DC Machines: DC Generators (part 2)

Armature Windings

Lap-Wound Armatures:

Lap-wound armatures are used in machines designed for low voltage and high current. These armatures are generally constructed with large wire because of high current. A good example of where lap-wound armatures are used is in the starter motor of almost all automobiles. One characteristic of machines that use a lap-wound armature is that they have as many pairs of brushes as there are pairs of poles. The windings of a lap-wound armature are connected in parallel. This permits the current capacity of each winding to be added and provides a higher operating current. Lap-wound armatures have as many parallel paths through the armature as there are pole pieces.

Wave-Wound Armatures:

Wave-wound armatures are used in machines designed for high voltage and low current. These armatures have their windings connected in series. When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same. A good example of where wave-wound armatures are used is in the small generator in hand-cranked megohmmeters. Wave-wound armatures never contain more than two parallel paths for current flow regardless of the number of pole pieces, and they never contain more than one set of brushes (a set being one brush or group of brushes for positive and one brush or group of brushes for negative).

Frogleg-Wound Armatures:

Frogleg-wound armatures are probably the most used. These armatures are used in machines designed for use with moderate current and moderate voltage. The windings of a frogleg-wound armature are connected in series-parallel. Most large DC machines use frogleg-wound armatures.

--19 Frogleg-wound armatures are connected in series-parallel. These windings are generally used in machines intended for medium voltage and current operation.

Brushes

The brushes ride against the commutator segments and are used to connect the armature to the external circuit of the DC machine. Brushes are made from a material that is softer than the copper bars of the commutator. This permits the brushes, which are easy to replace, to wear instead of the commutator. The brush leads are generally marked A1 and A2 and are referred to as the armature leads.

Pole Pieces

The pole pieces are located inside the housing of the DC machine. The pole pieces provide the magnetic field necessary for the operation of the machine. They are constructed of some type of good magnetic conductive material such as soft iron or silicon steel. Some DC generators use permanent magnets to provide the magnetic field instead of electromagnets. These machines are generally small and rated about one horsepower or less. A DC generator that uses permanent magnets as its field is referred to as a magneto.

--20 Pole pieces are constructed of soft iron and placed on the inside of the housing.

Field Windings

Most DC machines use wound electromagnets to provide the magnetic field. Two types of field windings are used. One is the series field and the other is the shunt field. Series field windings are made with relatively few turns of very large wire and have a very low resistance. They are so named because they are connected in series with the armature. The terminal leads of the series field are labeled S1 and S2. It’s not uncommon to find the series field of large horsepower machines wound with square or rectangular wire. The use of square wire permits the windings to be laid closer together, which increases the number of turns that can be wound in a particular space. Additionally, smaller square and rectangular wire can be used to yield the same surface area of larger round wire.

--21 Square wire permits more turns than round wire in the same area. 48 turns 80 turns Core material

--22 A square wire of equal size contains more surface area than round wire. Additional area of square wire.

--23 Both series and shunt field windings are contained on each pole piece. Shunt field winding; Series field winding

Shunt field windings are made with many turns of small wire. Because the shunt field is constructed with relatively small wire, it has a much higher resistance than the series field. The shunt field is intended to be connected in parallel with, or shunt, the armature. The resistance of the shunt field must be high because its resistance is used to limit current flow through the field. The shunt field is often referred to as the "field," and its terminal leads are labeled F1 and F2.

When a DC machine uses both series and shunt fields, each pole piece contains both windings. The windings are wound on the pole pieces in such a manner that when current flows through the winding it produces alternate magnetic polarities. In --23, two pole pieces form north magnetic polarities and two form south magnetic polarities. A DC machine with two field poles and one interpole. Interpoles are discussed later in this unit.

--24 A two-pole DC machine with one interpole.

--25 The series field is connected in series with the armature.

--26 Schematic drawing of a series generator.

--27 Residual magnetism produces an initial voltage used to provide a current flow through the load.

--28 If more load is added, current flow and output voltage increase.