Armature Reaction in DC Machines

This article explains armature reaction in dc motor and armature reaction in dc generator.

Usually, we assume that the only flux acting in a dc machine is that due to the field poles alone, called field flux Φ_f . But, since dc armature contains coils and when current passes through them, they produce mmf ( mmf=Number of turns×current ) which produces magnetic flux (as flux=mmf/reluctance). This flux is called armature flux Φ_a .

So, the actual flux in a dc machine will be the resultant of field flux and armature flux.

Actually, the armature flux so produced has two undesirable effects on 
main field flux. These are

1.        Net reduction in the main field flux per pole and
2.        Distortion of the main field flux  

Reduction in main field flux per pole reduces the generated voltage (E_g=PΦZN/60A) in dc generator and reduces the torque (T_a=KΦI_a) developed in armature of dc motor as flux appears in expressions of both E_g and T_a . 

The distortion of main flux influences the limits of successful commutation in dc machines as it may cause circulating currents and sparking at the brushes.

The phenomenon answering  what is armature reaction is illustrated by the flux figures shown below.

Figure 1 shows flux distribution when there is no armature current. Let the phasor representing main flux be Φ_f .

Figure 2 shows flux distribution when there is only armature current. Let the phasor representing armature flux be Φ_a .

But, when a dc machine is in use, both main flux and armature flux exist simultaneously and therefore, the resultant flux is due to the superposition of the two fluxes. Let the resultant flux be represented by Φ_R .

Magnetic Neutral Axis

A magnetic neutral axis (MNA) is an imaginary axis perpendicular to the resultant magnetic field. As visible from figure 3, due to armature reaction, MNA has shifted by an angle θ in a direction of rotation for a generator and against the direction of rotation for a motor.

As commutator is always placed on MNA and now that there is non-zero flux at original MNA ∴ this flux interferes with commutation.

Net reduction in flux per pole

Why there is a net reduction in main flux per pole when the addition of Φ_a to Φ_f  at one end of a pole is equal to the subtraction of Φ_a from Φ_f  at the other end of the same pole as appearing from figures ?

Indeed, there won't be any reduction if there were no magnetic saturation in pole core. But actually, magnetic saturation does occur at the field additive pole end and as a consequence, the strengthening effect is less than the weakening effect and the resultant flux is decreased from its no-load value.

  The resultant waveform is shown as the superposition of above two           cases. It has the two important points –

Ø reduction in flux per pole due to magnetic saturation of the     field pole (weakening effect of armature reaction) and

Ø non-zero flux at commutator position (distortion effect of armature reaction.

continue to Methods to reduce Armature Reaction