Q6Electrical Machine Design

Space factor: the (window) space factor Kw is defined as the ratio of the net copper conductor cross-sectional area to the total available window area in a transformer or slot area in a rotating machine, Kw = (copper area)/(total window or slot area), accounting for the portion of the total space occupied by insulation, clearances, and structural elements rather than current-carrying copper, as discussed in more detail (with typical numerical ranges) elsewhere in this paper.

Rating of a machine: the rating of an electrical machine is the output (kW, kVA, or HP) that it can deliver continuously (or according to its specified duty cycle, as classified per IS:4722/IEC 60034-1) without exceeding its permissible temperature rise limit, at rated voltage, frequency, and other specified operating conditions; a machine's rating is thus fundamentally a thermal/insulation-limited quantity, not merely a statement of its maximum instantaneous capability, and correctly matching a machine's duty-cycle rating to its intended application load profile (as discussed for various standard duty types elsewhere in this paper) is essential to avoid either premature insulation failure (under-rated machine) or uneconomical oversizing (over-rated machine).

Choice of specific magnetic loading and influencing factors: as discussed in detail elsewhere in this paper, the choice of specific magnetic loading (Bav) involves balancing machine size (favoring higher Bav) against iron loss, magnetizing current/power factor, and risk of saturation (all favoring lower Bav), with the actual choice influenced by: the core material's saturation and loss characteristics (better materials permit higher Bav for the same loss budget); the permissible core loss and overall efficiency target; the type of machine (induction motors, needing self-supplied magnetizing current, generally use somewhat lower Bav than separately-excited synchronous machines); and the operating frequency (higher frequency requires lower Bav to control core loss, since core losses increase with frequency).

Factors influencing overall machine performance (beyond magnetic loading alone): additional factors that influence a machine's performance for a given design include the specific electric loading chosen (jointly with Bav determining the D²L product and hence size), the L/D ratio selected (affecting cost, cooling surface area, and the mechanical/electrical trade-offs of a long-thin versus short-fat machine geometry), the number of poles and slots chosen (affecting winding distribution, harmonic content, and susceptibility to parasitic effects like cogging/crawling as discussed elsewhere in this paper), and the cooling method employed (directly setting the practically achievable permissible electric loading for a given temperature rise limit), all of which interact together (not independently) to determine the final realized performance of the completed machine design.