Design an alternator by moving sliders.
Salient-pole or cylindrical rotor is the first question, because they are two different machines with different dimension rules and a different gap. Set the rating and the speed, and the armature, the magnetic circuit, the poles and the whole field system are designed live.
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Both halves of the machine
An alternator is two design problems joined at the air gap: an armature that has to produce a given voltage at a given frequency, and a field system that has to produce the flux the armature assumed, against the mmf the iron and the gap demand. A tool that does the first and stops has designed a stator bolted to a blank rotor.
Here both are designed, and the join is the interesting part: the gap is sized from the short-circuit ratio you asked for, the pole and the field winding from the mmf that gap and that iron actually need, and the ratio the finished machine delivers is then read back off its own open- and short-circuit characteristics. It comes out a few per cent away from the one you ordered, every time, on any machine — and so does the textbook’s own design problem, which orders 1.3 and delivers 1.41. The two are not the same quantity, and the studio says so instead of painting a sound design red.
Salient pole or cylindrical rotor
The rotor choice is the first control, above everything else, because it is not a setting. A salient-pole machine is a low-speed, many-poled hydro or engine-driven generator with projecting poles, a shaped gap and a damper winding. A cylindrical-rotor turbo-alternator is a two- or four-pole machine at 3000 or 1500 rpm whose rotor is a solid forging with the winding buried in slots, limited by peripheral speed and centrifugal stress rather than by flux. The dimension rules, the gap and the drawings all differ.
Method and honest limits
Closed-form classical design after A. K. Sawhney, A Course in Electrical Machine Design, Chapter 12, pinned in CI against Examples 12.1–12.7 and 12.11–12.19 and both of the chapter’s Design Problems. Thirteen errata in that chapter are pinned both ways — what the page prints and what its own inputs give.
The steel data is one dynamo sheet sampled at five flux densities, every point a reading the book itself printed in a worked design and used to reach a published answer. It is not a grade library, it does not vary with lamination thickness or frequency, and the hysteresis and eddy parts are not separated. Outside the sampled range the studio says the figure is out of range rather than extrapolating, because the knee is the entire content of a magnetisation curve.
Common questions
How are the main dimensions of an alternator chosen?
From the output equation Q = C₀·D²L·ns with C₀ = 11·Bav·ac·Kw×10⁻³, giving D²L = Q/(C₀·ns). For a salient-pole machine the speed is fixed by the poles, p = 2f/ns, and the bore usually follows from the ratio of core length to pole pitch; for a turbo-alternator the peripheral speed Va = π·D·ns is the binding constraint instead, because a two-pole rotor at 3000 rpm is limited by what the forging will stand (Sawhney Ch. 12).
What is the short-circuit ratio of an alternator?
The ratio of the field current needed for rated voltage on open circuit to that needed for rated current on short circuit — in effect the inverse of the per-unit synchronous reactance. It decides the machine’s regulation, its stability and how much field copper it needs. Sawhney §12.10 gives 0.5–0.7 for a turbo-alternator, raised to 1.0–1.5 where capacitive loading is expected, and 1.0–1.5 for a salient-pole hydro machine.
Why is the air gap of a salient-pole machine shaped rather than uniform?
Because a uniform gap under a salient pole gives a flat-topped flux wave full of harmonics. Widening the gap towards the pole tips makes the air-gap flux distribution closer to a sinusoid, which is what the whole emf calculation assumes. The studio draws the taper as designed, so the shape is visible rather than asserted.
Why does the alternator design use a coil span of 83.3 per cent?
Five-sixths of a pole pitch is the one span that puts both the fifth and the seventh harmonic on cos 75°, nearly killing both at once — which is why it appears in design after design. The book prints it as "8 33 per cent" on p. 744; 8.33 % would be a 15° coil generating almost nothing, so the figure is 83.3 %, and the erratum is pinned in a test.
How is the field winding of an alternator designed?
From the mmf the magnetic circuit needs. The ampere-turns are summed round the path — gap, teeth, armature core, pole body and yoke — using the steel’s own B-H readings; the field ampere-turns at no load follow from the short-circuit ratio, and the winding is then sized from the exciter voltage, the permissible current density and the space available on the pole. The studio publishes the open-circuit and short-circuit characteristics it builds from that, and reads the short-circuit ratio off them.
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