Iii6 Threephase Power

Three-phase power is the form of power most often distributed to industrial users. This form of transmission has three advantages over single-phase systems: (1) less copper is required to supply a given power at given voltage; (2) if the load of each phase of the three-phase source is identical, the instantaneous output of the alternator is constant; and (3) a three-phase system produces a magnetic field of constant density that rotates at the line frequency—this greatly reduces the complexity of motor construction.

The author realizes that both delta systems and wye systems exist, but will concentrate on four-wire wye systems as being representative of internal distribution systems. This type of internal distribution system allows the customer both single-phase and three-phase service. Our focus will be on measuring power and determining power factor in four-wire three-phase wye-connected systems.

III.6.1 The Four-Wire Wye-Connected System

Figure III.19 shows a generalized four-wire wye-connected system. The coils represent the secondary windings of the transformers at the site substation while the generalized loads represent phase loads that are the

Table III.1 How to Select Capacitor Ratings for Induction Motors/Source: 1.

Table III.1 How to Select Capacitor Ratings for Induction Motors/Source: 1.

Fig. III.19 Generalized four-wire wye-connected system. The coils A, B, and C represent the three transformer secondaries at the site substation; while Za, ZB. and Zc are the generalized loads seen by each phase.

TABLE IV—220-, 440-, 550-V0LT MOTORS, TOTALLY-ENCLOSED, FAN-COOLED, GENERAL ELECTRIC TYPE KG {NEMA DESIGN "C"), HIGH STARTING TORQUE, NORMAL STARTING CURRENT

Induction

Nominal Motor Sp««d in Rpm and Numbir of Pol«»

0 0

Post a comment