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  • DC Motor Current Draw Variation Between Motors

    Discussion in 'The main mechanical design forum' started by JonGreene, Mar 11, 2015.

    1. JonGreene

      JonGreene New Member

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      Hi All,

      This is my first post. I am a senior mechanical engineering student looking for some help on my senior design project, any help is greatly appreciated!

      We purchased these 12V DC window motors (https://www.parallax.com/product/28962) for the purpose of motorizing a stander (similar to a wheel chair, with the exception that the patient is placed in an upright standing position). The specifications and data for the motors is very limited as you may find on the website, but Parallax does provide a current draw value at no load of 1.5 A.

      The issue: through our bench testing, we have found that one motor will draw 1.5 A under no load, and the other will draw 2.5 A.

      The question: Our professor believes a difference in 1A of draw current under no load is 'normal' production variation for DC motors. Is this a reasonable assumption or should we contact Parallax and ask for a replacement?

      Thank you!
       
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    3. dpongracz

      dpongracz New Member

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      You fail to mention if these DC motors are new or used. DC motors used in vehicles are brush motors. A typical Brush DC Motor contains six components: Stator, Rotor/Armature, Commutator, and Brushes, an axle, and magnets. A no load variation of over 50% is quite large. My experience with worn DC brush motors is that the brush wears down and the spring force pushing it against the commutator is reduced causing arching and excessive wear. The commutator becomes tarnished and conductive materials can deposit in the insulator spaces between the conductors. Cleaning the commutator and the insulator slots between conductors as well as installing new brushes usually restores the motor to normal. If the motors are new, I would contact the manufacturer.
      David Pongracz
      Senior Mechanical design engineer and distinguished member of technical staff
      Alcatel-Lucent Bell Laboratories, retired
       
    4. JonGreene

      JonGreene New Member

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      Hi dpongracz, thank you for your response!

      The motors are indeed used, but unfortunately no further detail is available on 'how used' they are..they were used by the previous years' design team.

      Taking your advice, we disassembled the motors and inspected the brushes; we didn't find a gap between the brushes and the commutator, the spring seemed to be providing enough force to maintain contact. We then used compressed air to blow out any debris between the conductors: this brought current draw down from 2.5 A to 1.7 A on the troubled motor!

      A more thorough cleaning and inspection is the next step for us, to see how close we can bring the current draws back to normal. Thank you for your direction, it is greatly appreciated!
       
    5. PWASS

      PWASS Well-Known Member

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      I'm guessing that they are geared motors, if so, the gearboxes might be worn and need differing amounts of torque to run without load. One might be 'stiffer' than the other and therefore need more motor current to run.
       

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