconnected winding will have some ability to do its own
thing.

It won't be easy to get anything like an accurate angle out of the
voltages. If you brought all 3 connections out to some sliprings, you
could get a position good to about a third of a turn just by looking
to see which pin is at the +V or -V voltage. You only really need two
connections because if two of them aren't the third must be.
I do not think that OP is talking about brush motors, i think 'e is talking
about something like these:
www.hobby-lobby.com/brushless-motors.htm
www.acehobby.com/ace/m_brushless.htm
secure.hobbyzone.com/catalog/ HZ/eflite/eflite_brushless/
http://www.mcg-net.com/electric_motors/brushless-motors.html
--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  
--Schiller
.

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