Geometrical Neutral Axis (GNA) and Magnetic Neutral Axis (MNA)

What are the Geometrical neutral axis (GNA) and Magnetic neutral axis (MNA)?

The Geometrical Neutral axis (GNA) bisects the angle between the two poles of a machine. Whereas the Magnetic Neutral axis (MNA) is the perpendicular axis on the magnetic flux line. Also machine has a “Direct (D) axis” and “Quadrature (Q) axis”.

What is a Geometrical Neutral Axis (GNA)?

A GNA or the Geometrical neutral axis is the fixed perpendicular axis to the magnetic north and South poles. So its position changes only when the position of the poles is changed.

What is the Magnetic Neutral Axis (MNA)?

A MNA or Magnetic neutral axis is defined as the axis on which there is no Flux generated in the armature conductors. Because they move parallel to the main field flux lines. And in the formula, we have multiple of the sinθ. Which says when theta is zero flux linkage is zero in the armature windings. That is why there is no flux generated when the conductors are on the MNA. Sometimes this MNA shifts due to the distortion in the main field.

Why do MNA shifts?

As the Armature reaction weakens the main field, this weak main field also affects the generated emf in the case of the DC generator. As MNA is Always perpendicular to the field EMF, the MNA also gets distorted along with the field EMF. In short, MNA is shifted only when the Machine is loaded.

The Magnetic Neutral Axis coincides with the Geometrical Neutral Axis only when there is no armature reaction or no load.

Why do brushes are placed on the neutral position or MNA?

Brushes are always placed on the neutral axis which is the magnetic neutral axis (MNA). Because the armature current reversal takes happens on MNA, as a result, the commutation process takes place easily. If we place brushes elsewhere, and when a current reversal is happening, there will be also an opposite generated current, and SPARKING (🔥) happens which may damage the machine ⚡.

GNA and MNA coinsides for normal flux
GNA and MNA coincide for normal flux
GNA and MNA differs due to distorted flux
GNA and MNA differ due to the distorted flux
  • So, The perpendicular axis between the magnetic North and magnetic South poles is GNA. It never changes until the position of the poles change.
  • There are infinite Axes of symmetry but only one magnetic neutral axis.
  • But MNA is the axis along which induced EMF is ZERO.

Direct axis and Quadrature axis

The direct axis is the axis along the axis of the rotor whereas the axis perpendicular to the D or Direct axis is Q or Quadrature axis.

Below are the flux linkages available to these axes.

AXISFlux linkagesRate of change of flux linkages
(i.e EMF induced)
Along d-axis
(direct axis)
MinimumMaximum
(motion is perpendicular to flux)
θ = 90°
Along q-axis
(quadrature axis)
MaximumMinimum/ZERO
(motion is parallel to flux)
θ = 0°

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