Now we are going to discuss about armature winding in details. Before going through this section, we
should understand some basic terms related to armature winding of DC generator
The pole pitch is defined as the peripheral distance between centers of two adjacent poles in DC machine.
This distance is measured in term of armature slots or armature conductor come between two adjacent
pole centers. Pole Pitch is naturally equal to the total number of armature slots divided by the number of
poles in the machine.
If there are 96 slots on the armature periphery and 4 numbers of poles in the machine, the numbers of
armature slots come between two adjacent poles centre’s would be 96/4 = 24. Hence, the pole pitch of
that DC machine would be 24.
As we have seen that, pole pitch is equal to total numbers of armature slots divided by total numbers of
poles, we alternatively refer it as armature slots per pole.
Coil of dc machine is made up of one turn or multi turns of the conductor. If the coil is made up of single
turn or a single loop of conductor, it is called single turn coil. If the coil is made up of more than one turn
of a conductor, we refer it as a multi-turn coil. A single turn coil will have one conductor per side of the
coil whereas, in multi turns coil, there will be multiple conductors per side of the coil. Whatever may be
the number of conductors per side of the coil, each coil side is placed inside one armature slot only. That
means all conductors of one side of a particular coil must be placed in one single slot only. Similarly, we
place all conductors of opposite side of the coil in another single armature slot.
Coil span is defined as the peripheral distance between two sides of a coil, measured in term of the
number of armature slots between them. That means, after placing one side of the coil in a particular slot,
after how many conjugative slots, the other side of the same coil is placed on the armature. This number
is known as coil span.
If the coil span is equal to the pole pitch, then the armature winding is said to be full – pitched. In this
situation, two opposite sides of the coil lie under two opposite poles. Hence emf induced in one side of
the coil will be in 180o
phase shift with emf induced in the other side of the coil. Thus, the total terminal
voltage of the coil will be nothing but the direct arithmetic sum of these two emfs. If the coil span is less
than the pole pitch, then the winding is referred as fractional pitched. In this coil, there will be a phase
difference between induced emf in two sides, less than 1800
. Hence resultant terminal voltage of the coil is
vector sum of these two emf’s and it is less than that of full-pitched coil.
In practice, coil pitch (or Span) as low as eight tenth of a Pole Pitch, is employed without much serious
reduction in emf. Fractional pitched windings are purposely used to effect substantial saving in copper of
the end connection and for improving commutation.