I have a 2-part question about electromagnetism and motors

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    Electromagnetism Motors
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Discussion Overview

The discussion centers around the construction and mathematical analysis of a DC shunt wound motor. Participants explore concepts related to electromagnetism, including the Biot-Savart law, magnetic flux, voltage production, and motor design considerations. The scope includes theoretical calculations, practical construction challenges, and the underlying physics principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks clarification on converting measurements from Teslas to Webers to find flux density and the appropriate unit conversions.
  • Another participant questions the formula for calculating voltage produced when cutting through magnetic flux lines.
  • There is discussion about the construction of the motor, including the use of laminated steel, commutators, and carbon brushes.
  • Participants express differing levels of understanding and experience in motor construction, with one participant mentioning collaboration with a more experienced mechanic.
  • Concerns are raised about the disruption of magnetic flux due to commutator gaps and the potential need for adjustments to minimize arcing.
  • A formula for calculating Back EMF is provided, but there is a request for clarification on how to determine average magnetic flux.
  • One participant mentions the relative permeability of the iron core and its implications for motor design.
  • There are suggestions to study existing motors for practical insights into construction and design.
  • Discussion includes considerations of using a single commutator versus multiple segments and the implications for motor functionality.

Areas of Agreement / Disagreement

Participants express varying levels of agreement on certain technical aspects, but multiple competing views remain regarding the best approach to motor construction and the calculations involved. The discussion does not reach a consensus on several technical questions, including the optimal design choices and formulas to use.

Contextual Notes

Participants highlight limitations in their understanding of specific calculations and design principles, indicating a reliance on further research and practical experimentation. There are unresolved questions regarding the effects of commutator design on motor performance and the exact mathematical relationships involved in voltage production.

Who May Find This Useful

This discussion may be useful for individuals interested in DIY motor construction, students studying electromagnetism, and those seeking practical insights into the design and operation of DC motors.

  • #31
Yes. It takes more energy to demagnetize the core than you spent magnetizing it, so every time you demagnetize and reverse the polarity of flux in a ferromagnetic material, some energy will be lost to magnetic hysteresis.
 
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  • #32
I didn't even think of that, back to the drawing board.. Thank you again, always something insightful!
 
  • #33
this guy's videos might help you get started

you'll need headphones, his accent i find difficult but i do like the first presentation.

 
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  • #34
I listened, but that presentation is not in English and trying to read subtitles and watch the examples is confusing.
 
  • #35
James Birt said:
that presentation is not in English
Oops sorry about that
i thought it was my old ears on this terrible laptop speaker !

will look for a proper one...

for starters try



and
 
Last edited:
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  • #36
Haha, old school, I like it.
 
  • #37
What the video calls the neutral plane (I call it the geometric neutral plane) contains the motor's axis of rotation and is perpendicular to lines of field flux when no armature current is flowing.

Armature reaction (caused by the magnetic field that arises from armature current) tends to rotate field lines against the direction of motor rotation. The video terms this rotated neutral plane the adjusted neutral plane. I call it the electrical neutral plane.

When an armature conductor passes through the electrical neutral plane, it's moving parallel to magnetic lines of force, and hence no back EMF is induced as a result of the conductor's motion. However, some additional back EMF is self-induced as armature current decreases (caused by armature inductance). These two phenomena work together to rotate the electrical neutral plane in the same direction.

When voltage in an armature conductor is zero (that is, when it passes through the electrical neutral plane), this is the ideal point at which to commutate the current.
 
  • #38
David Lewis said:
Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)
Correction: Average magnetic flux should be average magnetic flux density. (The SI unit for flux density is tesla.)
 

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