Signal in a phase on a DC motor

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

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.

Mark

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

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.

amounts of noise.

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

I do not think that OP is talking about brush motors, i think 'e is talking about something like these: