DC motor EMF spike-to-RPM ratio

Discussion in 'General Electronics Chat' started by boriz, Aug 6, 2009.

  1. boriz

    Thread Starter New Member

    Jul 16, 2009
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    Hi.

    When observing the back EMF spikes produced by a small DC motor on a CRO, how many spikes make a rev?

    My gut says two. But I need a better explanation.

    Thanks.
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    There will be more than 2. They are commutation events caused by the brushes connecting to the new segments on the commutator. It depends somewhat on the number of segments on the commutator, tiny DC (toy) motors generally have 2 brushes and 3 commutator segments (to 3 coils), so you get 6 pulses per rev.

    Here's a circuit to detect the commutation pulses and send logic level pulses to a PIC etc;
    http://www.romanblack.com/encoder.htm
     
  3. boriz

    Thread Starter New Member

    Jul 16, 2009
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    0
    6! That makes no sense to me. Can you explain please? Thanks for the link, but all it says on actual RPM measurement is: “Speed measurement is best performed by measuring the number of pulses over a timed period and not by measuing the time between 2 pulses!”
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    1,305
    There are 2 brushes, connecting to 3 copper segments in a circle (the commutator). One full motor rotation gives 6 separate events.
     
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