# Signal in a phase on a DC motor

Discussion in 'Electronic Design' started by [email protected], Jun 4, 2007.

1. ### Guest

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!

It sounds like they are referring to a brushless DC motor. Try a
search on BLDC or brushless DC for an introduction. "most small DC
motors" seems an exageration to me but it may be correct or it may
depend on how you define small.

Myself I'd quibble about them actually being DC but the terminology has
stuck.

Robert

3. ### redbellyGuest

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

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.

Mark

p.s Note to Robert A: since the OP refers to "most" small DC motors, I
think they mean the standard, brushed type.

4. ### MooseFETGuest

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.

5. ### MarraGuest

amounts of noise.

6. ### MarraGuest

I did a project once to try and get tacho pulses from the commutator
and it worked fine at a set frequency but needed lots of filtering.

I suppose you could add another brush as a pickup. That does assume
that a brushed motor has three phases wound on the rotor which seems a
questionable assertion although more likely true on a small motor.

Well it depends on the population they are thinking of. If your
population is dominated by PC cooling fans and disk drive motors you'll
certainly think most small DC motors are brushless.

I really don't know what dominates, BLDC, stepper, PM... I wouldn't be
surprised if the most dominant was universal.

Robert

8. ### joseph2kGuest

I do not think that OP is talking about brush motors, i think 'e is talking