Choice of specific electric loading in induction machine

Specific electric loading ($ac$) is the total stator ampere-conductors per metre of gap periphery:

$$ac = \frac{6 I_{ph} T_{ph}}{\pi D}$$

Typical values for induction motors are 5000–45000 A/m. Its choice depends on:

1. Copper loss and temperature rise: a higher $ac$ means more copper per unit surface, so more $I^2R$ loss and a higher temperature rise. Good ventilation allows a higher $ac$.
2. Operating voltage: high-voltage machines need thicker insulation, so less slot space is left for copper and a lower $ac$ must be used.
3. Size and cost: from $Q = C_0D^2Ln_s$, a higher $ac$ gives a smaller and cheaper machine.
4. Overload capacity: a higher $ac$ means more turns and higher leakage reactance, which lowers the maximum torque and overload capacity.
5. Diameter of the machine: larger machines (larger $D$) have more space for copper and can use a higher $ac$.
6. Space factor and slot design: a high $ac$ needs more copper in the slots. Deeper slots raise slot leakage, and wider slots weaken the teeth.

Example: a motor with 1800 conductors carrying 11.16 A on a 0.276 m bore has $ac = 1800 \times 11.16/(\pi \times 0.276) \approx 23000$ A/m, a normal value for a medium motor.

In short, a high value saves material but raises temperature rise and leakage reactance. The designer chooses a value consistent with the required overload capacity and cooling.