is connected across phases B and C in series. Thus, the voltage across each load is larger than the voltage across a single phase. In a wye-connected ac generator, the three  start  ends  of  each  single-phase  winding  are connected  together  to  a  common  neutral  point  and  the opposite,  or  finish,  ends  are  connected  to  the  line terminals, A, B, and C. These letters are always used to designate the three phases of a three-phase system, or the three line wires to which the ac generator phases comect. A   three-phase,   wye-connected   ac   generator supplying three separate loads is shown in figure 3-13. When unbalanced loads are used, a neutral may be added as shown in the figure by the broken line between the common neutral point and the loads. The neutral wire serves as a common return circuit for all three phases and maintains a voltage balance across the loads. No current flows in the neutral wire when the loads are balanced.  This  system  is  a  three-phase,  four-wire  circuit and  is  used  to  distribute  three-phase  power  to shorebased installations. The three-phase, four-wire system is not used aboard ship, but it is widely used in industry and in aircraft ac power systems. Delta  Connection A  three-phase  stator  may  also  be  connected  as shown in figure 3-12, view C. This is called the delta connection. In a delta-connected ac generator, the start end of one phase winding is connected to the finish end of the third; the start of the third phase winding is connected to the finish of the second phase winding; and the start of the second phase winding is connected to the finish of the first phase winding. The three junction points are connected to the line wires leading to the load. A  three-phase,  delta-connected,  ac  generator  is depicted in figure 3-14. The generator is connected to a  three-phase,  three-wire  circuit,  which  supplies  a three-phase, delta-connected load at the right-hand end of   the   three-phase   line.   Because   the   phases   are connected  directly  across  the  line  wires,  phase  voltage Figure 313.—Three-phase ac generatar showing neutral connection. Figure 3-14.-Three-phase, delta-connected system. is equal to line voltage. When the generator phases are properly  connected  in  delta,  no  appreciable  current flows within the delta loop when there is no external load connected to the generator. If anyone of the phases is  reversed  with  respect  to  its  correct  connection,  a short-circuit current flows within the windings of no load, causing damage to the windings. TRANSFORMERS A transformer is a device that has no moving parts and that transfers energy from one circuit to another by electromagnetic   induction.   The   energy   is   always transferred without a change in frequency, but usually with   changes   in   voltage   and   current.   A   step-up transformer receives electrical energy at one voltage and delivers it at a higher voltage. Conversely, a step-down transformer  receives  energy  at  one  voltage  and  delivers it at a lower voltage. Transformers require little care and maintenance  because  of  their  simple,  rugged,  and durable  construction.  The  efficiency  of  transformers  is high.  Because  of  this,  transformers  are  responsible  for the more extensive use of alternating current than direct current. The   conventional   constant-potential transformer is designed to operate with the primary connected across a constant-potential source and to provide  a  secondary  voltage  that  is  substantially constant from no load to full load. Various  types  of  small,  single-phase  transformers are  used  in  electrical  equipment.  In  many  installations, transformers are used on switchboards to step down the voltage  for  indicating  lights.  Low-voltage  transformers are included in some motor control panels to supply control circuits or to operate overload relays. Instrument  transformers  include  potential,  or voltage,   transformers   and   current   transformers. Instrument transformers are commonly used with ac instruments when high voltages or large currents are to be  measured. 3-9