1-13
As you can see, eddy current losses are kept low when the core material is made up of many thin
sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The laminations
are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of
the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The
insulation value need not be high because the voltages induced are very small.
Most generators use armatures with laminated cores to reduce eddy current losses.
Q15. How can eddy current be reduced?
Hysteresis Losses
Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature
core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When
the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of
the magnetic particles, as they try to align themselves with the magnetic field, produces molecular
friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes
armature resistances to increase.
To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc
generator armatures. After the steel has been formed to the proper shape, the laminations are heated and
allowed to cool. This annealing process reduces the hysteresis loss to a low value.
THE PRACTICAL DC GENERATOR
The actual construction and operation of a practical dc generator differs somewhat from our
elementary generators. The differences are in the construction of the armature, the manner in which the
armature is wound, and the method of developing the main field.
A generator that has only one or two armature loops has high ripple voltage. This results in too little
current to be of any practical use. To increase the amount of current output, a number of loops of wire are
used. These additional loops do away with most of the ripple. The loops of wire, called windings, are
evenly spaced around the armature so that the distance between each winding is the same.
The commutator in a practical generator is also different. It has several segments instead of two or
four, as in our elementary generators. The number of segments must equal the number of armature coils.
GRAMME-RING ARMATURE
The diagram of a GRAMME-RING armature is shown in figure 1-12, view A. Each coil is
connected to two commutator segments as shown. One end of coil 1goes to segment A, and the other end
of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to
segment B. The rest of the coils are connected in a like manner, in series, around the armature. To
complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every
other coil.
