Stator Slots

Number of poles Number of stator slots Number of rotor slots Skew angle (degrees)... 

For large motors, the practice is to wind the stator with formed coils (Figure 9.2). The coils are pre-formed and cured before insertion into the stator slots. They are insulated with resin-rich glass and mica paper tapes. The process of impregnation is therefore termed resin-rich insulation. The completed formed wound stator is then heated to remove trapped moisture and finally impregnated in varnish class F or H as required, under vacuum and pressure. The stator is then cured in an oven as described above. The process of insulation and curing conforms to powerhouse insulation requirements. This practice facili-... 

Generator field temperature high Stator slot temperature high... 

Protection against electrodynamic forces. These are caused by transient currents, such as on faults, and mainly affect the overhangs or the parts of the windings that fall outside the stator slots. These parts are specially braced and strengthened at the time of manufacture. 

Since the standard insulation level (BIL) of a machine, equipment or a system is already defined, according to Tables 11,6. 14.1, 32.1(A), 13.2 and 13.3. the mtichines are aecordingly designed for this btisic insulation (BIL) only. When the prospective surges are expected to be more severe than this, separate protection becomes imperative. This is particularly important for a rotating machine w hich, besides being a dry equipment, also has only a limited space within the stator slots and hence has the smallest BIL of all, as is evident from Table I 1.6. compared to Tables 14.1, 32.1(A) and 13.2. For its comprehensive protection it can be considered in two parts,... 

A low armature leakage reactance Xa requires the number of stator slots per phase to be kept small, and a high utilisation of conductors per slot. Double layer slots are most often used for high voltage machines. 

The armature leakage reactance is very much dependent upon the stator slot dimensions. It can be shown that ... 

Detailed hydrodynamic studies have been performed for pump-fed in-line mixers with rotors and stators comprised of teeth with slots between them. LeClair (1995) reported an early attempt for a Kady mixer. Although the simulation was quite simplistic and considered only a small section with assumed perfect symmetry, the results revealed a complex circulation pattern in the stator slot. 

Because the tooth depth was small compared to the mixer diameter, it was initially believed that the flow field was two dimensional. Figure 8-9 shows mean velocity vectors, resulting from a 2D simulation, in the quadrant closest to the exit pipe. These results are angularly resolved in that the flow field changes as the rotor passes the stator. The direction of rotor rotation is clockwise and an extremely complex flow pattern is revealed in the stator slots and volute. Circulation cells in the stator slots allow reentrainment of volute fluid back into... 

Figure 8-11 shows the total mass flow rate exiting each of stator slots 1 to 7 (refer to Figure 8-8) predicted by the 2D simulation, as a function of rotor tooth position. As the rotor tooth blocks the stator slot, the flow rate drops rapidly and does not recover until the rotor tooth passes the slot completely. The maximum... 

Figure 8-11 Mass flow rate exiting stator slots 1 to 7 as a function of time or rotor position. The rotor tooth completely blocks the stator slot at time step 9 and the slot begins to open again at time step 21. Results of 2D CFD simulation. Stator slot numbers are defined in Figure 8-8.

Rotor-stator Slotted ring or Either, often 1 < ds < Sparse data for... 

The movement of rotor teeth around the stator produces a clogging effect, resulting into vibrations and noise. To reduce this effect, the common practice is not to provide the rotor slots parallel to the shaft axis but at an angle. This practice is known as rotor bar skew . A proper skewing can also improve the starting torque and reduce the starting cunent, in addition to the effects of space... 

Excessive vibrations according to international codes can cause mechtinieal failure in the insulation by loosening wedges, overhangs, blocks and other supports that hold the stator and the rotor windings or rotor bars in their slots. Vibrations also tend to harden and embrittle copper windings and may eventually break them when they become loose (see also Sections 1 1.4.6 and I 1.4.7). 

This is a resin-rich system. The stator is wound with pre-formed coils on similar lines as noted above. After thorough testing on each coil for the polarization index and dissipation factor (tan S), as discussed in Sections 9.5 and 9.6, and the impulse voltage withstand test, as discussed in Section 11.4.9, the individual coils are completely cured and toughened before inserting them into the slots. The rest of the process is as noted earlier. 

Embedded temperature detectors are resistance temperature detectors (RTDs) or resistance thermometers or thermocouples, built within the machine during manufacture at points that are not accessible when the machine has been assembled. This method is generally employed for the likely hot spots of a machine such as the slot portion and the overhangs of the stator windings. 

Note The embedded lemperalure deleclor melhod is inappropriate for stator windings, which have only one coil side per slot, in such ciises the resistance melhod must be used with the same limits of temperature rise. For checking the temperature of such a winding in service, an embedded detector at the bottom of the slot is of little use because it would give mainly the temperature of the iron core. A detector placed between the coil and the wedge will follow the temperature of the winding much more closely and is, therefore, better for check tests, although the temperature there may also be a little less than the actual one. 

Total pressure surveys were made downstream of the stators in both the radial and circumferential directions to determine the effect of coolant on stator losses. The wake traces for the stator with discrete holes and the stator with trailing edge slots show that there is a considerable difference in total... 

The characteristic length of the dispersion chamber, e.g., the slot width between rotor and stator in dispersion mills or the nozzle diameter in high-pressure homogenizers (utilizing high-speed fluid shear), will be denoted as d. ... 

Colloid Mills. Colloid mills are another form of rotor stator mill. A colloid mill is composed of a conical rotor rotating in a conical stator (Fig. 8.5). The surface of the rotor and stator can be smooth, rough, or slotted. The spacing between the rotor and stator is adjustable by varying the axial location of the rotor to the stator. The gap can be as little as a few hundred microns to a couple of millimeters.18 Varying the gap varies not only the shear imparted to the particles but also the mill residence time and the power density applied. Particle size is affected by adjusting the gap and the rotation rate. It is possible to produce particles in the 1-10 pm size range. 

For the toothed rotor/stator mills, there are localized areas of high shear where the most energy is imparted to the particles, and breakage is believed to occur, through a combination of shear and collision. Therefore, useful parameters that quantify mill performance will reflect these effects. Three typical parameters used in rotor-stator mill scale up are tip speed (rotation rate of rotor x rotor circumference), shear rate (tip speed/distance between rotor and stator), and shear frequency (rotation rate x number of slots on rotor x number of slots on stator).Rotor-stator types typically differ in the number of rows of teeth and the number of teeth in each row of rotor and stator, though other geometric differences are possible. When rotor-stators of different design are available for study, shear frequency appears to have... 

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