in the slot with the core loss in the teeth. In either type of winding, slot dimensions have another important influence. The greater the ratio of slot depth to slot width, the higher the stator reactance, resulting in lower motor torque and locked-rotor current. A wider, shallower slot reverses those tendencies.
7. Performance – torque, lockedrotor current, temperature rise, and (particularly since 1992 in the U.S.) full-load efficiency. 8. Number of units involved. Coil configuration depends upon both voltage rating and motor size. The larger the machine Figure 6. A completed form winding of the conventional double-layer type, in at any voltage, the higher the sta- which all coils are of the same shape. — Electrical Apparatus file photo tor current, and the larger the required conductor size. Also, the “end turns” – the projection of each coil beyond the ends of the stator core – become longer, and therefore more difficult to adequately support by restraining the movement of the coil or of individual coil wires against the magnetic forces of starting current. Why is random winding the universal norm for the smaller machines? The main reasons: 1. A random winding can be installed or replaced without special tooling, and for high production rates can be installed by machine. 2. Low cost of coil manufacturer, requiring no rigid forming or taping. 3. Operating voltages are low enough to minimize insulation requirements. Although the published statement: “. . . form-wound stator . . . windings are used if the rated voltage is 2,300 or more” incorrectly implies that form winding is never used for “low” voltages (600 and below), form winding is customary at 2300 volts and above. But motor size and coil conductor arrangements may dictate form winding for “large” machines at any voltage. Some European motors have been designed for round-wire usage at voltages above 4,000 (although the coil construction is not “random”). The added coil insulation thickness required, and the smaller market potential at higher voltages, have led to design practices for the higher voltages that differ in several respects from standardized (NEMA or IEC) “low voltage” motors. Inevitably, rectangular slots distributed radially around the stator bore result in tooth widths that are distinctly non-uniform – wider at the bottom, narrower at the top. The designer must balance the copper loss
Winding practices around the world Around the world, formed coil design has become fairly standardized. Some variation in the shaping has been found useful to facilitate coil insertion into stator slots, notably in 2 pole machines. But all coils in the set are of the same shape (Figure 6). That practice is widely followed for all sizes and all voltages. This is called a “two-layer” winding, because there are two separate coil sides in each slot, parts of two separate coils. An exception is the form winding method variously termed “pushed-through” or “concentric” winding – see Figures 7 and 8 (although no two-dimensional portrayal is adequate). This is a “single-layer concentric” winding, with only one coil side per slot. Such designs were once fairly common for large machines in Britain. A U.S. author more than 30 years ago said of the push-through winding that it was Please turn to next page
Open ends of "hairpin" shaped and joined after coil is inserted
Figure 7. A “hairpin” or “push-through” formed coil, which can be inserted endwise into a semi-closed (“bridged”) stator slot; the open ends are then bent to be similarly joined.
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Figure 8. A form winding in progress years ago using “pushthrough” coils of the type shown in Figure 7 (half of then inserted from one end of the stator, the other half from the other end). Typical voltage ratings: 3,300 or 11,000. — Electrical Apparatus file photo
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