Three-Phase Connections

Electrical power systems are normally three-phase systems connected using wye or delta connections. In wye connections the three phases are connected to form a letter Y with a neutral point at the intersection of the three phases. In delta connections the three phases are connected to form a Greek letter delta (A). Delta systems do not have a neutral hence delta systems are 3-wire systems. A Y connection has a neutral and thus it is a 4-wire system. 

Generators typically are Y-connected to provide three phases and a neutral for a 3-phase, 4-wire system. The neutral can be grounded or ungrounded, but a grounded neutral is usually preferred. 

Transformers can be delta-connected on both the primary side and the secondary side for a 3-phase, 3-wire system or delta on the primary side and Y on secondary side for a 3-phase, 4-wire system. Transformers can also be Y connected on both primary and secondary sides, but such is not 

Apparent power is the total power of a circuit and is measured in VA or kVA (1,000 VA). It is obtained by multiplying voltage and current. 

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With regard to the schematic in Figure 9.1, consider the following interfaces the liquid and solid, the liquid and gas, and the solid and gas. At the point a the three phases connect and the forces must balance if the system is at equilibrium. The forces can be resolved into components parallel to the substrate and... 

Heating cycles and power inputs are tabulated in Fig. 145 and 146. Induction heatings for containers are preferably designed in accordance with standardized inputs in single-phase connection 10, 16, 25, 40 and 63 kVA, in three-phase connection 100, 125, 160 and 200 kVA. 

Fig. 6. Connectivity patterns for a diphasic soHd showing zero-, one-, two-, or three-dimensional connectivity of each phase to itself. In the 3—1 composite, for instance, the shaded phase is three-dimension ally connected. Arrows are used to indicate the connected directions.

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Check the starter connections and contacts in all three phases. Also check proper contacts and brush pressure in slip-ring motors. If these are satisfactory and the motor still does not pick up, check the suitability of the motor for the type of load and switching method. 

Power input to a three-phase machine may be measured by two single-phase wattmeters, connected as in the two-w attmeter method. (Section 11.4.3). Alternatively i single polyphase wattmeter may be used. 

When the primary of a three-phase two-winding transformer, having its secondary wound for a three-phase open delta, is connected across an unbalanced supply system, a residual voltage across the open delta will appear. This is the principle on which this transformer is based (Figure 15.4(a)). As discussed in Section 21.2.2, and illustrated in Figure 21.7, the phasor sum of all the three line to ground voltages in a three-phase balanced system is zero, i.e. 

A three-phase three-wire, star connected isolated neutral system,... 

Determine the discharge device for the discharge of a three-phase 6.6 kV, 50 Hz, 1000 kVAr, yconnected capacitor bank, connected in units of 10 x 100 kVAr each, through an automatic p.f. correction relay, having a closing cycle of 10 seconds. Data available from the capacitor manufacturer,... 

These transformers are three-phase and may be connected for zig-zag or star/delta connections (Section 20.9.1). The delta may also be made open type by inserting a resistor across it to help adjust the zero-sequence impedance, if required. 

These are meant to be used with a capacitor to tune a filter circuit, with resonances in the audio frequency range for reducing and filtering the harmonics or communication frequencies. They provide a near short-circuit for the required harmonics to filter them out of circuit. They may be single-phase or three-phase and connected in series or parallel of the capacitor circuit and may have a fixed or variable reactance, rated continuously with saturated magnetic characteristics. They may incur heavy losses. 

The other place where the constitution is not fully defined is where there is a horizontal line on the phase diagram. The lead-tin diagram has one line like this - it runs across the diagram at 183°C and connects (Sn) of 2.5 wt% lead, L of 38.1% lead and (Pb) of 81% lead. Just above 183°C an alloy of tin -i- 38.1% lead is single-phase liquid (Fig. 3.5). Just below 183°C it is two-phase, (Sn) -i- (Pb). At 183°C we have a three-phase mixture of L -I- (Sn) -I- (Pb) but we can t of course say from the phase diagram what the relative weights of the three phases are. 

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

The common connection of all three armature windings in the Y connection allows a fourth, or neutral, conductor to be used. This neutral point is often grounded in transmission and distribution circuits. Such a circuit is termed a three-phase, four-wire circuit. 

Just as the armature coils of a three-phase generator may be connected in a A or Y configuration, the circuit loads may be connected in a A or Y configuration. The A-load configuration may be supplied from a source that is connected in either A or Y. The Y connection may include a neutral (fourth) wire, connected at the common connection of the circuit. 

Sro.5[(Zr6B)Clii.34(2)l2.66] have been characterized by X-ray diffraction. The Zre octahedra in these phases are three-dimensionally connected by exo-iodide atoms, which simultaneously bridge three octahedra. The resulting con-... 

Liquid-Fluid Equilibria Nearly all binary liquid-fluid phase diagrams can be conveniently placed in one of six classes (Prausnitz, Licntenthaler, and de Azevedo, Molecular Thermodynamics of Fluid Phase Blquilibria, 3d ed., Prentice-Hall, Upper Saddle River, N.J., 1998). Two-phase regions are represented by an area and three-phase regions by a line. In class I, the two components are completely miscible, and a single critical mixture curve connects their criticsu points. Other classes may include intersections between three phase lines and critical curves. For a ternary wstem, the slopes of the tie lines (distribution coefficients) and the size of the two-phase region can vary significantly with pressure as well as temperature due to the compressibility of the solvent. 

The phase equilibrium for pure components is illustrated in Figure 4.1. At low temperatures, the component forms a solid phase. At high temperatures and low pressures, the component forms a vapor phase. At high pressures and high temperatures, the component forms a liquid phase. The phase equilibrium boundaries between each of the phases are illustrated in Figure 4.1. The point where the three phase equilibrium boundaries meet is the triple point, where solid, liquid and vapor coexist. The phase equilibrium boundary between liquid and vapor terminates at the critical point. Above the critical temperature, no liquid forms, no matter how high the pressure. The phase equilibrium boundary between liquid and vapor connects the triple point and the... 

In contrast to these we have the equilibrium processes of sublimation, absorption, dissolution, precipitation, evaporation, and condensation, throngh which the physical states of solid, Uqnid, and gas are connected. For example, the common crystallization of salts from sea water involves all three phases. Distillation, which is essential for prodncing organic solvents, is a two-step evaporation (liquid => gas) condensation (gas => Uqnid) process. 

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