# Is a Brushed DC motor Sinusoidal or Trapezoidal. ?



## major (Apr 4, 2008)

RIPPERTON said:


> The rotor current in a DC motor is alternating but do the brushes and commutator achieve a sine or trap wave form ?


Closer to trapezoid.


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## RIPPERTON (Jan 26, 2010)

Hmm.
So the rotor is AC the field is DC...
how many phases does the rotor have then ?


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## dcb (Dec 5, 2009)

RIPPERTON said:


> how many phases does the rotor have then ?


probably the nearest analogy to "phase" in a dc motor is the rotor segment count divided by number of poles. i.e. what is the angular distance between magnetic "steps", i guess.

Also all the segments are in series (assumption, I should probably double check) so only the segments passing over the brush are doing interesting current things, and are probably dominated by the other segments as they transition, so it *should* be mostly dc, again imho.


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## major (Apr 4, 2008)

RIPPERTON said:


> Hmm.
> So the rotor is AC the field is DC...
> how many phases does the rotor have then ?


I suppose it would be the number of commutator segments divided by the number of pole pairs. Kind of weird because result of that math is often a fractional number, like 16.5. (for a 33 bar comm in a 4 pole motor)


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## dcb (Dec 5, 2009)

here is a guy who is testing fuel pumps by scoping the voltage drop on the relay, clever, comparing each ripple to the others to spot a pump on its way out (electrically).

http://autolabscopediagnostics.com/fuelpump.htm

but this is a pm motor, and the armature is the only current, so it is dc with a bit of ripple basically (from the segment(s) crossing the brushes and reversing current). More commutator bars means less ripple. Fewer poles means individual segments reverse direction less often for a given rpm. Only a two pole two commutator bar (or 4 pole/4bar) motor would see truly ac per segment. Otherwise the rest of the segments (inductors) in the active pole will jam the incoming segments current.

I was curious myself what the life of a segment was like, so I took my best guess based on the motor current waveform from that page (attached), basically trapezoidal, like major said, with ripple on top.


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