- Armature Reaction Definition: Armature reaction in a DC machine is the impact of the armature flux on the main magnetic field, altering its distribution and strength.
- Cross Magnetization: Cross magnetization due to armature current affects the magnetic field by shifting the magnetic neutral axis, leading to efficiency issues.
- Brush Shift: Shifting brushes can help manage armature reaction but has limitations, making it practical only for small machines.
- Inter Poles: Inter poles help neutralize the armature reaction and provide the necessary commutation voltage, preventing sparking and damage.
- Compensating Winding: Compensating windings balance the armature reaction’s effects, maintaining stable magnetic fields and improving overall machine performance.
Armature reaction is the armature mmf acting on the main field. At no load the magnetic neutral axis sits on the geometrical neutral axis, so the brushes sit there too. On load the armature flux lies along the interpolar axis and looks roughly triangular. That brush-axis mmf cross-magnetizes the main field. Flux crowds at the trailing pole tip in a generator and at the leading pole tip in a motor.
So the armature flux distorts and, with saturation, can also weaken the main field. In a DC motor the leading tips get more flux and the trailing tips get less.
What is Leading and Trailing Pole tip?
The leading tip is the edge the armature conductors meet first. The trailing tip is the other edge. On a clockwise motor the lower North tip and the upper South tip lead. Generating rotation is the opposite, so those tips swap. Cross-magnetizing mmf then shifts the magnetic neutral axis with load, with rotation in a DC generator and against rotation in a DC motor. If the brushes stay on the old axis, motor back emf or generated voltage falls and commutation sparks. The coil under the brush then leaves one pole influence for the other, so current reverses fast and a large reactance voltage appears. Three practical counters are used:
Brush Shift
One fix is to move the brushes with rotation when generating and against rotation when motoring. That shift also cuts air-gap flux, so generated voltage falls and motor speed rises. The demagnetizing mmf (magneto motive force) from the shift is given by:
Where,
Ia = armature current,
Z = total number of conductors,
P = total number of poles,
β = angular shift of carbon brushes (in electrical Degrees).
The brushes would have to move again at every load, rotation or motoring/generating change. Brush shift is therefore left to very small machines, with the rocker locked at normal load and one mode. Factories rarely use it.
Inter Pole
Medium and large DC machines therefore use interpoles. They are long, narrow poles on the interpolar axis. In generating they take the polarity of the next main pole in the rotation order. In motoring they take the pole that has just passed. Their mmf cancels armature mmf on that interpolar axis. Because they are in series with the armature, reversing armature current also reverses the interpoles.
Armature-reaction mmf already lies on that interpolar axis. The same poles also inject a commutating voltage that cancels reactance voltage (L × di/dt) in the coil under the brush, so sparking stays down.
The interpole winding always carries armature current, so it tracks load, rotation and motoring/generating. The poles are kept narrow so they act mainly on the commutating coil. A wide base delays saturation and keeps the interpoles following current changes.
Compensating Winding
Commutation is not the only armature-reaction trouble. Heavy load can push flux density very high at the generator trailing tip and the motor leading tip.
The coil under that tip can then induce enough voltage to flash between neighbouring commutator bars, helped by the hot air already around the brushes.
That flash can run bar to bar until the commutator rings from brush to brush. Fast load swings also put L× di/dt across adjacent bars. The flash starts under the pole centre (highest coil inductance) and can run the same ring fire. The case is worse when generating load is falling or motoring load is rising, because generated emf and L× di/dt then add. Pole-face compensating windings are the usual cure.
A compensating winding is pole-face conductors, parallel to the shaft, carrying armature current opposite to the armature conductors under that pole arc. Full compensation puts the main field shape back. Armature-circuit inductor also falls, so current can change faster. The winding tracks load, rotation and motoring/generating because it is in the armature circuit. Commutation is easier too, because the interpoles no longer have to cancel armature mmf under the pole arc.
Compensating windings typical uses:
- Large machines that see heavy overloads or plugging
- Small motors that reverse suddenly and accelerate hard.
NOTE:
- Cross-magnetizing armature reaction comes mainly from conductors under the pole arc. At high load that crowding can over-flux the generator trailing tip and the motor leading tip. Saturation in the shoe means the gain on the crowded side is smaller than the loss on the rest of the shoe, so net flux per pole falls. Engineers call this the demagnetizing effect of cross-magnetizing armature reaction. Compensating windings are fitted to cancel it.
- Interpole and compensating windings sit in series with the armature winding, on opposite sides of the armature circuit.
- Interpoles mainly aid commutation. Compensating windings hold net air-gap flux nearer a constant value by cancelling the rise or fall from armature reaction.
