Star, connected winding

In the event of single phasing, in star-connected windings, one of the phases of each positive and negative sequence components will counter-balance each other, to produce zero current in the open phase (Figure 12.8(a)). The magnitude of these components in the two healthy phases are equal, i.e. I, = 1, and the maximum heat of the stator as in equation (12.4)... 

Figure 12.8 Stator and line currents on single phasing for a star-connected winding...

A delta-connected or an ungrounded star-connected winding should also be protected through a restricted ground fault scheme, otherwise it will remain unprotected. There is no zero sequence or residual current in such a winding to detect a ground fault. 

Many of the electrical engineering textbooks that include the subject of motors in their contents describe the eqnivalent circnit of an indnction motor as a series and parallel combination of resistances and reactances, see References 1 to 8. The eqnivalent circnit nsnaUy defines the situation for one of the three phases and so care needs to be taken to ensure that the final resnlts obtained apply to the complete motor. Care is also necessary in nsing the ohmic data from mannfacturers, they may have either star winding valnes or delta winding valnes and the choice may not be obvious. The equivalent circuit of most practical use is shown in Figure 5.1 for one star connected winding, where -... 

The first letter denotes the source of power from a star-connected winding. T denotes that the star point of the source is solidly connected to earth, which is usually at a location very near to the winding. 1 denotes that the star point and the winding are isolated from earth by their design and... 

Heat generated in a star-connected stator winding... 

For more than one capacitor bank one RVT for each bank will be necessary for the correct discharge of each bank, (Figure 26.5). An RVT, if provided with an additional star-connected tertiary winding rated for 110 V line to line, can aiso check the discharge condition and its duration and if it meets the design requirements. This can even be monitored remotely through a relay and control panel. 

High voltage supphes within a plant are invariably arranged as three-phase star-connected systems. The star point of the transformer secondary winding or the generator stator winding is... 

The three-phase windings of an a.c. generator may be star connected or delta connected as shown in Fig. 2.54. The important relationship between phase and line currents and voltages is also shown. The square root of 3 (Vs) is simply a constant for three-phase circuits, and has a value of 1.732. The delta connection is used for electrical power transmission because only three conductors are required. Delta connection is also used to connect the windings of most three-phase motors because the phase windings are perfectly balanced and, therefore, do not require a neutral connection. 

For large motors, 1000 kW and above, if they are provided with star windings the three neutral end leads must be connected to a separate terminal box to enable mounting of CTs for differential protection. The neutral terminal box need not be phase segregated. 

Figure 1. Lines of constant mass and varying chemical composition for the computed Wolf-Rayet models of 3, 5, 7, 10, 15, 20, SO, 40, and 60Mq in the HR diagram (solid lines). The pure helium stars are connected through a dashed line, while the extreme helium poor stars are connected through a dotted line. Also the HRD positions after applying a correction for the partly optically thick stellar wind on the effective temperature are shown. Furthermore, the theoretical zero age main sequence (ZAMS) is indicated, together with schematic evolutionary tracks for stars of 15, 30, and IOOMq. The crosses and circles correspond to HRD positions of observed WNE and WC stars, respectively, according to Smith and Willis (198S, Astron. Astrophys. Suppl. 54,229).

Rapid rotation and mass-loss connection in Be stars was suggested for the first time by Struve(1931), which necessarily leads to the requirement of break-up velocity in Be stars. However, Vsin i statistics suggests almost all Be stars are well below the break-up velocity. The additional forces have been searched for so far i.e., stellar wind, magnetic field, mass accretion, and so on. At the moment, none of them can succeed in explaining the episodic mass-loss in Be stars. 

Star windings are used when a neutral connection is required for earthing or for un-balanced loads (these are usually groups of single-phase loads placed between separate phases of the supply and its neutral. Each group may not be identical and hence the system will be unbalanced). 

In order to restrict the detection of earth faults to those within the stator winding, and those from the stator terminals to the switchgear current transformers, a sensitive relay (64) is used. Three current transformers are used in the stator live lines and one in the star to NER connection. All four current transformers are connected in parallel such that any unbalance in the currents due to an internal fault is detected by the restricted earth fault relay (64). A sensitive high impedance relay is used to achieve an instantaneous response. However, if a high impedance is connected across a current transformer it is possible that very high voltages will appear across the impedance. This is due to the action of... 

The wound rotor consists of a laminated cylinder of silicon steel with copper coils embedded in slots around the circumference. The windings may be connected in star or delta and the end connections brought out to slip rings mounted on the shaft. Connection by carbon brushes can then be made to an external resistance to improve starting. 

Consider a cross-section on a plane transversal to the machine s longitudinal axis of rotation o, as suggested in Fig. 8.3a. Because of the usual modeling assumptions, the magnetic phenomena are considered to be uniformly distributed along the longitudinal axis, hence they are analyzed with a 2D approach on the plane of the cross-section [9, chapter 10], In Fig. 8.3 some notation is fixed for further reference. The cartesian (jc, y)-coordinate system (axes at 90° electrical to each other) is fixed to the stator. The 3-phase (star- or Y-connected) stator winding shown in... 

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