Parallel vs Radial Magnetic Fields
HSC Physics Syllabus
- investigate the operation of a simple DC motor to analyse:
– the functions of its components
– production of a torque `\tau = nIAB_(_|_) sin \theta`
– effects of back emf
Why Use Radial Magnets?
This video investigates the operation of a simple DC motor. It focuses on the effect of using a radial magnet and why this is preferred over parallel magnets.
Radial Magnets vs Parallel Magnets
Parallel magnetic fields are uniform as the magnetic field lines are always parallel and equidistant to one another.
Radial magnetic fields produce a non-uniform radial field. In a radial magnetic field, the field lines are not parallel.
The type of magnetic field has a significant effect on the operation of a motor. Recall that torque of a motor is given by `\tau = nIAB \sin \theta`. As the coil rotates in a parallel field, the angle `\theta` between the area vector and the magnetic field lines will change. This causes the magnitude of torque to oscillate between a maximum and zero in the form of an absolute value sinusoid.
However, magnetic field lines of radial magnets remain perpendicular to the area vector throughout its rotation i.e. angle `\theta` remains constant. This causes torque to be constant at its maximum value. This is the main reason why motors tend to utilise radial magnets to allow for a 'smoother' operation.
Figure shows the changes in torque of a simple DC motor using parallel versus radial magnets throughout one revolution of armature rotation starting from the horizontal position.
Regardless the type of magnets used, torque of a DC motor is temporarily zero when the current flow through the coil is zero. This occurs when the split-ring commutators momentarily lose contact with the brush.
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