Advantages of rotating field As everywhere a.c. is used, the generation level of a.c. voltage may be higher as 11 KV to 33 KV. This gets induced in the armature. For stationary armature large space can be provided to accommodate large number of conductors and the insulations. It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. So high voltage armature is generally kept stationary. This avoids the interaction of mechanical and electrical stresses.

It is easier to collect larger currents at very high voltage from a stationary member than from the slip ring and brush assembly. The voltage required to be supplied to the field is very low (110 V to 220 V d.c.) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating. The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature as stationary.

Due to low voltage level on the field side, the insulation required is less and hence field system has very low inertia. It is always better to rotate low inertia system than high inertia, as efforts required to rotate low inertia system are always less. Rotating field makes the overall construction very simple. With simple, robust mechanical construction and low inertia of rotor, it can be driven at high speeds. So greater output can obtained from an alternator of given size

If field is rotating, to excite it be external d.c. supply two slip rings are enough. Once each for positive and negative terminals. As against this, in three phase rotating armature the minimum number of slip rings required are three and can not be easily insulated due to high voltage levels. The ventilation arrangement for high voltage side can be improved if it is kept stationary.

AC Windings Conductor. A length of wire which takes active part in the energy-conversion process is a called a conductor. Turn. One turn consists of two conductors. Coil. One coil may consist of any number of turns. Coil –side. One coil with any number of turns has two coil-sides.

Single- layer and double layer windings. Single- layer winding One coil-side occupies the total slot area Used only in small ac machines

Double- layer winding Slot contains even number (may be 2,4,6 etc.) of coil-sides in two layers Double-layer winding is more common above about 5kW machines

Pole – pitch. A pole pitch is defined as the peripheral distance between identical points on two adjacent poles. Pole pitch is always equal to 180 o Coil–span or coil-pitch. The distance between the two coil-sides of a coil is called coil-span or coil-pitch. It is usually measured in terms of teeth, slots or electrical degrees.

Chorded-coil. If the coil-span (or coil-pitch) is equal to the pole-pitch, then the coil is termed a full-pitch coil. In case the coil-pitch is less than pole-pitch, then it is called chorded, short-pitch or fractional-pitch coil

In AC armature windings, the separate coils may be connected in several different manners, but the two most common methods are lap and wave In poly phase windings it is essential that the generated emfs of all the phases are of equal magnitude The waveforms of the phase emfs are identical The frequency of the phase emfs are equal

The windings used in rotating electrical machines can be classified as Concentrated Windings All the winding turns are wound together in series to form one multi-turn coil All the turns have the same magnetic axis Examples of concentrated winding are – field windings for salient-pole synchronous machines – D.C. machines – Primary and secondary windings of a transformer

Distributed Windings All the winding turns are arranged in several full-pitch or fractional-pitch coils These coils are then housed in the slots spread around the air-gap periphery to form phase or commutator winding Examples of distributed winding are – Stator and rotor of induction machines – The armatures of both synchronous and D.C. machines

if the number of slots per pole per phase is an integer, then the winding is called an integral-slot winding In case the number of slots per pole per phase (q) is not an integer, the winding is called fractional-slot winding.

Winding Factor (Coil Pitch and Distributed Windings) Pitch Factor or Coil Pitch: The ratio of phasor (vector) sum of induced emfs per coil to the arithmetic sum of induced emfs per coil is known as pitch factor (Kp) or coil span factor (Kc) which is always less than unity. Let the coil have a pitch short by angle θ electrical space degrees from full pitch and induced emf in each coil side be E,

If the coil would have been full pitched, then total induced emf in the coil would have been 2E. when the coil is short pitched by θ electrical space degrees the resultant induced emf, E R in the coil is phasor sum of two voltages, θ apart =2cos/2

Advantages Shortens the ends of the winding and therefore there is a saving in the conductor material Reduce the effects of distorting harmonics, and thus the waveform of the generated voltage is improved and making it approach a sine wave

The distribution factor is always less than unity. Let no. of slots per pole = Q and no. of slots per pole per phase = m. Induced emf in each coil side = Ec Angular displacement between the slots, β=180/ The emf induced in different coils of one phase under one pole are represented by side AC, CD, DE, EF… Which are equal in magnitude (say each equal Ec) and differ in phase (say by β o ) from each other.

If bisectors are drawn on AC, CD, DE, EF… they would meet at common point (O). The point O would be the circum center of the circle having AC, CD, DE, EF…as the chords and representing the emfs induced in the coils in different slots.

E.M.F. EQUATION OF AN ALTERNATOR OR AC GENERATOR. Let, P= No. of poles Z= No. of Conductors or Coil sides in series/phase i.e. Z= 2T…Where T is the number of coils or turns per phase (Note that one turn or coil has two ends or sides) f = frequency of induced e.m.f in Hz ф = Flux per pole (Weber) N = Rotor speed (RPM)

In one revolution of the rotor i.e. in 60/N seconds, each conductor is cut by a flux of Pф Weber. dф= фP and also dt= 60/N seconds then induced e.m.f per conductor ( average) = dф/ dt= Pф/(60/N) =P N ф/60…..(a) But We know that f = PN/120 or N= 120f/P Putting the value of N in Equation (a)… We get the average value of e.m.f per conductor is = Pф/60 x 120 f/P = 2f ф Volts. —  {N= 120f/P} If there are Z conductors in series per phase, then average e.m.f per phase = 2fфZ Volts= 4fфT Volts ….{Z=2T}

Also we know that Form factor= RMS Value/Average Value… RMS value= Form factor x Average Value, = 1.11 x 4fфT = 4.44fфT Volts. ( Note that it is exactly the same equation as the e.m.f equation of the transformer) And the actually available voltage per phase = 4.44 K c K d fфT