CONSTRUCTION OF D.C. MACHINE

• •    A dc machine can either be operated as a generator or as a motor.

• •    Therefore the construction is same for both dc generator and dc motor.

• •    The sectional view of the dc machine is shown in the following Figure.

The main parts in a dc machine are

1. Yoke or magnetic frame

2. Magnetic poles

3. Field coils

4. Inter poles (or) Commutator poles

5. Armature and Armature coils

6. Commutator and Brushes

7. Bearings and End covers





Yoke or magnetic frame

• •    It is made of cast iron for small machines. For large machines, it is made of cast steel.

Function of yoke

i)    It provides mechanical support for the machine and acts as a cover
for the machine

ii)    It forms the portion of a magnetic circuit.

Magnetic poles

• •    The field magnet consists of pole cores and pole shoes.

• •    The pole cores and pole shoes are built with thin laminations of annealed steel and are held together using rivets or under hydraulic pressure.

• •    These magnetic poles are fitted to the yoke using screws.

• •    An advantage of pole cores built up of laminations is that eddy current losses in the pole faces are minimized.

Function of pole shoes

(i)    They spread out the flux in the air gap and also reduce the reluctance of the magnetic path at its larger cross section.

(ii)    They support the exciting coils or field coils.

      Field coils

• •    Field coils are wound with enameled copper wire. Sometimes cotton insulation is used.

• •    The coils are dried in vaccum and then impregnated with an insulating compound.

• •    Sufficient space is left between the layers for ventilating purposes.

• •    The exciting coils on the inter poles are connected in series with the armature.

• •    So they carry the full armature current and are made up of a few turns of heavy conductor.

Interpoles or Commutating poles

Inter poles are present in the high capacity dc machines.

Function of Interpoles or Commutating poles

 (i) To improve commutation and

(ii) To reduce armature reaction

Armature and Armature coils

•    When the armature is revolving, it is subjected to alternating magnetization.
•    This causes hysteresis loss.
•    To minimize this loss, low hysteresis steel containing a few percentage of silicon is used in the armature.
•    To reduce the eddy current loss, laminations are provided .

•    The armature coils are wound with single cotton covered or double cotton covered or enameled insulated wires.
•    These coils are put into the insulated slots of the armature to avoid short circuit between the armature conductors and core.
•    The slots are closed by fiber or wooden wedges to prevent the conductors from plying out due to the centrifugal force caused by the rotation of the armature.
•    The armature is impregnated with varnish and dried. In medium and small machines, circular conductors are used.
•    In large machines, rectangular strips of conductors are used for winding the armature coils.

Commutator

•    It is made of wedge shaped segments of hard drawn (or) drop forged copper insulated from each other by thin layers of built-up mica.
•    The segments are held together by clamping them using Vshaped end rings and insulated from the segments by V shaped micanite rings.
•    Each segment is provided with a riser to which the leads of the armature coils are soldered.

Brushes

•    The purpose of brush is to carry the current from the commutator to the external circuit.
•    It is made of carbon or copper. For low voltage machines, it is made of copper-carbon compound.
•    The brushes are placed in the brush holder, which is kept pressed against the commutator by a spring.
•    The brush holder is fitted to a spindle, which is insulated from the machine.
•    Connections from the brushes are taken by means of flexible pigtails, made of copper ribbons.

Bearings and End covers

•    End covers are made of cast iron or fabricated steel are fitted to both ends of the yoke.
•    To these end covers either ball bearings or roller bearings are fitted and armature shaft is mounted over these bearings.
•    These bearings are lubricated with grease or hard oi


Armature winding fundamentals

•    In modern dc machines, drum type of armature windings is provided. In this type, the coils are wound and placed into the insulated slots of the armature.
•    A coil with more than one turn is known as multi turn coil.
•    The coils are always placed in two slots which are approximately one pole pitch apart.
•    The portion of a coil placed inside a slot is called coil side and each coil will have number of conductors.
•    The distance between the two coil sides of a coil is termed as coil pitch, which is expressed in terms of
number of slots.
•    The ends of a coil are connected to two different commutator segments on the commutator.
•    The distance in terms of number of commutator segments, between the commutator segments to which the ends of a coil are connected is called commutator pitch.
•    The double layer windings are present in the dc machine. In double layer winding there will be two coil sides belonging to two different coils in each slot.
•    If a coil side of one coil is laid in the top layer of a slot, the other coil side of the same coil occupies the bottom layer of the slot usually one pole pitch.

Drum type of winding has two types.

1. Lap winding

2. Wave winding

Lap winding

•    In lap winding each coil is connected in series with next coil under the same pole pair.
•    If the start of one coil is connected to one particular commutator segment, the end of that coil and start of the next coil are connected to the next commutator segment either to the right or left of the first commutator segment.
•    This process is continued till all the coils are connected. Here the commutator pitch is always + 1.
•    In lap winding there are as many parallel paths as there are number of poles.
•    The emf induced in any generator is equal to the emf induced in all the coils in a parallel path.
•    Therefore lap winding is used in low voltage large current dc machines.