Choice of ampere conductors per meter in DC machine

Specific electric loading ($ac$) of a dc machine is the total armature ampere-conductors per metre of armature periphery:

$$ac = \frac{I_z Z}{\pi D}$$

where $I_z$ is the current in each conductor ($I_a/a$) and $Z$ the total conductors. Usual values are 15000–50000 A/m. Its choice depends on:

1. Temperature rise

Higher $ac$ means more copper loss per unit armature surface. Since heat is removed through that surface, temperature rise increases. Machines with good cooling (forced ventilation) can use a higher $ac$.

2. Speed of machine

A high-speed armature is better ventilated (fan action), so a higher $ac$ can be used.

3. Voltage

High-voltage machines need thicker insulation, which leaves less slot space for copper. Hence a lower $ac$ is used for high voltage.

4. Size of machine

Larger machines have deeper slots and more room for copper, so they can use a higher $ac$.

5. Armature reaction

Armature mmf per pole $= ac\,\pi D/(2P)$. A high $ac$ means strong armature reaction, so more field mmf (more field copper, larger poles) is needed to avoid excessive distortion and loss of flux.

6. Commutation

Reactance voltage is proportional to the ampere-conductors per slot and $ac$. A high $ac$ makes commutation difficult and may need interpoles.

7. Copper loss and efficiency

A higher $ac$ increases copper loss and lowers efficiency.

8. Size and cost

From $P_a = C_0D^2Ln$ with $C_0 = \pi^2B_{av}\,ac\times10^{-3}$, a higher $ac$ gives a smaller, cheaper machine.