Armature Reaction and the Shift of the Neutral Axis
Where must the brushes sit for no sparking?
In a DC machine running as a generator, when the main flux (vertical) and the armature cross flux (horizontal) combine, the resultant flux tilts in the direction of rotation, and the magnetic neutral axis where the flux is 0 (gold dashed line) shifts by the same amount. Commutation has to happen where the flux is 0, and if you place the brushes in each of three positions with the tabs, only one position makes the sparks disappear. Finding where that position lies is the question of this lesson.
The armature becomes a magnet too
Under load, current flows in the armature winding, and the axis of the armature magnetomotive force it creates points along the brush axis, that is, along the geometric neutral axis. If the main flux passes vertically from the poles into the armature, the armature flux runs horizontally, at right angles to it. This cross flux is the starting point of armature reaction.
The vector sum pushes the neutral axis
The main flux and the armature cross flux are at right angles, so the resultant flux tilts by their vector sum, and the magnetic neutral axis, where the flux density is 0, stays at right angles to the resultant and shifts from the geometric neutral axis by the same angle. In a generator it shifts in the direction of rotation; in a motor, where the armature current flows the other way, it shifts against the rotation. The shift grows with the armature current, and the tilt in the picture is an example at one load. At one pole tip the flux crowds in and saturates while the other side weakens, so the total flux can drop a little (the demagnetizing effect).
Three fixes that protect commutation
Moving the brushes to the new magnetic neutral axis gives clean commutation at that load, but the shift differs with every load, so they would have to be moved each time. Interpoles are small poles placed on the geometric neutral axis; wound in series with the armature, they strengthen and weaken with the load and give the commutating coil just the right opposing emf, eliminating sparks. Compensating windings, set in slots in the pole faces, cancel the armature cross flux right where it forms, so the flux distortion itself is prevented. The larger the machine, the more likely it uses both interpoles and compensating windings.
Back to the first screen
The sparks disappeared only when the brushes were moved in the direction of rotation onto the magnetic neutral axis. The armature cross flux combines with the main flux as vectors, tilts the resultant flux, and pushes the neutral axis, where the flux is 0, by the same amount. Commutation has to happen where the flux is 0, so brushes left in place are off and spark. Armature reaction shakes the promise of B1's commutator, switching the connection at the zero crossing, and so the zero crossing is held in place by shifting the brushes or by interpoles and compensating windings, which work regardless of load.
Armature reaction is both a headache of DC machines and where much of the craft of their design shows. With the neutral axis and flux in hand, the next step is the torque and speed produced by flux and armature current when the same machine runs as a motor. The next lesson covers the relations T ∝ Φ·Ia and N ∝ (V - IaRa)/Φ, and how speed is controlled by changing the field (MC-B3).