Slot ripples in alternator e.m.f. waves

A review of the literature of the subject shows the wide acceptance of the view that slot ripples in the open-circuit e.m.f. waveform of salient-pole synchronous machines are caused by the interaction of rotor permeance harmonics with those set up by the stator slot-openings. It is shown in the paper that this is usually only a secondary cause and that these ripples are due primarily to stator slot-frequency currents flowing in secondary circuits on the rotor; as a consequence the ripples may be generated with either a smooth or salient-pole rotor. It is further shown that e.m.f. ripples may be generated not only when the stator slots per pole-pair are even but also when they are odd or fractional. These e.m.f. ripples may correspond to the fundamental or a higher harmonic of the flux-density variation due to the stator slot-opening. Expressions are derived for the frequencies of the ripples in the special cases, such as turbo-alternators and synchronous motors with smooth-slotted rotors, where the squirrel-cage winding is placed in uniformly-indexed slots. Various methods for reducing or suppressing these ripples are discussed and corresponding formulae derived. The paper concludes with a detailed examination of a number of oscillograms and the appropriate data of the machines to which they refer.