True/False: armature reaction and ampere conductors

TRUE.

The armature mmf per pole is

$$AT_a = \frac{I_zZ}{2P} = \frac{ac\,\pi D}{2P}$$

So, for a given diameter and number of poles, armature mmf is directly proportional to the specific electric loading $ac$. A large $ac$ produces strong armature reaction: more cross-magnetisation, more distortion of the gap flux, more demagnetising effect at saturation, and poorer commutation. A larger field mmf (or compensating winding) is then needed.

Example: with $ac = 22000$ A/m, $D = 0.306$ m and $P = 4$, $AT_a = 22000 \times \pi \times 0.306/8 \approx 2640$ A per pole; doubling $ac$ doubles this armature mmf. Hence designers limit $ac$, or add interpoles and compensating windings in heavily loaded machines.