Eccentric Fractional Winding: Characteristics and Principles
Eccentric Fractional Winding Characteristics
Differentiating Characteristics of Eccentric Fractional Winding: A coil is considered fractional if the number of groups results in a non-integer value. In this case, all groups in the winding will be equal. However, if the number of coils per group is fractional, groups will be uneven and distributed around the periphery of the stator, with a repetition of the groups. These windings are generally used when changing from one polarity to another on the same stator. Polarity switching can occur when the number of coils per group results in a non-integer value, having an integer and a fractional part.
Coil Width and Construction
- Coil Width: The width is the calculated number of slots.
- Coils: Are formed by a specific number of turns, closed, and have a beginning and an end. They are composed of active sides and sliding heads.
- Sliding Heads: The speed difference between synchronous speed and the rotation speed of the armature (rotor). Slippage is necessary for an EMF to be generated in the rotor conductors.
Pole Pitch and Poles
- Pole Pitch: The distance between the axes of two adjacent poles of different polarity.
- Poles: Iron cores surrounded by coils. When traversed by an electric current, they produce a magnetic flux that gives rise to magnetic poles. Depending on the direction of the current, they can be north or south poles.
Stator Slots
Slots: Hollow spaces located along the circumference of the stator where the active sides of the coils are placed.
Determination of Phase Principles
Unlike DC windings, AC windings are open. Each phase winding has two free ends: a beginning and an end. The determination of the winding principles is directly related to the phase displacement, whether the system is single or three-phase. The beginning of a phase can be initiated in any slot of the 360° circumference of the machine. The placement of the beginning of the first phase will determine the location of the other phases, respecting the lag. In three-phase windings, this lag is determined by calculating the gap, which is 120°.
Concentric and Fractional Windings
Concentric Electrical Coils: A fractional winding occurs when the number of slots per pole and phase is fractional. These windings are usually used when adapting from one polarity to another on the same stator, resulting in a non-integer number of coils per group, with an integer and a fractional part. If the number of coils is an integer, groups are formed in the same phase, with, for example, 3 coils and 2 coils alternately. The total number of coils of one phase will be the value of the number of coils per group multiplied by the number of groups.
Difference Between Concentric and Eccentric Windings
Unlike concentric coils, in eccentric windings, all coils are equal. They are classified as overlapping and wavy. Windings are usually connected and embedded in poles, indicating that the coil groups have both poles in the same winding. They can be calculated and executed in one or two layers. Eccentric windings in 1 and 2 layers can reduce the pole pitch. In the case of a single layer, the pole pitch can be reduced, but always respecting the value. In two-layer embedded windings, the value of the pole pitch can be odd or even.