Three-Phase Concentric Motor Winding Techniques
Three-Phase Concentric Winding Systems
Three-phase concentric winding systems, sometimes referred to as Akella coils, consist of concentric groups where all coils within a group are wound in the same direction and typically placed in the same slots but are of different sizes. A notable drawback of this system is the necessity for multiple molds for each coil group, as each individual coil within the group has a unique size.
Coil Group Formation & Placement
The coil groups are strategically placed around the stator’s perimeter, with their arrangement depending on the number of phases and poles. The distribution varies based on whether the number of groups per phase is even or odd:
- Even Number of Groups per Phase: When the number of groups per phase (m) is even, they are distributed equidistantly along the stator’s perimeter. This distribution is often divided into two identical parts. For instance, with two groups per phase, each group is positioned on opposite sides of the shaft, maintaining equal distance from each other.
- Odd Number of Groups per Phase: When the number of groups per phase (m) is odd, the groups are placed equidistantly along the entire circumference of the stator.
Connecting Coil Groups in a Phase
In concentric windings, coil groups within a single phase can be interconnected in two primary ways:
- Connection per Pole (Alternating Polarity): In this method, the finish of one group connects to the finish of the next (F-F), and the start of one group connects to the start of the next (S-S). This typically results in what can be considered smaller effective pole groups with fewer coils each, but a greater number of such groups per phase to form the complete phase winding.
- Consistent Pole Connection (Series Connection for Same Polarity): Here, the finish of one group connects to the start of the next (F-S). This method generally leads to larger effective pole groups with more coils in each, and consequently, fewer such groups are needed to complete the phase winding.
Two-Speed Dahlander Windings
Asynchronous motors can achieve two distinct operational speeds using a single winding by altering the number of effective poles. This is accomplished by changing the internal connections of the winding, effectively reconfiguring each phase winding, often by dividing it into two halves. The pole switching is achieved within the winding itself, leading to a change in speed. The typical speed ratio obtained by this polarity switching method is 2:1. For example, if the motor’s lower speed is 750 RPM, its higher speed will be 1500 RPM. This specific type of winding connection is widely known as the Dahlander connection.
Determining Winding Phase Starts
Unlike DC machine windings, AC machine windings are typically open, meaning each phase of the winding has two free ends: a start and a finish. Correctly determining the starts of the windings is crucial and is directly related to the required phase displacement for the electrical system, whether it is single-phase or three-phase. The start of the first phase (e.g., U1) can be placed in any suitable slot along the 360° electrical circumference of the machine. Once the first phase’s start is positioned, the starts of the subsequent phases (e.g., V1, W1 for a three-phase system) are determined by respecting the necessary phase displacement. For standard three-phase windings, this displacement is 120 electrical degrees.
Whole and Fractional-Slot Windings
A winding is classified as a fractional-slot winding when the number of slots per pole per phase (q) is a fractional number (i.e., not an integer). These types of windings are often employed when adapting a stator for a different number of poles or to achieve specific performance characteristics, such as reducing torque ripple or noise. A consequence of fractional-slot design is that coil groups within the same phase do not all contain the same number of individual coils. Since the number of turns in a physical coil must be an integer, groups are formed with a varying number of coils (e.g., some groups might have N coils while others have N+1 coils, arranged in a repeating pattern). The total number of coils in one phase is the product of the average number of coils per group and the number of groups.
Concentric vs. Eccentric Windings
A key difference lies in coil geometry: in concentric windings, coils within a group have different sizes (diameters) but share a common center. In contrast, for eccentric windings, all coils are identical in shape and size. Eccentric windings are further classified into:
- Lap Windings (Overlapping): These are generally connected according to the number of poles. They can be designed and implemented in either single or double layers within the stator slots.
- Wave Windings.
In both single-layer and double-layer eccentric windings, the coil pitch (or coil span) can be shortened (i.e., reduced from full pitch, which spans a full pole). For single-layer lap windings, if the coil pitch is shortened, it must always result in an odd number of slots being spanned. Conversely, for double-layer lap windings, the value of the coil pitch can be either even or odd.