• A.C. windings design deals with assigning coils to slots and phases, establishing coil connections inside and between phases, and calculating the number of turns/coil and the wire gauge.
• The mmf of a winding means the spatial/time variation of ampere turns in slots along the stator periphery.
• A pure traveling mmf is the ideal; it may be approximately realized through intelligent placing of coils of phases in their slots.
• Traveling wave mmfs are capable producing ripple-free torque at steady state.
• The practical mmf wave has a spacial period of two pole pithces 2τ.
• There are p1electrical periods for one mechanical revolution.
• The speed of the traveling wave (fundamental) n1 = f1/p1; f1–current frequency.
• Two phases with step-wise mmfs, phase shifted by π/2p1 mechanical degrees or m phases (3 in particular), phase shifted by π/mp1 geometrical degrees, may produce a practical traveling mmf wave characterized by a large fundamental and small space harmonics for integer q. Harmonics of order ν = 5, 11, 17 are reverse in motion, while ν = 7, 13, 19 harmonics move forward at speed nν = fν/(νp1).
• Three-phase windings are built in one or two-layers in slots; the total number of coils equals half the number of stator slots Ns for single-layer configurations and is equal to Ns for two-layer windings.
• Full pitch and chorded coils are used; full pitch means π/p1 mechanical radians and chorded coils means less than that; sometimes elongated coils are used; single-layer windings are built with full pitch coils.
• Windings for induction machines are built with integer and, rarely, fractional number of slots/pole/phase, q.
• The windings are characterized by their mmf fundamental amplitude (the higher the better) and the space harmonic contents, (the lower, the better); the winding factor Kwν characterizes their performance.
• The star of slot emf phasors refers to the emf phasors in every slot conductor drawn with a common origin and based on the fact that the airgap field produced by the mmf fundamental is also a traveling wave; so, the emfs are sinusoidal in time.
• The star phasors are allocated to the three-phases to produce three resultant phasors 1200 (electrical) apart; in partly symmetric windings, this angle is only close to 1200.
• Pole-changing windings may be built by changing the direction of connections in half of each phase or by dividing each phase into a few sections (multiples of 3, in general) and switching them from one phase to another.
• Pole-changing is used to modify the speed as n1 = f1/p1.
• New pole-changing windings need only two single throw switches while standard ones need more costly switches.
• Single phase supplies require two-phase windings–at least to start; a capacitor (or a resistor at very low power) in the auxiliary phase provides a traveling mmf for a certain slip (speed).
• Reducing harmonics content in mmfs may be achieved by varying the number of turns per coil in various slots and phases; this is standard in twophase (single-phase supply) motors and rather new in three-phase motors, to reduce the noise level.
• Two phase (for single phase supply) pole-changing windings may be obtained from three-phase such windings by special connections.
• Rotor three-phase windings use full pitch coils in general.
• Rotor cage mmfs have a fundamental (µ = p1) and harmonics (in mechanical angles) µ = knr± p1, Nr – number of rotor slots.
• The rotor cage may be replaced by an Nr/p1 phase winding with one conductor per slot.
• The rotor mmf fundamental amplitude F2m varies with slip (speed) up to F1m(of the stator) and so does the phase shift angle between them γ1,2. The angle γ1,2 varies in the interval γ1,2∈ (0, ±π/4).
• For skewed rotor slots a part of rotor mmf, variable along stator stack length (shaft direction), remains uncompensated; this mmf is called here the “skewing” mmf which may produce heavy saturation levels at low speed (and high rotor currents).
• The sum of the stator and rotor mmfs, which solely exist in absence of skewing constitutes the so called “magnetisation” mmf and produces the main (useful) flux in the machine.
• Even this magnetising mmf varies with slip (speed) – for constant voltage and frequency; however it decreases with slip (reduction in speed); this knowledge is to be used in later Chapters (5, 6, 8).