The Magnetic Field of Synchronous Machines:

Slides to Complement Class Notes

by Constantine J. Hatziadoniu

Stator Carries a 3-phase ac winding, Rotor carries a dc winding

Machine with Salient Poles

The Rotor is supplied via slip rings

One phase with concentrated winding:Flux distribution is rectangular

One-phase distributed winding: Flux space distribution is closer to sinusoidal

Three phase stator: positive sequence currents

Phase a is excited by a sinusoidal current: Air-gap field is a sinusoidal (space) distribution pulsating in time

Phase a field is a two-pole magnet; The magnetic axis passes through the middle (axis) of the winding

Phase b bi-polar field

Phase c bi-polar field

All three fields added in the air-gap: The resulting field is a rotating sinusoidal distribution

The combined field (dashed line) moving around the air gap

…Continued

Instances of the rotating field: Field rotates ones around the air-gap within a complete electric period

The winding arrangement of a four-pole machine

Phase a of the 4-pole machine: The spatial distribution varies twice in a full cycle

Phase b of the 4-pole machine

Phase c of the 4-pole machine

The three distributions shown for the 4-pole machine

Combined field is a rotating distribution

The 4-pole field rotates ½ times around in one electric period—the machine speed is reduced to half the speed of a 2-pole machine

The Rotor Field of the 4-pole machine

Rotor and Stator Fields rotate in step inside the airgap