Lecture Notes on DC Motors

Jul 28, 2024

Notes on DC Motors and Permanent Magnets

Introduction to DC Motors

  • Focus on types of VDC motors
  • Examining those using permanent magnets versus coils.

Magneto Motive Force (MMF) and Equivalent Electric Circuits

  • MMF in a coil with current (I):
    • MMF = N * I
    • Represented as an equivalent voltage source in electric circuits.
  • Permanent Magnet Behavior:
    • Acts as a non-ideal current source delivering magnetic flux instead of current.

Analyzing Circuits with Current Sources

  • Ideal Current Source:
    • Horizontal line on current vs. voltage graph signifying constant current output regardless of voltage changes.
    • Relationship determined by Ohm’s Law (V = I * R).
  • Non-Ideal Current/Voltage Characteristics:
    • In a real current source circuit, voltage (V_X) in the resistor is dependent on the circuit’s characteristics as described by the intersection of the source and resistor lines.

Creating Flux in Magnetic Circuits

  • Use a coil with current to apply MMF and establish flux.
  • Magnetic circuit can be replaced with an electric circuit model under certain conditions:
    • Valid only when magnetic reluctance from air gap is considerably higher than that of the magnetic core.

Permanent Magnets and Hysteresis

  • Hard Iron:
    • Exhibits a wide hysteresis loop, ideal for permanent magnets.
  • Applying MMF on hard iron leads to:
    • Flux increases until saturation.
    • When MMF is reduced to zero, remains magnetized (remanent flux).
    • Requires negative MMF to demagnetize back to zero flux (coercive force).

Behavior of Permanent Magnets in Circuits

  • When replacing coils with permanent magnets:
    • Current Source Replacement:
      • Permanent magnet's MMF replaces the coil to create necessary flux.
  • Analyze region of operation between remanent flux and coercive force.
  • Air gap reluctance significantly influences MMF applied by the magnet:
    • Increasing air gap results in lower effective MMF from the magnet.

Constructing a DC Motor

  • Traditional method uses coils to create magnetic flux:
    • Example: Two coils each with 1000 turns producing magnetic flux via 2A current.
  • Replacing Coils with Permanent Magnets:
    • Permanent magnet situated in the motor creates necessary flux in the air gap.
  • Motor parts:
    • North and South poles aligned with shaft and rotor connections.
  • Connections:
    • Only two terminal connections lead out from the motor (armature connections A1 and A2).

Conclusion

  • Overview of analyzing DC Motors with permanent magnets.
  • Importance of understanding MMF and magnetic behavior for effective motor design.

Note: This lecture covered the theoretical and practical aspects of DC motors functioning with respect to both coils and permanent magnets, how to analyze circuits related to them, and further explored magnetic characteristics.