Section : [1.3], [1.3.1] , and [1.3.2]

1.3 TYPES OF STATOR WINDING CONSTRUCTION

Three basic types of stator winding structures are employed over the range from 1 kW to more than 1000 MW:

1. Random-wound stators
2. Form-wound stators using multiturn coil
3. Form-wound stators using Roebel bars

In general, random-wound stators are typically used for machines less than several hundred kW. Form-wound coil windings are used in most large motors and many generators rated up to 50 to 100 MVA. Roebel bar windings are used for large generators. Although each type of construction is described below, some machine manufacturers have made hybrids that do not fit easily into any of the above categories; these are not discussed in this blog.

1.3.1 Random-Wound Stators

Random-wound stators consist of round, insulated copper conductors (magnet wire or winding wire) that are wound continuously (by hand or by a winding machine) through slots in then stator core to form a coil (Figure 1.5). Figure 1.5 shows that most of the turns in the coils can be easily seen. Each turn (loop) of magnet wire could, in principle, be placed randomly against any other turn of magnet wire in the coil, independent of the voltage level of the turn, thus the term “random.” Since a turn that is connected to the phase terminal can be adjacent to a turn that is operating at low voltage (i.e., at the neutral point), random-wound stators usually operate at voltages less than 1000 V. This effectively limits random-wound stators to machines less than several hundred kW or HP.

1.3.2 Form-Wound Stators—Coil Type

Form-wound stators are usually intended for machines operating at 1000 V and above. Such windings are made from insulated coils that have been preformed prior to insertion in the slots in the stator core (Figure 1.6). The preformed coil consists of a continuous loop of magnet wire shaped into a coil (sometimes referred to as a diamond shape), with additional insulation applied over the coil loops.

Figure 1.5. Photograph of the end-winding and slots of a random-wound stator. (Courtesy TECOWestinghouse.)

(a)

(b)

 

 

Figure 1.6. (a) Photograph of a form-wound motor stator winding. (Courtesy TECO-Westinghouse.) (b)
A single form-wound coil being inserted into two slots.

Usually, each coil can have from two to 12 turns, and several coils are connected in series to create the proper number of poles and turns between the phase terminal and ground (or neutral); see Figure 1.4. Careful design and manufacture are used to ensure that each turn in a coil is adjacent to another turn with the smallest possible voltage difference. By minimizing the voltage between adjacent turns, thinner insulation can be used to separate the turns. For example, in a 4160 volt stator winding (2400 V line-to-ground), the winding may have 10 coils connected in series, with each coil consisting of 10 turns, yielding 100 turns between the phase terminal and neutral. The maximum voltage between adjacent turns is 24 V. In contrast, if the stator were of a random-wound type, there might be up to 2400 V between adjacent turns, since a phase-end turn may be adjacent to a neutral-end turn. This placement would require an unacceptably large magnet wire insulation thickness.