spark "suppression" possible on brushed DC motor?

Discussion in 'Physics' started by praondevou, Jan 14, 2012.

  1. praondevou

    Thread Starter AAC Fanatic!

    Jul 9, 2011
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    The OP of this thread asks for a spark suppression on a brushed DC motor.

    First of all, can a spark at brushes/the commutator better be described as a spark or as an arc? A spark is instantaneous, while an arc is sustained. At what duration do I say arc or spark?

    Let's say the brushes ar connected to two poles of a multi-pole DC motor. When the brush leaves the commutator the distance is so small that immediatly a spark begins to appear between both. That means current flow continues. When the spark stops, the inductance of the winding that was just energized causes a voltage to appear on that winding. When this voltage is high enough sparking can occur again.

    Is my reasoning correct so far?

    The voltage on the winding is inaccessible since it is on the rotor. So a true spark suppression would only be possible on the winding not the stator, as this website seems to confirm.

    A snubber on the stator would help to avoid propagation of transients on the power supply lines that's true, but what would it do to the length/intensity of the spark? There are often capacitors directly mounted on the motor terminals. How do they contribute to spark suppression? Do they at all? Or is their main function to suppress RF? Or to provide sustained current in high frequency PWM applications?

    I understand that having a capacitor at the motor terminals shunts the high voltage that would come from the winding through the sparks to them. Without it there is still the motor power supply wire inductance in series until we get to the main DC-bus caps.

    Informations on the web are contradictious. I tried to find a reliable source of information on this subject but failed. Something from a motor manufacturer maybe or a university.
    THIS document describes very well forms of arc suppression and that these methods have no or little effect on the spark generation.

    Can anybody shed some light on this? Some practical experience maybe? Did someone measure the length of sparks?

    Btw, the answer to question 9 in AACs http://www.allaboutcircuits.com/worksheets/dcmotor.html does not sufficiently explain why commutating a winding would cause a spark.
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    A lot of the sparking has to do with the number of armature poles and the way the armature is wound, "lap" or "wave" winding. Another thing is if the motor has to be run in both directions, clockwise and counter-clockwise.

    If a motor is only made for one direction, like a treadmill motor, the brushes are off-set a small amount from ture diagonal. The brush off-set helps to keep sparking to a minimum, but if the motor is run in the opposite direction there is much more sparking and lower performance.

    If a 'used' motor has been turning one direction most of its lifetime and then is made to run backward, it will spark mor because of the brush wear pattern.

    Don't know of any good way to suppress sparking though.
     
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  3. shortbus

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    Sep 30, 2009
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    To answer the question about the thread, I think he is changing the frequency of his PWM rather than the duty time. From what he wrote on ETO he's trying to make the motor run real slow, a 3000 RPM motor at 250-300 RPM if I remember right. And by lowering the frequency he thinks it will turn slower. Told them he didn't understand gearing.
     
  4. praondevou

    Thread Starter AAC Fanatic!

    Jul 9, 2011
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    This "the-same-subject-over-several-forums-posting thing" the OP is doing I don't understand.

    I think we would have everything in my company to measure spark duration on a brushed DC motor... Would be very time-consuming I guess.
     
  5. amilton542

    Active Member

    Nov 13, 2010
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    This can be referred to as armature reaction.

    In larger machines they use interpole windings in contrast to compensating windings in order to neutralise the reaction so the brushes lie in the neutral plane axis.

    It can also be achieved by means of a brush-rocker as to shift the brushes, depending on the function of the machine the direction of shift will differ.

    I often refer to a spark as electrostatic and an arc as some transient e.g. short-circuit.
     
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